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A Rossby wave breaking-induced enhancement in the tropospheric ozone over the Central Himalayan region
Atmospheric Environment 224 (2020) 117356
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A Rossby wave breaking-induced enhancement in the tropospheric ozone over the Central Himalayan region Kondapalli Niranjan Kumar a, d, *, Som Kumar Sharma a, Manish Naja b, D.V. Phanikumar c a
Physical Research Laboratory, Ahmedabad, India Aryabhatta Research Institute of Observational Sciences, Nainital, India c Department of Science and Technology, New Delhi, India d National Centre for Medium Range Weather Forecasting, Ministry of Earth Sciences, New Delhi, India b
H I G H L I G H T S
� Unusual enhancement of surface and tropospheric ozone in the Himalayan region. � Impact of Rossby Wave Breaking in the upper troposphere prior to ozone enhancement. � Vertical transport of ozone rich dry air from the stratosphere deep into troposphere. A R T I C L E I N F O
A B S T R A C T
Keywords: Tropospheric Ozone Rossby waves Potential Vorticity Tropospheric Emission Spectrometer Ozone Monitoring Instrument
The high-altitude regions in the Himalayas are prone to high ozone concentrations frequently resulting from diverse dynamical and transport mechanisms. Here, we report an unusual enhancement in the surface and tropospheric ozone concentrations over the central Himalayan region from ground-based and space-borne measurements in the month of December 2010. The surface ozone levels (~80 ppbv) on 18–19 December 2010 is observed to be two-fold higher relative to the seasonal average (December-January-February) of about 40–50 ppbv in the central Himalayan region. The space-borne measurements from Tropospheric Emission Spectrometer and Ozone Monitoring Instrument onboard Aqua satellite also show higher values in the tropo spheric column ozone over this region. The satellite observations indicate an increase in tropopause temperature of about 5 � C and decrease in tropopause altitude about 1 km during 18–19 December 2010 resulting in the occurrence of tropopause fold facilitating the stratospheric-tropospheric exchange processes over the study re gion. The plausible reason for the occurrence of tropopause fold and subsequent enhancement of tropospheric and surface ozone is found to be associated with the breaking Rossby waves in the upper troposphere. The wave breaking leads to the advection of high-PV (potential vorticity) air, with magnitudes of about 3–4 PVU, towards the central Himalayan region from high-latitudes. The vertical component of PV advection also shows a deep stratospheric intrusion of high-PV air into the troposphere. The isentropic transport of ozone across the folding tropopause due to the wave breaking is clearly depicted from the satellite and reanalysis datasets. Therefore, the present study has strong implications of upper tropospheric wave dynamics to the tropospheric and surface ozone over the Himalayan regions having complex topography.
1. Introduction The tropospheric ozone is one of the most important atmospheric constituents controlling air quality and global climate (e.g., Monks et al., 2015). Increase in tropospheric ozone concentration in recent times has become a major concern to the scientific community because of its central role in the tropospheric chemistry (e.g., Oltmans et al. 2006). It is
an air pollutant and greenhouse gas thereby influencing the air quality and climate (IPCC, 2013; Liu et al., 2019). The average lifetime of O3 is approximately 5–7 days in the free troposphere. However, depending upon the local meteorological conditions, large spatial and temporal variability exists in ozone concentration. Tropospheric ozone is only about 10% of total ozone, but it causes an adverse effect on human health as well as on crop yield and ecosystem (Zeng et al., 2008;
* Corresponding author. National Centre for Medium Range Weather Forecasting, Ministry of Earth Sciences, Noida, U.P, India. E-mail addresses: [email protected], [email protected] (K.N. Kumar). https://doi.org/10.1016/j.atmosenv.2020.117356 Received 11 September 2019; Received in revised form 11 January 2020; Accepted 15 February 2020 Available online 20 February 2020 1352-2310/© 2020 Elsevier Ltd. All rights reserved.
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Lelieveld et al., 2015; Lal et al., 2017). The major sources of the tropospheric ozone are in situ photochemical process, involving its precursor gases like CO, hydrocarbons with an important and critical role of NO–NO2(Crutzen et al., 1999; Sharma et al., 2017). Another source in downward transport of ozone from the stratosphere (Levy � et al., 1985; Skerlak et al., 2019). Similarly, ozone loss takes place via chemical reactions and surface depositions. A global effort, utilizing several global models indicates a large variability and uncertainty in the budget of tropospheric ozone (Wild, 2007). Horizontal and vertical transport of ozone and its precursor gases leads to further complication (Lal et al., 2013; Naja et al., 2016; Bhardwaj et al., 2018), particularly over regions having complex orography induced wave generations like in the Himalayan region (Phanikumar et al., 2017; Brunamonti et al., 2018). Apart from the photochemical processes, the non-local dynam ical processes, including stratospheric intrusions, are also well-recognized precursors in the context of wave forcing that controls the variability of tropospheric ozone (Lelieveld and Dentener, 2000; WMO, 2003; Lin et al., 2015). The downward flux of ozone from the stratosphere can be more prominent near the cyclonic regions and jet streams in the upper troposphere (Das et al., 2011; Leclair De Bellevue et al., 2006; Austin and Midgley, 1994; Holton et al., 1995; Stohl et al., 2000). The troughs and cut-off lows including the tropopause folds are also active regions of the stratospheric intrusion of the ozone (WMO, 1986). The ozone enhancement due to the Stratosphere-Troposphere Exchange (STE) processes has been studied in the presence of various meteorological events such as tropical cyclones, mountain waves, frontal disturbances, and remote forcing of sea surface temperatures (Hocking et al., 2007; Das, 2009; Nath et al., 2016; Phanikumar et al., 2017; Albers et al., � 2018; Edwards et al., 2018; Skerlak et al., 2019). For instance, Albers et al. (2018) reported stratospheric intrusions of ozone in the context of El Nino Southern Oscillation (ENSO)-related jet variability through wave breaking over the Pacific-North American region. Das et al. (2009) reported the stratospheric intrusion of ozone rich dry air into the troposphere in the vicinity of a tropical cyclone using the radar obser vations over the tropical latitudes. The upper-level fronts also cause significant enhancement in the surface ozone through rapid transport from the stratosphere, in the presence of tropopause folds, depending � upon the state of boundary layer stability and turbulence (Skerlak, 2014; � Skerlak et al., 2019). For instance, model simulations over complex to pographies such as the Rocky Mountains and Tibetan Plateau indicate the vertical transport of ozone into the surface will be inhibited either � completely or partially when the stable boundary layer exists (Skerlak et al., 2019). However, the turbulence in the deep boundary layer over the elevated topographies causes significant mixing enabling the vertical transport to the surface but still, the stratospheric air mass is diluted before it reaches the surface. Hence, the surface ozone enhancement is also critically dependent on the boundary layer processes during the � stratospheric intrusions (Skerlak et al., 2019). Bracci et al. (2012) have characterized various synoptic conditions leading to the transport of ozone over the Nepal region in the Himalayas. The authors have noted that 94% of cases of stratosphere-troposphere transport occur in the presence of stratospheric potential vorticity structures, upper-tropospheric jet, quasi-stationary ridges, and monsoon depressions during their observational period between the years 2006 and 2008. Ojha et al. (2014) studied the vertical distribution of ozone using the weekly balloon measurements in the central Himalayan re gion. It is noted that significant seasonality exists in the tropospheric ozone with higher magnitudes during spring followed by autumn. In contrast to the spring maximum of surface ozone as reported in the Himalayan region in previous studies, Ding and Wang (2006) reported summertime maximum over the Tibetan Plateau due to stronger sub tropical jet and subsequent stratospheric intrusions. The above studies over the complex Himalayan region also indicate occasional enhance ment in the ozone during some days in the mid- and upper-tropospheric ozone with a simultaneous reduction in the relative humidity due to
stratospheric intrusions. Also, the ozone build-up during springtime is strongly linked to the regional pollution in the Indo-Gangetic Plain (IGP) region and biomass burning in north India (Ojha et al., 2014). In a recent study by Phanikumar et al. (2017) reported an anomalous tropospheric ozone enhancement in the presence of tropopause fold formed due to orographic gravity wave breaking in the central Himalayan region. Therefore, a prior understanding of the main precursors of a given re gion is very important for improving ozone prediction models (Liu et al., 2018). Hence, in the present study, we report the interaction of planetaryscale Rossby waves with the atmospheric circulation and the ozone distribution that has been marginally studied over the central Himala yan region, despite the surface weather in these regions are strongly controlled by the wave forcing associated with the upper-tropospheric Rossby waves during winter (Niranjan Kumar et al., 2016). We detec ted an unusual high tropospheric and surface ozone concentration during December 2010 in the central Himalayan region using the ground-based and satellite observations. We also provide observational evidence of Rossby wave breaking and interaction with atmospheric circulation leading to the anomalous enhancement (depletion) of lower tropospheric (stratospheric) ozone by using ground-based and satellite datasets. Therefore, the present study investigates an unusual enhancement in the surface and tropospheric ozone due to Rossby wave breaking episode in the Himalayan region. In this study, we will describe the various datasets acquired for this study, followed by the results and discussion. Finally, we present the summary and conclusions and scope of this work. 2. Data and methodology 2.1. Surface ozone measurements For the present study, we utilize surface ozone measurements from a high-altitude site, Nainital(29.37⁰N, 79.45⁰; E, 1958 m amsl.) in the central Himalayan region by using UV photometric ozone analyzers of Environment S.A. (model O341M), France for the measurement of the ozone mixing ratio. The retrieval of ozone is based on the principle of absorption of ultraviolet radiation (wavelength ~ 254 nm) by the ozone molecules using Beer–Lambert’s law. The lower detection limit of the surface ozone is ~1 ppb with an accuracy of ~5%. More details of the instrument and analysis technique are outlined in earlier papers (e.g., Kumar et al. 2010, 2011). There is no industry in the Nainital town. Some non-combustible industries are in Rudrapur town, which is about 40 km south of Nainital. 2.2. Space-borne ozone observations We have used ozone measurements from two satellites one is the Tropospheric Emission Spectrometer (TES; Beer, 2006) and another one is Ozone Monitoring Instrument (OMI; Levelt et al., 2006), respectively to further strengthen our results from ground-based measurements. These two instruments are onboard NASA EOS (Earth Observing System) Aura satellite which has a ~705 km sun-synchronous polar orbit with an equator crossing time of ~13:45 LT for measuring the global vertical distribution of temperature, ozone and other tropospheric atmospheric constituents such as carbon monoxide (CO), methane (CH4) and water vapor (Beer et al., 2001; Beer, 2006). 2.2.1. Tropospheric Emission Spectrometer (TES) The TES is one of four instruments onboard Aura satellite operates in both nadir and limb modes, but a routine limb scan was terminated in May 2005. It measures infrared emission from the Earth’s atmosphere in the spectral band between 650 cm 1 and 3050 cm 1. The ozone is retrieved from the 9.6 μm absorption band using the 995–1070 cm 1 spectral range (Rodgers, 2000; Worden et al., 2004; Bowman et al., 2006). The observed radiance is gathered onto an array of 16 detectors 2
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that have a spatial pixel resolution of 5.3 km � 8.4 km. The TES ozone retrievals have been validated extensively using ozonesondes and other ground-based measurements in several previous reports (e.g., Worden et al., 2007; Nassar et al., 2008). In the present study, we have used the TES Level 3 (L3) daily data. The daily L3 data is generated using the Delaunay triangulations on a latitude/longitude grid plane followed by 2-D interpolations for a given pressure level (Osterman et al., 2009).
2.4. ERA5 reanalysis This data set is a fifth-generation reanalysis from ECMWF with several improvements than the previous versions including ERAI (Hersbach and Dee, 2016). The ERA5 data is produced from the 4D-Var data assimilation system, with high spatial (30 km) and temporal (hourly) resolutions than its previous version of ERAI (79 km, Dee et al., 2011) and uses advanced model physics. ERA5 data resolve Earth at mosphere in 137 levels from the surface to height of 80 km. The detailed description of the ERA5 reanalysis dataset can be found elsewhere in Copernicus Climate Change Service (C3S, 2017). ERA5 reanalysis dataset provides several atmospheric variables but, in this study, we have used daily aggregated ozone data from surface to the 100 hPa during the anomalous ozone enhancement on 19 Dec 2010 over the central Himalayan region.
2.2.2. Ozone Monitoring Instrument (OMI) The OMI measures ultraviolet (UV) and visible backscatter radiances in three different channels bands 270–310 nm (UV-1), 310–365 nm (UV2), and 350–500 nm (UV-3). OMI is a nadir view instrument with a spatial resolution 13km�24 km (along � across-track) at nadir and uses a push-broom technique, covers a cross-swath width of ~2600 km. OMI measures the tropospheric and stratospheric ozone composition along with other trace gases. The ozone profiles are retrieved from UV11radiance channels using the optimal estimation and vector linear ized discrete ordinate radiative transfer model techniques described in Spurr (2006). In the present study, we have used the OMI, Ozone profile (PROFOZ) product contains the retrieved total columnar ozone, tropo spheric and stratospheric columnar ozone, retrieved vertical profiles, and other auxiliary parameters. The OMI PROFZ product is found to be a highly reliable data source for measuring the tropospheric ozone (e.g. Hayashida et al., 2018 and references therein). More details of the ozone retrieval algorithm of OMI PROFOZ product can be found elsewhere in Liu et al. (2010).
3. Results and discussion 3.1. Spatio-temporal variability of ozone over the study region Fig. 1a–b depicts the topography map of the Indian subcontinent and adjoining countries also include the complex topography associated with the Himalayan region. The mountain range extends from west to east of the northern part of India between the latitudes 30⁰ and 40⁰. The study region, where the unusual enhancement of ozone is observed, is also shown in Fig. 1a (“solid rectangular box”). Fig. 1b shows the threedimensional view of the inset topography map of the study region. The location of the Aryabhata Institute of Observational Sciences (ARIES)Nainital (29.35⁰N, 79.45⁰E), where the surface-based ozone measure ments have been taken is also depicted by a thick solid circle. The ARIESNainital is located at the foothills of the Himalayan region at an altitude of ~1958 m above the mean sea level. Fig. 1c shows the temporal changes in the surface ozone and TES retrieved tropospheric columnar ozone in the month of December 2010. It is interesting to note the remarkable enhancement in surface ozone during December 18–22, 2010. The peak ozone concentration (~80 ppbv) reported during 18–19 December 2010 which is about twice as high as the winter (DJF) sea sonal mean surface ozone (~43 ppbv with a standard deviation of ~11 ppbv; Kumar et al., 2010) over this location.
2.3. ERA-interim reanalysis (ERAI) In order to further illustrate, the synoptic conditions and background dynamics during the unusual enhancement of the surface and tropo spheric ozone, we have analyzed various meteorological fields (hori zontal winds, temperature, vorticity, etc.) and ozone mixing ratio from the ERAI data during December 2010. More details of the ERAI rean alysis data product can be found elsewhere in Dee et al. (2011). ERAI data is produced in various spatial resolutions but for the present study, we have used the 1.5⁰ � 1.5⁰ latitude-longitude grid resolution data covering the pressure levels from the surface to 100hPa. (covering altitude region from surface to ~16 km).
Fig. 1. (a) The topography (in meters) map of India and adjoining countries, as generated from the dataset ETOPO2v2 [National Geophysical Data Center (NCDC), 2006]. (b) The threedimensional view of complex topography in the central Himalayan region [indicated by a rect angle in (a)] and the location of the ARIES, Nainital [29.35⁰N, 79.45⁰E] is shown by “solid black circle”. (c) The temporal evolution of tropospheric columnar ozone (“orange dashed line”) as measured by the Tropospheric Emission Spectrometer (TES) on A-train Aura satellite and surface ozone (“blue solid line”) measured by using UV photometric ozone analysers over a high-altitude site Nainital, in Central Himalayan region. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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stratospheric columnar ozone for understanding the vertical transport of ozone. Fig. 3 shows the stratosphere column ozone for the similar dates considered in Fig. 2. Ozone reduction is found in the stratospheric region while the troposphere ozone is enhanced as shown in Figs. 3 and 2, respectively. This indicates that there could be a vertically downward transport of ozone from the stratosphere and this will be further dis cussed in the following sections. The mechanisms for the stratospheric intrusion of ozone rich air into the troposphere has been studied in several previous studies all over the � globe (e.g., Kumar et al., 2010; Olsen et al., 2013; Skerlak et al., 2014; Albers et al., 2018). The intrusions along the tropopause are strongly governed by the tropopause folds where the dry stratospheric air in trudes irreversibly deep into the troposphere (Sprenger et al., 2003; � Skerlak, 2014). The tropopause folds occur both in summer and winter preferentially in subtropical latitudes, however, the winter tropopause folds are deeper than the summer folds (Sprenger et al., 2003). The tropopause folds facilitate the transport of ozone-rich air into the troposphere and occasionally to the surface (Ding and Wang, 2006; � Skerlak et al., 2019). Here, we have analyzed the tropopause height and temperature evolution in the month of December 2010and are shown in Fig. 4. The data is obtained from the Atmospheric Infrared Sounder (AIRS) onboard Aqua satellite observations during the ascending pass of the satellite. Fig. 4 clearly depicts an increase of about 5 � C of tropopause tempera ture on 19 December 2010 from 74 � C (mean tropopause temperature) to 69 � C. At the same time, the tropopause altitude decreases about 1 km from about 16.5 km (control days) to 15.5 km on the event day. Hence, Fig. 4 clearly confirms the occurrence of tropopause folding over the study region during the anomalous enhancement of ozone in the month of December 2010.
The seasonal variation of surface ozone concentrations in many of the global sites indicates a higher ozone concentration during the springtime (e.g. Tsutsumi et al., 1994; Oltmans et al., 2006; Naja et al., 2003; Ojha et al., 2014). Similarly, the maximum ozone is noted in the springtime over the Nainital region with a mean magnitude of about 67 ppbv with a standard deviation of 12 ppbv and a secondary peak during post-monsoon season strongly controlled by the prevailing wind pat terns over Nainital region (Kumar et al., 2010). While the seasonal mean during winter (DJF) over this region is about 43 ppbv. Therefore, the observed surface ozone concentrations in Fig. 1c are far higher than the winter seasonal average. Nevertheless, the peak ozone concentrations sometimes indicate higher than the 100 ppbv in some other seasons except for the winter season. Therefore, considering the surface ozone concentrations in the winter, this event is a highly anomalous event over the Nainital region. This is further supported by the TES observed tropospheric columnar ozone as displayed in Fig. 1c, which also indicates a peak value of ~60 DU on 19 December 2010. It should be noted that the TES pass in not covered the study region on 18 December 2010. The latitude-longitude map of tropospheric ozone from the OMI is retrieved further to understand the spatial-temporal extent of the anomalous event and shown in Fig. 2. The spatial resolution (2⸰ latitude � 4⸰ longitude) of TES L3 data is relatively poor, therefore, we have utilized the data from the OMI instrument which is also on the same Aura satellite. Fig. 2 shows the distribution of the columnar ozone in the troposphere during 17–23 December 2010. Fig. 2a–d shows the significant enhancement in the tropospheric column ozone over the Indo-Gangetic Plain (IGP) region and the central Himalayan region. The spatial distribution also elicits the enhanced ozone region lies to the south of the Nainital region and along the foothills of the central Hi malayan region with peak concentration is noted on 19 December 2010 from Fig. 2. It is also noted a decrease in the magnitude of the tropo spheric ozone on 23 December 2010 relative to the peak concentration. The variations of the surface ozone in this region could be associated with either transport from distant locations or the vertical transport from the stratosphere as the Nainital region is in the free troposphere. While the region is distant from other emission sources and local photochemistry contributes very little to ozone variations (Kumar et al., 2010). The enhancement of the tropospheric ozone is highly localized over the region 25–30⁰N and 78–85⁰E. Hence, we have also analyzed the
3.2. Subtropical jet and Rossby Wave Breaking The stratospheric and tropospheric interactions, such as the tropo pause folds are studied in connection with the breaking of Rossby waves that usually propagate along the jet streams in the upper troposphere (Allen et al., 2009 and references therein). These baroclinic large-scale planetary-scale waves predominantly propagate along the subtropical tropopause region. Therefore, understanding the variability of ozone
Fig. 2. Spatial distribution of the troposphere column ozone (DU) on (a) 17 December 2010, (b) 19 December 2010, (c) 21 December 2010, and (d) 23 December 2010 as retrieved from the Ozone Monitoring Instrument (OMI) onboard Aura Satellite. The location of the ARIES-Nainital is shown by a “solid black circle”. 4
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Fig. 3. Spatial distribution of the stratospheric column ozone (DU) on (a) 17 December 2010, (b) 19 December 2010, (c) 21 December 2010, and (d) 23 December 2010 as retrieved from the Ozone Monitoring Instrument (OMI) onboard Aura Satellite. The location of the ARIES-Nainital is shown by “solid black circle”.
Fig. 4. Temporal evolution of the tropopause temperature and height averaged over the study region retrieved from the Atmospheric Infrared Sounder (AIRS) observations during December 2010. The vertical dashed line indicates the day of unusual enhancement of the surface ozone over the Nainital and adjoining areas in the central Himalayan region.
strong cyclonic circulation centering on 60oE is clearly seen, which propagates eastward and reaches around 75oE on 17 Dec 2010 (Fig. 6b). This cyclonic circulation further propagates eastward and reaches around 85oEon 19 Dec 2010, with amplified equatorward wind anom alies around 80oE. It should also to be noted that the eastward propa gating circulation anomalies are faded after 19 Dec 2010. These cyclonic and anticyclonic circulations are associated with the troughs and ridges of the upper-tropospheric planetary-scale Rossby waves. Hence, Fig. 6 indicates that the Rossby wave breaking event occurred on 19 Dec 2010 supporting the previous arguments. Figs. 6c and 5c clearly show the amplification of the Rossby waves during the event day over the central Himalayan region. Indeed, the STJ act as a waveguide for the uppertropospheric Rossby waves and further facilitates the growth and amplification before the breaking. Further, it should be noted an anti cyclonic circulation between 50oE and 60oE to the east of the cyclonic circulation (Fig. 6f). This implies there must be another group of Rossby
including other atmospheric constituents across the tropopause region can be studied by leveraging the STJ and Rossby wave breaking. The spatial and temporal distribution of the zonal wind and meridional wind anomalies in the upper troposphere to visualize the evolution of sub tropical jet (STJ) and Rossby wave propagation, respectively and has been shown in Fig. 5. It further indicates that during control days i.e. prior to the ozone enhancement, the STJ does not show any significant variations. However, on 19 December 2010, the STJ splits into two parts and pushed poleward and equatorward on the eastern and western re gions, respectively, centering around 80oE. It is also interesting to note about the propagation of the Rossby waves before and after the event day. The eastward propagating Rossby waves faded after the event day (Fig. 5c–f), which indicates the Rossby wave breaking might have occurred on the very same day (Fig. 5c). In order to strengthen the above discussion, we have shown the anomalous wind vectors in the upper troposphere in Fig. 6. In Fig. 6a, a 5
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Fig. 5. (a–f), Spatial and temporal evolution of meridional wind anomalies (shaded) at 300 hPa pressure level during 15–25 Dec 2010. The zonal winds (contours levels shown at 30 m 1 and 60 ms 1; thick solid lines) in the upper troposphere are also overlayed.
Fig. 6. Spatio-temporal evolution of anomalous wind vectors in the upper troposphere at 300 hPa during 15-25 Dec 2010. The magnitude of wind vectors is indicated by different colours from 0-25 ms 1, with an interval of 5 ms 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
wave trains started from 25 Dec 2010. The Rossby wave breaking results in rapid and irreversible de formations of potential vorticity (PV) contours in an isobaric or isen tropic level (Wernli and Sprenger, 2007; Hoskins et al., 1985; McIntyre and Palmer, 1983). Therefore, we showed the day-to-day variability of the PV contours from ERAI reanalysis over the 300 hPa isobaric level in Fig. 7. The high values of PV can be seen above 30⁰N before and after the event day i.e., 19 December 2010. While Fig. 7c also indicates high PV intrusion from mid-latitudes to the low latitudes over the central Hi malayan region. The high PV air over the study region on the event day could be associated with the remains of the Rossby wave breaking process. Further supporting evidence of the breaking event can be seen from Fig. 8. Fig. 8a shows the time series of meridional wind anomalies at different longitudes in the upper tropospheric region averaged in the latitude region over the central Himalayas during 15–31 December 2010. The ridges and troughs associated with the Rossby waves can be clearly seen by the positive and negative wind anomalies, respectively. The time evolution of the wave phase with longitude indicates the
Rossby waves are propagating eastward at a speed of ~5 m/s. The remarkable feature from Fig. 8a is that the wave propagation is absent after 19 December 2010 over the central Himalayan longitudes. This is clear evidence of the wave breaking event that further supports Figs. 5 and 7. While it is also interesting to note that the Rossby wave breaking event persists approximately 4–5 days and later the wave propagation reinstated back from 25 December 2010. Hence, the wave breaking event on 19 December 2010 brings midlatitude high-PV air into the central Himalayan region and can affect the ozone distribution in the upper troposphere as seen in Figs. 6c and 7c. Nevertheless, it is also interesting to note the type of Rossby wave breaking (anticyclonic and cyclonic) occurred in this unusual event as the strength of the intrusion also depends on the type of wave breaking (Liu and Barnes, 2018). The anticyclonic wave breaking mostly occurs on the equatorward verge of the jet while cyclonic breaking occurs on the poleward side of the STJ. The STJ shifts poleward (Fig. 5c) during the wave breaking, which indicates a greater chance of the anticyclonic wave breaking occurrence in the present case. Furthermore, in our case, 6
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Fig. 7. Spatio-temporal evolution of potential vorticity in the upper troposphere at 300 hPa isobaric level. The top panel (a–c) indicates PV maps during 15–19 Dec 2010 while bottom panel (d–f) during 21–25 Dec 2010 with every 2-day interval.
Fig. 8. The time vs. longitude section of daily meridional wind anomalies at 200 hPa isobaric level averaged between the latitudes 28⁰N-32⁰N. (b) Vertical crosssection (Pressure-Longitude) of potential vorticity (PVU, thick solid lines) averaged over the latitudes 28⁰N-32⁰N in the central Himalayan region on 19 December 2010. (c) Hovm€ oller diagram showing the time vs longitude of vertical velocity (pressure velocity) anomalies at 500 hPa isobaric level in the month of December 2010.
the wave breaking also occurs in the equator side of the STJ. A similar type of anticyclonic wave breaking events on the equator side of the jet-stream in mid-latitudes is reported in Barnes and Hartmann (2012). The isobaric and equatorward advection of stratospheric air also has a strong downward vertical component (Thorncroft et al., 1993, Hoskins et al., 1985). Therefore, we also showed the vertical cross-section of the pressure velocity (Pa/s) and PV averaged over the latitude region similar to Fig. 8a but on 19 December 2010. We also overlayed the isentropes (constant potential temperature contours shown as dashed lines) in Fig. 8b since it is more efficient in the quantification of the mixing and transport in the extra-tropics. The exchange of constituents between the stratosphere and troposphere occurs along the isentropes across the � tropopause folds in the upper troposphere (Shapiro, 1980; Skerlak, 2014). Moreover, isentropic transport is more important for tropo � spheric ozone (Lelieveld and Dentener, 2000; Skerlak et al., 2019). Fig. 8b indicates the isentropes are sloped downward across the strong PV intrusion (thick solid lines) over the central Himalayan region.
Therefore, it is evident that there is an exchange of stratospheric ozone in the upper troposphere through STE processes. The strength of the exchange of ozone into the troposphere also depends on the downward intrusion of PV. The PV intrusion into the lower altitudes is more prominently seen with 2PVU contours seen until 400 hPa from the ERAI reanalysis data. It indicates that the high ozone rich dry air is descending from upper levels into the lower troposphere. The other notable features are the strong subsidence (as indicated by the pressure velocity(shaded) on the eastern side, while the convergence on the western side of the PV tongue. Therefore, more extreme weather events can be apparently seen in the west and eastern region of the high PV anomalies, respectively (Kumar et al., 2017, 2019). €ller The subsidence of stratospheric air can be seen in the Hovmo diagram with more clarity across the latitudes over the central Hima layan region. Hence, we have plotted in Fig. 8c, the time vs longitude distribution of vertical (pressure) velocity averaged between 28oN-32oN at 500 hPa isobaric level. Fig. 8c indicates the remarkable signatures of 7
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the propagation of Rossby waves to the east starting from 15 Dec 2010. During the wave breaking day, the wave propagation ceased that can be noted from the temporal evolution of vertical velocity from 15 to 19 Dec 2010. Fig. 8c clearly depicts the strong subsidence of air on 19 Dec 2010 over the longitudes between 78oE and 82oE. This is further supporting our previous arguments of stratospheric intrusions of dry air over the central Himalayan region shown in Fig. 1a and b. Fig. 8c also depicts another chain of Rossby waves propagating eastward starting from 25 Dec 2010. It is also important to see the vertical distribution of ozone for this strong event for a better quantitative description of the tropospheric ozone. Hence, we have analyzed the vertical cross-section of ozone in the troposphere over the central Himalayan region from satellite (TES) and reanalysis datasets (ERAI and ERA5) as shown in Fig. 9. The highresolution topography data obtained from the dataset ETOPO2v2 [Na tional Geophysical Data Center (NCDC), 2006] is also shown in the bottom panels of Fig. 9. The topography data is interpolated to the grid resolutions of satellite and reanalysis datasets. Fig. 9a indicates the vertical distribution of ozone from TES data. Here, we have used the TES data as it provided better longitudinal coverage of the event relative to the OMI satellite. Fig. 9a shows a remarkable descending of strato spheric ozone into the lower troposphere with maximum ozone over the eastern part of the Himalayan mountain range. The peak magnitude of the lower troposphere and surface ozone is about 70 DU, which indicates a huge transport of ozone from the stratosphere relative to climatolog ical means in the region. TES data clearly shows intrusion of high ozone up to about 700–800 hPa that is the altitude of the Nainital region and showed an enhancement of about 60–70 ppbv. This agrees with Fig. 1c on the event day. Fig. 9b indicates the vertical cross-section of ozone from the ERAI reanalysis data. The ERAI data also indicated a strong intrusion of the ozone rich air into the troposphere, which is closely following the PV contours indicated in Fig. 8b. However, the magnitude of ozone in the lower troposphere is lower than the TES observations. The quality of ozone data in the ERAI reanalysis is compared with other previous versions of ECMWF reanalysis datasets and satellite � measurements (Dragani, 2011; Sprenger and Wernli (2003); Skerlak,
� (Skerlak, 2014). In particular, the assimilation of the Global Ozone Monitoring Experiment (GOME) ozone profiles reduces the un certainties in the ERA-Interim reanalysis (Dragani, 2010, 2011). Although the ERAI reanalysis ozone data is in better agreement with independent observations still there are some uncertainties in tropical and extratropics about �5% at 5 hPa and �10% at 10 hPa compared to various satellite observations including SAGE, HALO, and MLS data (Dragani, 2011). Further, the vertical profile of ozone show errors up to 20% in the lower stratosphere. With the continued improvements in the model physics and extensive use of satellite data, ECMWF has come up with much-improved reanalysis dataset ERA5 (section 2.4). Hence, we have also shown the vertical distribution of ozone with an updated version of ECMWWF reanalysis data. Fig. 9c shows the ERA5 data significantly improved in the retrieving the lower troposphere ozone relative to the ERAI reanalysis. The stratospheric intrusion of ozone is also very promising in the ERA5 reanalysis and reasonable agrees with the TES satellite observations. Nevertheless, the magnitude of ozone is still lesser than the TES measured ozone below 600 hPa. It is important to mention that satellite and reanalysis datasets un doubtedly indicating a significant transport of the ozone from the stratosphere to the lower troposphere and to the surface during this anomalous event. Therefore, this study has significant implications for the wave-induced stratosphere-tropospheric exchange processes in the high-altitude Himalayan region, which could be of great importance for the future investigation of STE processes in the complex orographic terrains. 4. Summary and concluding remarks Understanding the variability of the tropospheric ozone is very much important for any given region due to its prominent role in the air quality, biogeochemical processes, and more importantly, it is a green house gas. Moreover, it is also imperative to know the precursors of extreme levels of ozone in the free tropospheric regions where the dy namics could be playing a major role. Here, we reported such an unusual enhancement of ozone in the surface and troposphere over the central Himalayan region. Past studies have reported the long-range transport is one of the main sources of surface ozone variability in this region, where the local photochemistry play a marginal role as this region is away from the emission sources. Due to the complex topography of the central
2014 and references therein). The ERAI reanalysis dataset undoubtedly much-improved version relative to earlier reanalysis datasets (ERA-15 and ERA-40), due to its improved spatial resolution, 4D-Var data assimilation scheme and enhanced induction of satellite observations
Fig. 9. (top panel) Vertical cross-section of ozone in the West-East direction averaged over latitudes 28oN-32oN in the central Himalayan region from (a) Tropo spheric Emission Spectrometer, (b)ERA-Interim, and (c) ERA5 datasets on 19 December 2010. (bottom panel) The topography interpolated with respect to the longitudinal grid resolutions of (a) TES, (b) ERA-interim, and (c) ERA5, in the West-East direction averaged over latitudes 28oN-32oN. 8
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Himalayan region, the gravity wave breaking in the upper troposphere resulting in vertical transport of ozone from the stratosphere has also been reported to cause high ozone levels in the surface levels. Never theless, the extreme levels of ozone at the surface and troposphere in the present case is not due to the above mentioned two mechanisms and linked with the planetary wave breaking in the upper troposphere. The summary and conclusions drawn from the present study are given below.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.atmosenv.2020.117356. References Albers, J.R., Perlwitz, J., Butler, A.H., Birner, T., Kiladis, G.N., Lawrence, Z.D., Dias, J., 2018. Mechanisms governing interannual variability of stratosphere-to-troposphere ozone transport. J. Geophys. Res.: Atmosphere 123, 234–260. https://doi.org/ 10.1002/2017JD026890. Allen, G., Vaughan, G., Brunner, D., T May, P., Heyes, W., Minnis, P., K Ayers, J., 2009. Modulation of tropical convection by breaking Rossby waves. Q. J. R. Meteorol. Soc. 135, 125–137. https://doi.org/10.1002/qj.349. Austin, J.F., Midgley, R.P., 1994. The climatology of the jet stream and stratospheric intrusions of ozone over Japan. Atmos. Environ. 28, 39–52. https://doi.org/ 10.1016/1352-2310(94)90021-3. Barnes, E.A., Hartmann, D.L., 2012. Detection of rossby wave breaking and its response to shifts of the midlatitude jet with climate change. J. Geophys. Res.: Atmosphere 117 (D9). https://doi.org/10.1029/2012JD017469. Beer, R., 2006. TES on the Aura mission: scientific objectives, measurements,and analysis overview. IEEE Trans. Geosci. Rem. Sens. 44 (5), 1102–1105. Beer, R., Glavich, T.A., Rider, D.M., 2001. Tropospheric emission spectrometer for the Earth observing system’s Aura satellite. Appl. Optic. 40, 2356–2367. https://doi. org/10.1364/AO.40.002356. Bhardwaj, P., Naja, M., Rupakheti, M., Lupascu, A., Mues, A., Panday, A.K., Kumar, R., Mahata, K.S., Lal, S., Chandola, H.C., Lawrence, M.G., 2018. Variations in surface ozone and carbon monoxide in the Kathmandu Valley and surrounding broader regions during SusKat-ABC field campaign: role of local and regional sources. Atmos. Chem. Phys. 18, 11949–11971. https://doi.org/10.5194/acp-18-11949-2018. Bowman, K.W., Rodgers, C.D., Sund-Kulawik, S., Worden, J.R., Sarkissian, E., Osterman, G., Steck, T., Lou, M., Eldering, A., Shephard, M., Worden, H., Lampel, M., Clough, S., Brown, P., Rinsland, C., Gunson, M., Beer, R., 2006. Tropospheric emission spectrometer: retrieval method and error analysis. IEEE Trans. Geosci. Rem. Sens. 44 (5), 1297–1307. https://doi.org/10.1109/TGRS.2006.871234. Bracci, A., Cristofanelli, P., Sprenger, M., Bonaf�e, U., Calzolari, F., Duchi, R., Laj, P., Marinoni, A., Roccato, F., Vuillermoz, E., Bonasoni, P., 2012. Transport of stratospheric air masses to the Nepal climate observatory–pyramid (Himalaya; 5079 m MSL): a synoptic-scale investigation. J. Appl. Meteor. Climatol. 51, 1489–1507. 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� The surface ozone levels are extremely high for a period of about 4–5 days during 18–22 December 2010 over the central Himalayan re gion. The peak magnitude (~80 ppbv) of surface ozone reported on 18–19 December 2010, which is two-fold high relative to the sea sonal mean ozone observed in the region. During this period, it has been noted that the amplification of the upper-tropospheric Rossby waves, specifically on 19 December 2010. � The strong undulations of these upper tropospheric waves cause the weakening of the jet stream and the jet split into two branches over the central Himalayan region. It has also been noted that the wave propagation in the upper troposphere is faded after 19 December 2010, indicating the wave breaking event occurred. � The Rossby wave breaking is further identified using the potential vorticity (PV) intrusions on isobaric levels in the upper troposphere. Hence, the wave breaking resulting in the transport of the high PV air from the high latitudes towards the central Himalayan region. Moreover, a strong downward vertical component of PV into the mid-troposphere is also triggered by the breaking event on 19 December 2010. The high PV intrusion, therefore, caused the transport of the ozone rich air into the lower troposphere, which is clearly depicted by the TES satellite observations and two reanalysis data products from ECMWF. � Hence, the present study clearly establishes the fact that the planetary-scale wave breaking is also an important factor for the unusual enhancement of ozone over this region. Furthermore, it is also important to extend this study for several cases in this region as this region is more important for regional and global climate. While it is also needed to study the future scenario of the frequency of such wave breaking events in this region for a better understanding of the ozone variability in the Himalayan region. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Kondapalli Niranjan Kumar: Conceptualization, Methodology, Data curation, Writing - original draft, Visualization, Investigation, Software, Validation. Som Kumar Sharma: Conceptualization, Meth odology, Supervision, Writing - review & editing. Manish Naja: Conceptualization, Methodology, Supervision, Writing - review & edit ing. D.V. Phanikumar: Conceptualization, Methodology, Supervision, Writing - review & editing. Acknowledgments Surface ozone observations at ARIES, Nainital are supported by the ISRO ATCTM project. We are also thankful to ECMWF reanalysis Research data analysis, computational and information systems labo ratory team for providing us the datasets required for our analysis. This work is supported by the Science and Engineering Research Board Department of Science and Technology (SERB -DST) and Department of Space, Govt., of India.
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Stohl, A., Spichtinger-Rakowsky, N., Bonasoni, P., Feldmann, H., Memmesheimer, M., Scheel, H., Trickl, T., Hübener, S., Ringer, W., Mandl, M., 2000. The influence of stratospheric intrusions on alpine ozone concentrations. Atmos. Environ. 34, 1323–1354. Thorncroft, C.D., Hoskins, B.J., McIntyre, M.E., 1993. Two paradigms of baroclinic-wave life cycle behaviour. Q. J. R. Meteorol. Soc. 119, 17–55. Tsutsumi, Y., Zaizen, Y., Makino, Y., 1994. Tropospheric ozone measurement at the top of Mt. Fuji. Geophys. Res. Lett. 21, 1727–1730. https://doi.org/10.1029/ 94GL01107. Wernli, H., Sprenger, M., 2007. Identification and ERA-15 climatology of potential vorticity streamers and cutoffs near the extratropical tropopause. J. Atmos. Sci. 64, 1569–1586. https://doi.org/10.1175/JAS3912.1. Wild, O., 2007. Modelling the global tropospheric ozone budget: exploring the variability in current models. Atmos. Chem. Phys. 7, 2643–2660. https://doi.org/10.5194/acp7-2643-2007. 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World Meteorological Organization (WMO), 2003. WMO scientific assessment of ozone depletion 2002. WMO Global Ozone Res. Monit. Proj. Rep. 47, 498. Geneva, Switzerland. Worden, J., Kulawik, S.S., Shephard, M.W., Clough, S.A., Worden, H., Bowman, K., Goldman, A., 2004. Predicted errors of tropospheric emission spectrometer nadir retrievals from spectral window selection. J. Geophys. Res. 109, D09308. https:// doi.org/10.1029/2004JD004522.
Worden, H.M., Logan, J.A., Worden, J.R., Beer, R., Bowman, K., Clough, S.A., Eldering, A., Fisher, B.M., Gunson, M.R., Herman, R.L., Kulawik, S.S., Lampel, M.C., Luo, M., Megretskaia, I.A., Osterman, G.B., Shephard, M.W., 2007. Comparisons of Tropospheric Emission Spectrometer (TES) ozone profiles to ozonesondes: methods and initial results. J. Geophys. Res. 112, D03309. https://doi.org/10.1029/ 2006JD007258. Zeng, G., Pyle, J.A., Young, P.J., 2008. Impact of climate change on tropospheric ozone andits global budgets. Atmos. Chem. Phys. 8, 369–387.
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A Rossby wave breaking-induced enhancement in the tropospheric ozone over the Central Himalayan region Kondapalli Niranjan Kumar a, d, *, Som Kumar Sharma a, Manish Naja b, D.V. Phanikumar c a
Physical Research Laboratory, Ahmedabad, India Aryabhatta Research Institute of Observational Sciences, Nainital, India c Department of Science and Technology, New Delhi, India d National Centre for Medium Range Weather Forecasting, Ministry of Earth Sciences, New Delhi, India b
H I G H L I G H T S
� Unusual enhancement of surface and tropospheric ozone in the Himalayan region. � Impact of Rossby Wave Breaking in the upper troposphere prior to ozone enhancement. � Vertical transport of ozone rich dry air from the stratosphere deep into troposphere. A R T I C L E I N F O
A B S T R A C T
Keywords: Tropospheric Ozone Rossby waves Potential Vorticity Tropospheric Emission Spectrometer Ozone Monitoring Instrument
The high-altitude regions in the Himalayas are prone to high ozone concentrations frequently resulting from diverse dynamical and transport mechanisms. Here, we report an unusual enhancement in the surface and tropospheric ozone concentrations over the central Himalayan region from ground-based and space-borne measurements in the month of December 2010. The surface ozone levels (~80 ppbv) on 18–19 December 2010 is observed to be two-fold higher relative to the seasonal average (December-January-February) of about 40–50 ppbv in the central Himalayan region. The space-borne measurements from Tropospheric Emission Spectrometer and Ozone Monitoring Instrument onboard Aqua satellite also show higher values in the tropo spheric column ozone over this region. The satellite observations indicate an increase in tropopause temperature of about 5 � C and decrease in tropopause altitude about 1 km during 18–19 December 2010 resulting in the occurrence of tropopause fold facilitating the stratospheric-tropospheric exchange processes over the study re gion. The plausible reason for the occurrence of tropopause fold and subsequent enhancement of tropospheric and surface ozone is found to be associated with the breaking Rossby waves in the upper troposphere. The wave breaking leads to the advection of high-PV (potential vorticity) air, with magnitudes of about 3–4 PVU, towards the central Himalayan region from high-latitudes. The vertical component of PV advection also shows a deep stratospheric intrusion of high-PV air into the troposphere. The isentropic transport of ozone across the folding tropopause due to the wave breaking is clearly depicted from the satellite and reanalysis datasets. Therefore, the present study has strong implications of upper tropospheric wave dynamics to the tropospheric and surface ozone over the Himalayan regions having complex topography.
1. Introduction The tropospheric ozone is one of the most important atmospheric constituents controlling air quality and global climate (e.g., Monks et al., 2015). Increase in tropospheric ozone concentration in recent times has become a major concern to the scientific community because of its central role in the tropospheric chemistry (e.g., Oltmans et al. 2006). It is
an air pollutant and greenhouse gas thereby influencing the air quality and climate (IPCC, 2013; Liu et al., 2019). The average lifetime of O3 is approximately 5–7 days in the free troposphere. However, depending upon the local meteorological conditions, large spatial and temporal variability exists in ozone concentration. Tropospheric ozone is only about 10% of total ozone, but it causes an adverse effect on human health as well as on crop yield and ecosystem (Zeng et al., 2008;
* Corresponding author. National Centre for Medium Range Weather Forecasting, Ministry of Earth Sciences, Noida, U.P, India. E-mail addresses: [email protected], [email protected] (K.N. Kumar). https://doi.org/10.1016/j.atmosenv.2020.117356 Received 11 September 2019; Received in revised form 11 January 2020; Accepted 15 February 2020 Available online 20 February 2020 1352-2310/© 2020 Elsevier Ltd. All rights reserved.
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Lelieveld et al., 2015; Lal et al., 2017). The major sources of the tropospheric ozone are in situ photochemical process, involving its precursor gases like CO, hydrocarbons with an important and critical role of NO–NO2(Crutzen et al., 1999; Sharma et al., 2017). Another source in downward transport of ozone from the stratosphere (Levy � et al., 1985; Skerlak et al., 2019). Similarly, ozone loss takes place via chemical reactions and surface depositions. A global effort, utilizing several global models indicates a large variability and uncertainty in the budget of tropospheric ozone (Wild, 2007). Horizontal and vertical transport of ozone and its precursor gases leads to further complication (Lal et al., 2013; Naja et al., 2016; Bhardwaj et al., 2018), particularly over regions having complex orography induced wave generations like in the Himalayan region (Phanikumar et al., 2017; Brunamonti et al., 2018). Apart from the photochemical processes, the non-local dynam ical processes, including stratospheric intrusions, are also well-recognized precursors in the context of wave forcing that controls the variability of tropospheric ozone (Lelieveld and Dentener, 2000; WMO, 2003; Lin et al., 2015). The downward flux of ozone from the stratosphere can be more prominent near the cyclonic regions and jet streams in the upper troposphere (Das et al., 2011; Leclair De Bellevue et al., 2006; Austin and Midgley, 1994; Holton et al., 1995; Stohl et al., 2000). The troughs and cut-off lows including the tropopause folds are also active regions of the stratospheric intrusion of the ozone (WMO, 1986). The ozone enhancement due to the Stratosphere-Troposphere Exchange (STE) processes has been studied in the presence of various meteorological events such as tropical cyclones, mountain waves, frontal disturbances, and remote forcing of sea surface temperatures (Hocking et al., 2007; Das, 2009; Nath et al., 2016; Phanikumar et al., 2017; Albers et al., � 2018; Edwards et al., 2018; Skerlak et al., 2019). For instance, Albers et al. (2018) reported stratospheric intrusions of ozone in the context of El Nino Southern Oscillation (ENSO)-related jet variability through wave breaking over the Pacific-North American region. Das et al. (2009) reported the stratospheric intrusion of ozone rich dry air into the troposphere in the vicinity of a tropical cyclone using the radar obser vations over the tropical latitudes. The upper-level fronts also cause significant enhancement in the surface ozone through rapid transport from the stratosphere, in the presence of tropopause folds, depending � upon the state of boundary layer stability and turbulence (Skerlak, 2014; � Skerlak et al., 2019). For instance, model simulations over complex to pographies such as the Rocky Mountains and Tibetan Plateau indicate the vertical transport of ozone into the surface will be inhibited either � completely or partially when the stable boundary layer exists (Skerlak et al., 2019). However, the turbulence in the deep boundary layer over the elevated topographies causes significant mixing enabling the vertical transport to the surface but still, the stratospheric air mass is diluted before it reaches the surface. Hence, the surface ozone enhancement is also critically dependent on the boundary layer processes during the � stratospheric intrusions (Skerlak et al., 2019). Bracci et al. (2012) have characterized various synoptic conditions leading to the transport of ozone over the Nepal region in the Himalayas. The authors have noted that 94% of cases of stratosphere-troposphere transport occur in the presence of stratospheric potential vorticity structures, upper-tropospheric jet, quasi-stationary ridges, and monsoon depressions during their observational period between the years 2006 and 2008. Ojha et al. (2014) studied the vertical distribution of ozone using the weekly balloon measurements in the central Himalayan re gion. It is noted that significant seasonality exists in the tropospheric ozone with higher magnitudes during spring followed by autumn. In contrast to the spring maximum of surface ozone as reported in the Himalayan region in previous studies, Ding and Wang (2006) reported summertime maximum over the Tibetan Plateau due to stronger sub tropical jet and subsequent stratospheric intrusions. The above studies over the complex Himalayan region also indicate occasional enhance ment in the ozone during some days in the mid- and upper-tropospheric ozone with a simultaneous reduction in the relative humidity due to
stratospheric intrusions. Also, the ozone build-up during springtime is strongly linked to the regional pollution in the Indo-Gangetic Plain (IGP) region and biomass burning in north India (Ojha et al., 2014). In a recent study by Phanikumar et al. (2017) reported an anomalous tropospheric ozone enhancement in the presence of tropopause fold formed due to orographic gravity wave breaking in the central Himalayan region. Therefore, a prior understanding of the main precursors of a given re gion is very important for improving ozone prediction models (Liu et al., 2018). Hence, in the present study, we report the interaction of planetaryscale Rossby waves with the atmospheric circulation and the ozone distribution that has been marginally studied over the central Himala yan region, despite the surface weather in these regions are strongly controlled by the wave forcing associated with the upper-tropospheric Rossby waves during winter (Niranjan Kumar et al., 2016). We detec ted an unusual high tropospheric and surface ozone concentration during December 2010 in the central Himalayan region using the ground-based and satellite observations. We also provide observational evidence of Rossby wave breaking and interaction with atmospheric circulation leading to the anomalous enhancement (depletion) of lower tropospheric (stratospheric) ozone by using ground-based and satellite datasets. Therefore, the present study investigates an unusual enhancement in the surface and tropospheric ozone due to Rossby wave breaking episode in the Himalayan region. In this study, we will describe the various datasets acquired for this study, followed by the results and discussion. Finally, we present the summary and conclusions and scope of this work. 2. Data and methodology 2.1. Surface ozone measurements For the present study, we utilize surface ozone measurements from a high-altitude site, Nainital(29.37⁰N, 79.45⁰; E, 1958 m amsl.) in the central Himalayan region by using UV photometric ozone analyzers of Environment S.A. (model O341M), France for the measurement of the ozone mixing ratio. The retrieval of ozone is based on the principle of absorption of ultraviolet radiation (wavelength ~ 254 nm) by the ozone molecules using Beer–Lambert’s law. The lower detection limit of the surface ozone is ~1 ppb with an accuracy of ~5%. More details of the instrument and analysis technique are outlined in earlier papers (e.g., Kumar et al. 2010, 2011). There is no industry in the Nainital town. Some non-combustible industries are in Rudrapur town, which is about 40 km south of Nainital. 2.2. Space-borne ozone observations We have used ozone measurements from two satellites one is the Tropospheric Emission Spectrometer (TES; Beer, 2006) and another one is Ozone Monitoring Instrument (OMI; Levelt et al., 2006), respectively to further strengthen our results from ground-based measurements. These two instruments are onboard NASA EOS (Earth Observing System) Aura satellite which has a ~705 km sun-synchronous polar orbit with an equator crossing time of ~13:45 LT for measuring the global vertical distribution of temperature, ozone and other tropospheric atmospheric constituents such as carbon monoxide (CO), methane (CH4) and water vapor (Beer et al., 2001; Beer, 2006). 2.2.1. Tropospheric Emission Spectrometer (TES) The TES is one of four instruments onboard Aura satellite operates in both nadir and limb modes, but a routine limb scan was terminated in May 2005. It measures infrared emission from the Earth’s atmosphere in the spectral band between 650 cm 1 and 3050 cm 1. The ozone is retrieved from the 9.6 μm absorption band using the 995–1070 cm 1 spectral range (Rodgers, 2000; Worden et al., 2004; Bowman et al., 2006). The observed radiance is gathered onto an array of 16 detectors 2
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that have a spatial pixel resolution of 5.3 km � 8.4 km. The TES ozone retrievals have been validated extensively using ozonesondes and other ground-based measurements in several previous reports (e.g., Worden et al., 2007; Nassar et al., 2008). In the present study, we have used the TES Level 3 (L3) daily data. The daily L3 data is generated using the Delaunay triangulations on a latitude/longitude grid plane followed by 2-D interpolations for a given pressure level (Osterman et al., 2009).
2.4. ERA5 reanalysis This data set is a fifth-generation reanalysis from ECMWF with several improvements than the previous versions including ERAI (Hersbach and Dee, 2016). The ERA5 data is produced from the 4D-Var data assimilation system, with high spatial (30 km) and temporal (hourly) resolutions than its previous version of ERAI (79 km, Dee et al., 2011) and uses advanced model physics. ERA5 data resolve Earth at mosphere in 137 levels from the surface to height of 80 km. The detailed description of the ERA5 reanalysis dataset can be found elsewhere in Copernicus Climate Change Service (C3S, 2017). ERA5 reanalysis dataset provides several atmospheric variables but, in this study, we have used daily aggregated ozone data from surface to the 100 hPa during the anomalous ozone enhancement on 19 Dec 2010 over the central Himalayan region.
2.2.2. Ozone Monitoring Instrument (OMI) The OMI measures ultraviolet (UV) and visible backscatter radiances in three different channels bands 270–310 nm (UV-1), 310–365 nm (UV2), and 350–500 nm (UV-3). OMI is a nadir view instrument with a spatial resolution 13km�24 km (along � across-track) at nadir and uses a push-broom technique, covers a cross-swath width of ~2600 km. OMI measures the tropospheric and stratospheric ozone composition along with other trace gases. The ozone profiles are retrieved from UV11radiance channels using the optimal estimation and vector linear ized discrete ordinate radiative transfer model techniques described in Spurr (2006). In the present study, we have used the OMI, Ozone profile (PROFOZ) product contains the retrieved total columnar ozone, tropo spheric and stratospheric columnar ozone, retrieved vertical profiles, and other auxiliary parameters. The OMI PROFZ product is found to be a highly reliable data source for measuring the tropospheric ozone (e.g. Hayashida et al., 2018 and references therein). More details of the ozone retrieval algorithm of OMI PROFOZ product can be found elsewhere in Liu et al. (2010).
3. Results and discussion 3.1. Spatio-temporal variability of ozone over the study region Fig. 1a–b depicts the topography map of the Indian subcontinent and adjoining countries also include the complex topography associated with the Himalayan region. The mountain range extends from west to east of the northern part of India between the latitudes 30⁰ and 40⁰. The study region, where the unusual enhancement of ozone is observed, is also shown in Fig. 1a (“solid rectangular box”). Fig. 1b shows the threedimensional view of the inset topography map of the study region. The location of the Aryabhata Institute of Observational Sciences (ARIES)Nainital (29.35⁰N, 79.45⁰E), where the surface-based ozone measure ments have been taken is also depicted by a thick solid circle. The ARIESNainital is located at the foothills of the Himalayan region at an altitude of ~1958 m above the mean sea level. Fig. 1c shows the temporal changes in the surface ozone and TES retrieved tropospheric columnar ozone in the month of December 2010. It is interesting to note the remarkable enhancement in surface ozone during December 18–22, 2010. The peak ozone concentration (~80 ppbv) reported during 18–19 December 2010 which is about twice as high as the winter (DJF) sea sonal mean surface ozone (~43 ppbv with a standard deviation of ~11 ppbv; Kumar et al., 2010) over this location.
2.3. ERA-interim reanalysis (ERAI) In order to further illustrate, the synoptic conditions and background dynamics during the unusual enhancement of the surface and tropo spheric ozone, we have analyzed various meteorological fields (hori zontal winds, temperature, vorticity, etc.) and ozone mixing ratio from the ERAI data during December 2010. More details of the ERAI rean alysis data product can be found elsewhere in Dee et al. (2011). ERAI data is produced in various spatial resolutions but for the present study, we have used the 1.5⁰ � 1.5⁰ latitude-longitude grid resolution data covering the pressure levels from the surface to 100hPa. (covering altitude region from surface to ~16 km).
Fig. 1. (a) The topography (in meters) map of India and adjoining countries, as generated from the dataset ETOPO2v2 [National Geophysical Data Center (NCDC), 2006]. (b) The threedimensional view of complex topography in the central Himalayan region [indicated by a rect angle in (a)] and the location of the ARIES, Nainital [29.35⁰N, 79.45⁰E] is shown by “solid black circle”. (c) The temporal evolution of tropospheric columnar ozone (“orange dashed line”) as measured by the Tropospheric Emission Spectrometer (TES) on A-train Aura satellite and surface ozone (“blue solid line”) measured by using UV photometric ozone analysers over a high-altitude site Nainital, in Central Himalayan region. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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stratospheric columnar ozone for understanding the vertical transport of ozone. Fig. 3 shows the stratosphere column ozone for the similar dates considered in Fig. 2. Ozone reduction is found in the stratospheric region while the troposphere ozone is enhanced as shown in Figs. 3 and 2, respectively. This indicates that there could be a vertically downward transport of ozone from the stratosphere and this will be further dis cussed in the following sections. The mechanisms for the stratospheric intrusion of ozone rich air into the troposphere has been studied in several previous studies all over the � globe (e.g., Kumar et al., 2010; Olsen et al., 2013; Skerlak et al., 2014; Albers et al., 2018). The intrusions along the tropopause are strongly governed by the tropopause folds where the dry stratospheric air in trudes irreversibly deep into the troposphere (Sprenger et al., 2003; � Skerlak, 2014). The tropopause folds occur both in summer and winter preferentially in subtropical latitudes, however, the winter tropopause folds are deeper than the summer folds (Sprenger et al., 2003). The tropopause folds facilitate the transport of ozone-rich air into the troposphere and occasionally to the surface (Ding and Wang, 2006; � Skerlak et al., 2019). Here, we have analyzed the tropopause height and temperature evolution in the month of December 2010and are shown in Fig. 4. The data is obtained from the Atmospheric Infrared Sounder (AIRS) onboard Aqua satellite observations during the ascending pass of the satellite. Fig. 4 clearly depicts an increase of about 5 � C of tropopause tempera ture on 19 December 2010 from 74 � C (mean tropopause temperature) to 69 � C. At the same time, the tropopause altitude decreases about 1 km from about 16.5 km (control days) to 15.5 km on the event day. Hence, Fig. 4 clearly confirms the occurrence of tropopause folding over the study region during the anomalous enhancement of ozone in the month of December 2010.
The seasonal variation of surface ozone concentrations in many of the global sites indicates a higher ozone concentration during the springtime (e.g. Tsutsumi et al., 1994; Oltmans et al., 2006; Naja et al., 2003; Ojha et al., 2014). Similarly, the maximum ozone is noted in the springtime over the Nainital region with a mean magnitude of about 67 ppbv with a standard deviation of 12 ppbv and a secondary peak during post-monsoon season strongly controlled by the prevailing wind pat terns over Nainital region (Kumar et al., 2010). While the seasonal mean during winter (DJF) over this region is about 43 ppbv. Therefore, the observed surface ozone concentrations in Fig. 1c are far higher than the winter seasonal average. Nevertheless, the peak ozone concentrations sometimes indicate higher than the 100 ppbv in some other seasons except for the winter season. Therefore, considering the surface ozone concentrations in the winter, this event is a highly anomalous event over the Nainital region. This is further supported by the TES observed tropospheric columnar ozone as displayed in Fig. 1c, which also indicates a peak value of ~60 DU on 19 December 2010. It should be noted that the TES pass in not covered the study region on 18 December 2010. The latitude-longitude map of tropospheric ozone from the OMI is retrieved further to understand the spatial-temporal extent of the anomalous event and shown in Fig. 2. The spatial resolution (2⸰ latitude � 4⸰ longitude) of TES L3 data is relatively poor, therefore, we have utilized the data from the OMI instrument which is also on the same Aura satellite. Fig. 2 shows the distribution of the columnar ozone in the troposphere during 17–23 December 2010. Fig. 2a–d shows the significant enhancement in the tropospheric column ozone over the Indo-Gangetic Plain (IGP) region and the central Himalayan region. The spatial distribution also elicits the enhanced ozone region lies to the south of the Nainital region and along the foothills of the central Hi malayan region with peak concentration is noted on 19 December 2010 from Fig. 2. It is also noted a decrease in the magnitude of the tropo spheric ozone on 23 December 2010 relative to the peak concentration. The variations of the surface ozone in this region could be associated with either transport from distant locations or the vertical transport from the stratosphere as the Nainital region is in the free troposphere. While the region is distant from other emission sources and local photochemistry contributes very little to ozone variations (Kumar et al., 2010). The enhancement of the tropospheric ozone is highly localized over the region 25–30⁰N and 78–85⁰E. Hence, we have also analyzed the
3.2. Subtropical jet and Rossby Wave Breaking The stratospheric and tropospheric interactions, such as the tropo pause folds are studied in connection with the breaking of Rossby waves that usually propagate along the jet streams in the upper troposphere (Allen et al., 2009 and references therein). These baroclinic large-scale planetary-scale waves predominantly propagate along the subtropical tropopause region. Therefore, understanding the variability of ozone
Fig. 2. Spatial distribution of the troposphere column ozone (DU) on (a) 17 December 2010, (b) 19 December 2010, (c) 21 December 2010, and (d) 23 December 2010 as retrieved from the Ozone Monitoring Instrument (OMI) onboard Aura Satellite. The location of the ARIES-Nainital is shown by a “solid black circle”. 4
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Fig. 3. Spatial distribution of the stratospheric column ozone (DU) on (a) 17 December 2010, (b) 19 December 2010, (c) 21 December 2010, and (d) 23 December 2010 as retrieved from the Ozone Monitoring Instrument (OMI) onboard Aura Satellite. The location of the ARIES-Nainital is shown by “solid black circle”.
Fig. 4. Temporal evolution of the tropopause temperature and height averaged over the study region retrieved from the Atmospheric Infrared Sounder (AIRS) observations during December 2010. The vertical dashed line indicates the day of unusual enhancement of the surface ozone over the Nainital and adjoining areas in the central Himalayan region.
strong cyclonic circulation centering on 60oE is clearly seen, which propagates eastward and reaches around 75oE on 17 Dec 2010 (Fig. 6b). This cyclonic circulation further propagates eastward and reaches around 85oEon 19 Dec 2010, with amplified equatorward wind anom alies around 80oE. It should also to be noted that the eastward propa gating circulation anomalies are faded after 19 Dec 2010. These cyclonic and anticyclonic circulations are associated with the troughs and ridges of the upper-tropospheric planetary-scale Rossby waves. Hence, Fig. 6 indicates that the Rossby wave breaking event occurred on 19 Dec 2010 supporting the previous arguments. Figs. 6c and 5c clearly show the amplification of the Rossby waves during the event day over the central Himalayan region. Indeed, the STJ act as a waveguide for the uppertropospheric Rossby waves and further facilitates the growth and amplification before the breaking. Further, it should be noted an anti cyclonic circulation between 50oE and 60oE to the east of the cyclonic circulation (Fig. 6f). This implies there must be another group of Rossby
including other atmospheric constituents across the tropopause region can be studied by leveraging the STJ and Rossby wave breaking. The spatial and temporal distribution of the zonal wind and meridional wind anomalies in the upper troposphere to visualize the evolution of sub tropical jet (STJ) and Rossby wave propagation, respectively and has been shown in Fig. 5. It further indicates that during control days i.e. prior to the ozone enhancement, the STJ does not show any significant variations. However, on 19 December 2010, the STJ splits into two parts and pushed poleward and equatorward on the eastern and western re gions, respectively, centering around 80oE. It is also interesting to note about the propagation of the Rossby waves before and after the event day. The eastward propagating Rossby waves faded after the event day (Fig. 5c–f), which indicates the Rossby wave breaking might have occurred on the very same day (Fig. 5c). In order to strengthen the above discussion, we have shown the anomalous wind vectors in the upper troposphere in Fig. 6. In Fig. 6a, a 5
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Fig. 5. (a–f), Spatial and temporal evolution of meridional wind anomalies (shaded) at 300 hPa pressure level during 15–25 Dec 2010. The zonal winds (contours levels shown at 30 m 1 and 60 ms 1; thick solid lines) in the upper troposphere are also overlayed.
Fig. 6. Spatio-temporal evolution of anomalous wind vectors in the upper troposphere at 300 hPa during 15-25 Dec 2010. The magnitude of wind vectors is indicated by different colours from 0-25 ms 1, with an interval of 5 ms 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
wave trains started from 25 Dec 2010. The Rossby wave breaking results in rapid and irreversible de formations of potential vorticity (PV) contours in an isobaric or isen tropic level (Wernli and Sprenger, 2007; Hoskins et al., 1985; McIntyre and Palmer, 1983). Therefore, we showed the day-to-day variability of the PV contours from ERAI reanalysis over the 300 hPa isobaric level in Fig. 7. The high values of PV can be seen above 30⁰N before and after the event day i.e., 19 December 2010. While Fig. 7c also indicates high PV intrusion from mid-latitudes to the low latitudes over the central Hi malayan region. The high PV air over the study region on the event day could be associated with the remains of the Rossby wave breaking process. Further supporting evidence of the breaking event can be seen from Fig. 8. Fig. 8a shows the time series of meridional wind anomalies at different longitudes in the upper tropospheric region averaged in the latitude region over the central Himalayas during 15–31 December 2010. The ridges and troughs associated with the Rossby waves can be clearly seen by the positive and negative wind anomalies, respectively. The time evolution of the wave phase with longitude indicates the
Rossby waves are propagating eastward at a speed of ~5 m/s. The remarkable feature from Fig. 8a is that the wave propagation is absent after 19 December 2010 over the central Himalayan longitudes. This is clear evidence of the wave breaking event that further supports Figs. 5 and 7. While it is also interesting to note that the Rossby wave breaking event persists approximately 4–5 days and later the wave propagation reinstated back from 25 December 2010. Hence, the wave breaking event on 19 December 2010 brings midlatitude high-PV air into the central Himalayan region and can affect the ozone distribution in the upper troposphere as seen in Figs. 6c and 7c. Nevertheless, it is also interesting to note the type of Rossby wave breaking (anticyclonic and cyclonic) occurred in this unusual event as the strength of the intrusion also depends on the type of wave breaking (Liu and Barnes, 2018). The anticyclonic wave breaking mostly occurs on the equatorward verge of the jet while cyclonic breaking occurs on the poleward side of the STJ. The STJ shifts poleward (Fig. 5c) during the wave breaking, which indicates a greater chance of the anticyclonic wave breaking occurrence in the present case. Furthermore, in our case, 6
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Fig. 7. Spatio-temporal evolution of potential vorticity in the upper troposphere at 300 hPa isobaric level. The top panel (a–c) indicates PV maps during 15–19 Dec 2010 while bottom panel (d–f) during 21–25 Dec 2010 with every 2-day interval.
Fig. 8. The time vs. longitude section of daily meridional wind anomalies at 200 hPa isobaric level averaged between the latitudes 28⁰N-32⁰N. (b) Vertical crosssection (Pressure-Longitude) of potential vorticity (PVU, thick solid lines) averaged over the latitudes 28⁰N-32⁰N in the central Himalayan region on 19 December 2010. (c) Hovm€ oller diagram showing the time vs longitude of vertical velocity (pressure velocity) anomalies at 500 hPa isobaric level in the month of December 2010.
the wave breaking also occurs in the equator side of the STJ. A similar type of anticyclonic wave breaking events on the equator side of the jet-stream in mid-latitudes is reported in Barnes and Hartmann (2012). The isobaric and equatorward advection of stratospheric air also has a strong downward vertical component (Thorncroft et al., 1993, Hoskins et al., 1985). Therefore, we also showed the vertical cross-section of the pressure velocity (Pa/s) and PV averaged over the latitude region similar to Fig. 8a but on 19 December 2010. We also overlayed the isentropes (constant potential temperature contours shown as dashed lines) in Fig. 8b since it is more efficient in the quantification of the mixing and transport in the extra-tropics. The exchange of constituents between the stratosphere and troposphere occurs along the isentropes across the � tropopause folds in the upper troposphere (Shapiro, 1980; Skerlak, 2014). Moreover, isentropic transport is more important for tropo � spheric ozone (Lelieveld and Dentener, 2000; Skerlak et al., 2019). Fig. 8b indicates the isentropes are sloped downward across the strong PV intrusion (thick solid lines) over the central Himalayan region.
Therefore, it is evident that there is an exchange of stratospheric ozone in the upper troposphere through STE processes. The strength of the exchange of ozone into the troposphere also depends on the downward intrusion of PV. The PV intrusion into the lower altitudes is more prominently seen with 2PVU contours seen until 400 hPa from the ERAI reanalysis data. It indicates that the high ozone rich dry air is descending from upper levels into the lower troposphere. The other notable features are the strong subsidence (as indicated by the pressure velocity(shaded) on the eastern side, while the convergence on the western side of the PV tongue. Therefore, more extreme weather events can be apparently seen in the west and eastern region of the high PV anomalies, respectively (Kumar et al., 2017, 2019). €ller The subsidence of stratospheric air can be seen in the Hovmo diagram with more clarity across the latitudes over the central Hima layan region. Hence, we have plotted in Fig. 8c, the time vs longitude distribution of vertical (pressure) velocity averaged between 28oN-32oN at 500 hPa isobaric level. Fig. 8c indicates the remarkable signatures of 7
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the propagation of Rossby waves to the east starting from 15 Dec 2010. During the wave breaking day, the wave propagation ceased that can be noted from the temporal evolution of vertical velocity from 15 to 19 Dec 2010. Fig. 8c clearly depicts the strong subsidence of air on 19 Dec 2010 over the longitudes between 78oE and 82oE. This is further supporting our previous arguments of stratospheric intrusions of dry air over the central Himalayan region shown in Fig. 1a and b. Fig. 8c also depicts another chain of Rossby waves propagating eastward starting from 25 Dec 2010. It is also important to see the vertical distribution of ozone for this strong event for a better quantitative description of the tropospheric ozone. Hence, we have analyzed the vertical cross-section of ozone in the troposphere over the central Himalayan region from satellite (TES) and reanalysis datasets (ERAI and ERA5) as shown in Fig. 9. The highresolution topography data obtained from the dataset ETOPO2v2 [Na tional Geophysical Data Center (NCDC), 2006] is also shown in the bottom panels of Fig. 9. The topography data is interpolated to the grid resolutions of satellite and reanalysis datasets. Fig. 9a indicates the vertical distribution of ozone from TES data. Here, we have used the TES data as it provided better longitudinal coverage of the event relative to the OMI satellite. Fig. 9a shows a remarkable descending of strato spheric ozone into the lower troposphere with maximum ozone over the eastern part of the Himalayan mountain range. The peak magnitude of the lower troposphere and surface ozone is about 70 DU, which indicates a huge transport of ozone from the stratosphere relative to climatolog ical means in the region. TES data clearly shows intrusion of high ozone up to about 700–800 hPa that is the altitude of the Nainital region and showed an enhancement of about 60–70 ppbv. This agrees with Fig. 1c on the event day. Fig. 9b indicates the vertical cross-section of ozone from the ERAI reanalysis data. The ERAI data also indicated a strong intrusion of the ozone rich air into the troposphere, which is closely following the PV contours indicated in Fig. 8b. However, the magnitude of ozone in the lower troposphere is lower than the TES observations. The quality of ozone data in the ERAI reanalysis is compared with other previous versions of ECMWF reanalysis datasets and satellite � measurements (Dragani, 2011; Sprenger and Wernli (2003); Skerlak,
� (Skerlak, 2014). In particular, the assimilation of the Global Ozone Monitoring Experiment (GOME) ozone profiles reduces the un certainties in the ERA-Interim reanalysis (Dragani, 2010, 2011). Although the ERAI reanalysis ozone data is in better agreement with independent observations still there are some uncertainties in tropical and extratropics about �5% at 5 hPa and �10% at 10 hPa compared to various satellite observations including SAGE, HALO, and MLS data (Dragani, 2011). Further, the vertical profile of ozone show errors up to 20% in the lower stratosphere. With the continued improvements in the model physics and extensive use of satellite data, ECMWF has come up with much-improved reanalysis dataset ERA5 (section 2.4). Hence, we have also shown the vertical distribution of ozone with an updated version of ECMWWF reanalysis data. Fig. 9c shows the ERA5 data significantly improved in the retrieving the lower troposphere ozone relative to the ERAI reanalysis. The stratospheric intrusion of ozone is also very promising in the ERA5 reanalysis and reasonable agrees with the TES satellite observations. Nevertheless, the magnitude of ozone is still lesser than the TES measured ozone below 600 hPa. It is important to mention that satellite and reanalysis datasets un doubtedly indicating a significant transport of the ozone from the stratosphere to the lower troposphere and to the surface during this anomalous event. Therefore, this study has significant implications for the wave-induced stratosphere-tropospheric exchange processes in the high-altitude Himalayan region, which could be of great importance for the future investigation of STE processes in the complex orographic terrains. 4. Summary and concluding remarks Understanding the variability of the tropospheric ozone is very much important for any given region due to its prominent role in the air quality, biogeochemical processes, and more importantly, it is a green house gas. Moreover, it is also imperative to know the precursors of extreme levels of ozone in the free tropospheric regions where the dy namics could be playing a major role. Here, we reported such an unusual enhancement of ozone in the surface and troposphere over the central Himalayan region. Past studies have reported the long-range transport is one of the main sources of surface ozone variability in this region, where the local photochemistry play a marginal role as this region is away from the emission sources. Due to the complex topography of the central
2014 and references therein). The ERAI reanalysis dataset undoubtedly much-improved version relative to earlier reanalysis datasets (ERA-15 and ERA-40), due to its improved spatial resolution, 4D-Var data assimilation scheme and enhanced induction of satellite observations
Fig. 9. (top panel) Vertical cross-section of ozone in the West-East direction averaged over latitudes 28oN-32oN in the central Himalayan region from (a) Tropo spheric Emission Spectrometer, (b)ERA-Interim, and (c) ERA5 datasets on 19 December 2010. (bottom panel) The topography interpolated with respect to the longitudinal grid resolutions of (a) TES, (b) ERA-interim, and (c) ERA5, in the West-East direction averaged over latitudes 28oN-32oN. 8
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Himalayan region, the gravity wave breaking in the upper troposphere resulting in vertical transport of ozone from the stratosphere has also been reported to cause high ozone levels in the surface levels. Never theless, the extreme levels of ozone at the surface and troposphere in the present case is not due to the above mentioned two mechanisms and linked with the planetary wave breaking in the upper troposphere. The summary and conclusions drawn from the present study are given below.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.atmosenv.2020.117356. References Albers, J.R., Perlwitz, J., Butler, A.H., Birner, T., Kiladis, G.N., Lawrence, Z.D., Dias, J., 2018. Mechanisms governing interannual variability of stratosphere-to-troposphere ozone transport. J. Geophys. Res.: Atmosphere 123, 234–260. https://doi.org/ 10.1002/2017JD026890. Allen, G., Vaughan, G., Brunner, D., T May, P., Heyes, W., Minnis, P., K Ayers, J., 2009. Modulation of tropical convection by breaking Rossby waves. Q. J. R. Meteorol. Soc. 135, 125–137. https://doi.org/10.1002/qj.349. Austin, J.F., Midgley, R.P., 1994. The climatology of the jet stream and stratospheric intrusions of ozone over Japan. Atmos. Environ. 28, 39–52. https://doi.org/ 10.1016/1352-2310(94)90021-3. Barnes, E.A., Hartmann, D.L., 2012. Detection of rossby wave breaking and its response to shifts of the midlatitude jet with climate change. J. Geophys. Res.: Atmosphere 117 (D9). https://doi.org/10.1029/2012JD017469. Beer, R., 2006. TES on the Aura mission: scientific objectives, measurements,and analysis overview. IEEE Trans. Geosci. Rem. Sens. 44 (5), 1102–1105. Beer, R., Glavich, T.A., Rider, D.M., 2001. Tropospheric emission spectrometer for the Earth observing system’s Aura satellite. Appl. Optic. 40, 2356–2367. https://doi. org/10.1364/AO.40.002356. Bhardwaj, P., Naja, M., Rupakheti, M., Lupascu, A., Mues, A., Panday, A.K., Kumar, R., Mahata, K.S., Lal, S., Chandola, H.C., Lawrence, M.G., 2018. Variations in surface ozone and carbon monoxide in the Kathmandu Valley and surrounding broader regions during SusKat-ABC field campaign: role of local and regional sources. Atmos. Chem. Phys. 18, 11949–11971. https://doi.org/10.5194/acp-18-11949-2018. Bowman, K.W., Rodgers, C.D., Sund-Kulawik, S., Worden, J.R., Sarkissian, E., Osterman, G., Steck, T., Lou, M., Eldering, A., Shephard, M., Worden, H., Lampel, M., Clough, S., Brown, P., Rinsland, C., Gunson, M., Beer, R., 2006. Tropospheric emission spectrometer: retrieval method and error analysis. IEEE Trans. Geosci. Rem. Sens. 44 (5), 1297–1307. https://doi.org/10.1109/TGRS.2006.871234. Bracci, A., Cristofanelli, P., Sprenger, M., Bonaf�e, U., Calzolari, F., Duchi, R., Laj, P., Marinoni, A., Roccato, F., Vuillermoz, E., Bonasoni, P., 2012. Transport of stratospheric air masses to the Nepal climate observatory–pyramid (Himalaya; 5079 m MSL): a synoptic-scale investigation. J. Appl. Meteor. Climatol. 51, 1489–1507. 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A new perspective on MST radar observations of stratospheric intrusions into troposphere associated with tropical cyclone. Geophys. Res. Lett. 36, L15821. https://doi.org/10.1029/2009GL039184. Das, S.S., Sijikumar, S., Uma, K.N., 2011. Further investigation on stratospheric air intrusion into the troposphere during the episode of tropical cyclone: numerical simulation and MST radar observations. Atmos. Res. 101 (4), 928–937. Dee, D.P., Uppala, S.M., Simmons, A.J., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U., Balmaseda, M.A., Balsamo, G., Bauer, P., Bechtold, P., Beljaars, A.C.M., van de Berg, L., Bidlot, J., Bormann, N., Delsol, C., Dragani, R., Fuentes, M., Geer, A.J., Haimberger, L., Healy, S.B., Hersbach, H., H� olm, E.V., Isaksen, L., Kållberg, P., K€ ohler, M., Matricardi, M., McNally, A.P., Monge-Sanz, B.M., Morcrette, J.-J., Park, B.-K., Peubey, C., de Rosnay, P., Tavolato, C., Th� epaut, J.-N., Vitart, F., 2011. 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� The surface ozone levels are extremely high for a period of about 4–5 days during 18–22 December 2010 over the central Himalayan re gion. The peak magnitude (~80 ppbv) of surface ozone reported on 18–19 December 2010, which is two-fold high relative to the sea sonal mean ozone observed in the region. During this period, it has been noted that the amplification of the upper-tropospheric Rossby waves, specifically on 19 December 2010. � The strong undulations of these upper tropospheric waves cause the weakening of the jet stream and the jet split into two branches over the central Himalayan region. It has also been noted that the wave propagation in the upper troposphere is faded after 19 December 2010, indicating the wave breaking event occurred. � The Rossby wave breaking is further identified using the potential vorticity (PV) intrusions on isobaric levels in the upper troposphere. Hence, the wave breaking resulting in the transport of the high PV air from the high latitudes towards the central Himalayan region. Moreover, a strong downward vertical component of PV into the mid-troposphere is also triggered by the breaking event on 19 December 2010. The high PV intrusion, therefore, caused the transport of the ozone rich air into the lower troposphere, which is clearly depicted by the TES satellite observations and two reanalysis data products from ECMWF. � Hence, the present study clearly establishes the fact that the planetary-scale wave breaking is also an important factor for the unusual enhancement of ozone over this region. Furthermore, it is also important to extend this study for several cases in this region as this region is more important for regional and global climate. While it is also needed to study the future scenario of the frequency of such wave breaking events in this region for a better understanding of the ozone variability in the Himalayan region. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Kondapalli Niranjan Kumar: Conceptualization, Methodology, Data curation, Writing - original draft, Visualization, Investigation, Software, Validation. Som Kumar Sharma: Conceptualization, Meth odology, Supervision, Writing - review & editing. Manish Naja: Conceptualization, Methodology, Supervision, Writing - review & edit ing. D.V. Phanikumar: Conceptualization, Methodology, Supervision, Writing - review & editing. Acknowledgments Surface ozone observations at ARIES, Nainital are supported by the ISRO ATCTM project. We are also thankful to ECMWF reanalysis Research data analysis, computational and information systems labo ratory team for providing us the datasets required for our analysis. This work is supported by the Science and Engineering Research Board Department of Science and Technology (SERB -DST) and Department of Space, Govt., of India.
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