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Tropical plumes due to potential vorticity intrusions over Indian sector
Atmospheric Research 172–173 (2016) 28–36
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Tropical plumes due to potential vorticity intrusions over Indian sector M. Sandhya a,b, S. Sridharan a,⁎, M. Indira Devi b a b
National Atmospheric Research Laboratory, Pakala Mandal Chittoor District, Andhra Pradesh, India Department of Physics, Andhra University, Visakhapatnam, Andhra Pradesh, India
a r t i c l e
i n f o
Article history: Received 1 May 2015 Received in revised form 18 December 2015 Accepted 30 December 2015 Available online 7 January 2016 Keywords: Tropical plumes Potential vorticity Specific humidity Tropical–extratropical interactions
a b s t r a c t Four cases of potential vorticity (PV) intrusion events over Indian sector (March 2009, April 2010, April 2010 and March 2014) which lead to the generation of tropical plumes (TP) are presented. The PV intrusions are identified from the threshold value of ERA (European Centre for Medium Range Weather Forecasting (ECMWF) Reanalysis)-interim PV at 350 K isentropic level greater than 1.4 potential vorticity unit (PVU) (1 PVU = 10−6 Km2kg−1 s−1) at 13.5°N. These PV intrusions trigger convection to the east, which is identified from the low (b270 K) infrared brightness temperature. It is noted that the spatial extent of convection is large in these cases and is similar to the TP structures commonly present over eastern Pacific and Atlantic sectors during northern winter. The Meteosat IR imagery also confirms the occurrence of tropical plumes over Indian sector. The TPs play a major role in the transport of moisture from lower latitudes to higher latitudes. The ERA-interim specific humidity averaged for 200–300 hPa shows large scale moisture transport from lower to higher latitudes tracking the plume structure. Apart from these, interannual and seasonal variations of the occurrence of TP in connection with the PV intrusion events over Indian sector for the years 2000–2014 are presented. It is found that the number of occurrence of TP is more during February–April and all the PV intrusions do not lead to the TP structures. The life time of majority of TP over Indian sector is found to be 1–2 days and all the TP are not precipitative. Unlike reported earlier, the PV intrusions having broad trough are also leading to TP over Indian sector, whereas the PV intrusions having narrow trough (less than 3° longitude band) do not lead to TP. Besides, the occurrence of TP does not relate to even the depth of penetration of PV trough. It is demonstrated that the occurrence of TP is due to the poleward advection associated with the PV intrusion. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tropical plumes (TP) are continuous bands of upper tropospheric clouds crossing 15°N and extending at least 2000 km in length (McGuirk and Ulsh, 1990). These upper level disturbances are important in the poleward transport of momentum and kinetic energy from low latitudes and play an important role in atmospheric general circulation (Knippertz, 2007). They are the indicators for tropical– extratropical interactions with large moisture transport and these structures affect local and global radiation budget (Tubi and Dayan, 2014, Frohlich et al., 2013). There are studies, which reported extreme precipitation events in the outer tropics or subtropics during the poleward moisture transport associated with the TP structures (Yoneyama and Parsons, 1999, Fink and Knippertz, 2003, Knippertz and Martin, 2005, 2007 to state a few). The number of TP occurrences is more in the central and eastern Pacific sectors followed by Atlantic sector (McGuirk et al., 1987) during northern hemispheric winter, when upper level westerlies are prevailing (Webster and Holton, 1982). McGuirk and Ulsh (1990) studied the occurrence of TP during October 1983–April 1984 ⁎ Corresponding author. Tel.: +91 8585 272124; fax: +91 8585 272018. E-mail address: [email protected] (S. Sridharan).
http://dx.doi.org/10.1016/j.atmosres.2015.12.020 0169-8095/© 2016 Elsevier B.V. All rights reserved.
and they observed approximately ten plume structures over northern Pacific during northern winter. Thepenier and Cruette (1981) noted approximately 145 TP structures stretching from the east of subtropical Pacific Ocean to Western Europe from 1976 to 1978 with maximum frequency of occurrence in December. An average of 30–40 TP structures occur per year over North Africa and subtropical North Atlantic (Zohdy, 1991). Frohlich et al. (2013) obtained the first global climatology of TP structures using 10.8 μm gridded brightness temperature data for the period 1983–2006. They have noted that the TP occurrence is largely confined to Pacific and Atlantic sectors during boreal winter. The distribution of TP structures is similar during boreal summer, however with lower frequencies, except for monsoon influenced regions. They also noted that TP occurrence frequencies are higher during boreal winter all over the globe except over North Indian Ocean (30°E–110°E) and North West Pacific (110°E–180°E), where the TP frequencies are more in boreal summer due to Indian summer monsoon. The TPs often develop when the downstream of extratropical upper-level troughs propagates into low latitudes, particularly over the wintertime eastern north Pacific and north Atlantic (Frohlich et al., 2013). The penetration of Rossby waves from midlatitude to tropical central eastern Pacific (TCEP) (PV intrusion) is an important source for the TP (Blackwell, 2000). However, Blackwell (2000) could simulate the TP
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structures over TCEP region by using a dry barotropic model and concluded that moist convective processes were not important for the generation of TP structures. They also noted that the plumes were completely absent in the absence of tropical convergent forcing and cyclonic Rossby wave sources. There are a few observational studies, which postulate that advection of PV ahead of low latitude upper level troughs, forces ascent and thereby generates spatially extended cloud band (McGuirk et al., 1988, Kiladis, 1998). However the exact physical mechanism for the TP structure generation is still not well explored. There are a few studies which suggest the relation between extension of mid-latitude upper level troughs and the occurrence of TP (Anderson and Oliver, 1970, Thepenier and Cruette, 1981). Knippertz (2007) suggested that the relation between TP and upper level disturbances appear to vary between different parts of TP, at different times of the evolution and from case to case. Extratropical stratospheric air with high PV can enter into the tropical troposphere through the tropopause fold at the western side of the upper level trough associated with jet streams (Kentarchos et al., 1999). These PV intrusions are common in the eastern Pacific and Atlantic sectors (Waugh and Polvani, 2000) during northern winter. Recently, Sandhya and Sridharan (2014) noted more PV intrusion during pre-monsoon months (March–May) over Indian sector. They found that though frequency of occurrence of PV intrusion is less compared to eastern Pacific and Atlantic sectors, the intrusions play a major role in triggering convection over Indian sector during premonsoon months. There are many studies over Pacific and Atlantic sectors which noted TP structures associated with PV intrusion (Wernli and Sprenger, 2007, McGuirk et al., 1987). However, TP structures associated with the PV intrusions have not been reported over Indian sector. The criteria for identifying TP is the occurrence of continuous middle to upper tropospheric clouds, which cross 15°N from either north or south with length ~ 2000 km (McGuirk et al., 1987). According to the brightness temperature value in IR (Tir) and water vapour (Twv) channels, there are four cloud categories (Roca and Ramanathan, 2000). The clouds are referred as deep convective if Tir ≤ 260 K, middle clouds if 260 K b Tir ≤ 270 K, low clouds (or clear sky) if Tir N 270 K and Twv N 246 K and cirrus if Tir N 270 K and Twv ≤ 246 K. In the present study, TP are identified over the Indian sector from the combination of merged IR brightness temperature value Tir ≤ 270 K and Meteosat-IR image (Meteosat images are available from June 2005 only). The cloud bands having minimum extension of 1500 km in length and crossing 15°N over Indian sector are considered as TP in the present study. A detailed statistics on TP over Indian sector is presented and their
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characteristics are described. Their links with the shape of the PV intrusion troughs are presented. 2. Data sets 2.1. NCEP/CPC 4 km global-merged IR brightness temperature data The 4 km merged (60°N–60°S) IR brightness temperature data, from all available geostationary satellites GOES-8/10, METEOSAT-7/5 & GMS are taken from the site http://mirador.gsfc.nasa.gov for the present study. The availability of METEOSAT-7/5 provides unique opportunity of global coverage. The data provided are corrected for zenith angle dependence, since IR temperature is affected by the geometric effects and radiometric path extension. The hourly data are available from 7 February 2000 to present. The data at 1200 UTC are taken for the present study. 2.2. ERA-interim data sets European Centre for Medium Range Weather Forecasting (ECMWF) reanalysis (ERA) interim data sets are results from analysis conducted at 6-h intervals available for a 1.5° × 1.5° latitude–longitude grid and are prepared by ECMWF using their variational data assimilation system (Berrisford et al., 2009). These data sets are currently available in the website data-portal.ecmwf.int/data/d/interim_daily/for 15 isentropic levels and 37 pressure levels. In the present study, potential vorticity at 350 K isentropic level and specific humidity averaged for the pressure levels 200–300 hPa are used. The 350 K isentropic level has traditionally been used to identify the occurrence of PV intrusions into the tropical troposphere (Waugh and Polvani, 2000). The PV intrusion events are normally identified by fixing a threshold value in the range of 1–3 potential vorticity unit (PVU) (1 PVU = 10−6 Km2kg−1 s−1) at 350 K isentropic level (~12 km height). Though there is no universally accepted threshold value, different authors suggested different values, namely 1.4 PVU (Manney et al., 1995), 1.5 PVU (Mohankumar, 2008) and 2.0 PVU (Waugh and Polvani, 2000) and in the present study, we keep the minimum threshold value of 350 K isentropic level PV at 13.5°N as 1.4 PVU to identify the intrusion. 3. Results and discussion Fig. 1a–c shows latitude–longitude cross-section of PV at 350 K isentropic level for the days 7–9 March 2009. The PV intrusion event
Fig. 1. Latitude–longitude cross-section of PV at 350 K isentropic level and merged brightness temperature (Tir) for the days 7–9 March 2009.
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Fig. 2. Latitude–longitude cross-section of PV at 350 K isentropic level and merged brightness temperature (Tir) for the days 7–9 April 2010.
can be identified from the tongue of high PV greater than 1.4 PVU on these days. On 7 March 2009, the high PV tongue reaches 12°N in the longitude band 63°E–66°E. On subsequent days (8–9 March 2009), it reaches further to 10.5°N. Normally, convection gets triggered ahead (eastward side) of the PV intrusion as a result of decreased static stability and enhanced vertical motion (Funatsu and Waugh, 2008). Fig. 1d–f shows latitude–longitude cross-section of merged infrared brightness temperature (Tir) for the above mentioned PV intrusion days. On 7 March 2009, Tir is ~300 K over 0°–30°N and 50°E–80°E, indicating no convection. On the same day, convection can be inferred from low value of Tir (b230 K) over 0°–7°N, 80°E–90°E. However, it does not appear to be due to the PV intrusion, as the intrusion is at 12°N and far west to the localised convection region. On 8 March 2009, convective regime shifts to higher latitude (~11°N) over 80°E–82°E and on 9 March 2009, it is (Fig. 1f) over the region 4°N–25°N, 77°E–85°E with Tir b 240 K indicating the existence of spatially extended deep convective clouds. However Tir value less than 270 K shows that the spatially extended clouds from low to high latitudes are middle or upper level clouds (Roca and Ramanathan, 2000). These large spatially extended cloud
bands are similar to the TP which are common in the central and eastern Pacific and relatively less common in Atlantic Ocean (McGuirk et al., 1987) during northern winter. Though localised convection events are present on 7–8 March 2009, they do not match with the criteria to be identified as TP. Fig. 2 shows latitude–longitude cross section of PV and Tir for the days 7–9 April 2010. On 7 April 2010, PV with value greater than 1.4 PVU reaches 10.5°N over 55°E–60°E. On subsequent days, the width of the PV trough gets narrowed at lower latitude. The Tir on 7 April 2010 is less than 240 K over 13°N–20°N and 67°E–74°E which is east of the PV intrusion region. However as the trough gets narrowed on subsequent days, the convection extends over a large spatial area of 0°–25°N and 0°–80°E on 8 April 2010 and further to 90°E on 9 April 2010. The low Tir value (b 240 K) shows that the spatially extended clouds from low to high latitudes are middle clouds. These large spatially extended cloud bands are similar to the TP. However the localised convection event on 7 April 2010 does not meet the criteria to be identified as TP. Fig. 3 shows latitude–longitude cross-section of PV and Tir for the days 28–30 April 2010. On 28 April 2010, there is no intrusion of high
Fig. 3. Latitude–longitude cross-section of PV at 350 K isentropic level and merged brightness temperature (Tir) for the days 28–30 April 2010.
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Fig. 4. Latitude–longitude cross-section of PV at 350 K isentropic level and merged brightness temperature (Tir) for the days 11–13 March 2014.
Fig. 5. Meteosat-IR imagery for the day (a) 9 March 2009 (b) 8 April 2010 (c) 30 April 2010 and (d) 12 March 2014.
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Fig. 6. Latitude–longitude cross-section of specific humidity averaged over 200–300 hPa for the day (a) 9 March 2009 (b) 8 April 2010 (c) 30 April 2010 and (d) 12 March 2014.
PV to lower latitudes. However, on subsequent days (29–30 April 2010), the PV with values greater than 1.4 PVU intrudes to lower latitudes 15°N and 12°N respectively. There are no TP structures or spatially extended convection on 28 April 2010, which can be inferred from high Tir (Tir N 300 K), though there is a localised convection (inferred from Tir b 200 K) over Indian subcontinent. On the day of PV intrusion (29 April 2010), convection can be inferred from low Tir (Tir b 230 K) (Fig. 3e) to the east of high PV tongue. However, the spatial extent of convection is larger from 0° to 25°N and 57°E to 90°E on 30 April 2010 (Fig. 3f) indicates generation of TP structures, consistent with the penetration of high PV tongue to lower latitudes and when the tongue width gets narrowed. Fig 4 shows latitude–longitude cross-section of PV and Tir for the days 11–13 March 2014. The high PV (PV N 1.4 PVU) tongue reaches 15°N on 12 March 2014. However, the penetration of high PV tongue to lower latitude is not clear on 11 March 2014 and 13 March 2014. The convection can be inferred over a large spatial area covering 0°–25°N and 53°E–87°E on 12 March 2014 to the east of PV intrusion region (Fig. 4e). The T ir is in the range 240–270 K indicating the presence of middle clouds. This large spatial extent of convection is similar to the TP structures present in the central and eastern Pacific and Atlantic sectors during northern winter. However, the TP structure
is completely absent when there is no PV intrusion on 11 March 2014 and 13 March 2014. McGuirk et al. (1987) defined TP based on visual inspection of IR satellite imagery. The TP are visible in IR satellite imagery as extended cloud bands from tropics to subtropics. Fig. 5a–b shows meteosat-IR images for the days 9 March 2009, 8 April 2010, 30 April 2010 and 12 March 2014 at 12 UTC. All the figures show TP like structures extending to 1500–2000 km over Indian sector. The TP plays a major role in the synoptic scale interaction between tropics and mid-latitudes (McGuirk et al., 1987, 1988). The role of tropical plumes in tropical–mid latitude interactions is studied by Davis (1981); Kininmonth (1983); Schroeder (1983) and Hill (1969). The plumes are important for the transport of moisture at upper troposphere (UT) over large latitudinal extent (Tubi and Dayan, 2014). The specific humidity (q) averaged at 200–300 hPa for these days are shown in Fig. 6a–d.The q is higher than 2 × 10−4 kg/kg over the region (0°–15°N, 70°–95°E) on 9 March 2009, (0°–20°N, 65°–85°E) on 8 April 2010, (0°–25°N, 65°–90°E) on 30 April 2010 and (0°–20°N, 70°–90°E) on 12 March 2014, which suggests that these TP events are associated with large scale moisture transport. The large spatial extent and the spatial structure almost track the cloud structure in the Meteosat-IR
Fig. 7. (a) Interannual variations of PV intrusion events at 13.5°N and 50°E–90°E and the occurrence of TP from 2004 to 2013. (b) Seasonal variations of the PV intrusions and the TP occurrence for the years 2004–2013.
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Table 1 Characteristics of PV intrusions from 2000 to 2014 leading to TP structures over Indian sector. Date
Spatial extent
Length to width ratio
Averaged precipitation (mm/day) (0oN–25oN 50oE–90oE)
Averaged precipitation (mm/day) (over plume area)
Life time (days)
1 March 2001
13°N–27°N 60°E–84°E 2°N–25°N 75°E–90°E 12°N–30°N 80°E–90°E 12°N–25°N 75°E–88°E 0°N–22°N 50°E–85°E 4°N–25°N 77°E–85°E 0°N–25°N 60°E–80°E 0°N–25°N 57°E–90°E 9°N–30°N 52°E–90°E 5°N–20°N 50°E–68°E 0°N–25°N 53°E–87°E
2.8
0.3
0.2
1
4
4.1
10.7
5
5.3
0.73
0.32
2
6
0.58
1.2
2
3.4
2
2.2
2
4
2.3
9.5
2
3.8
3.5
5.4
2
2.3
4.9
5.8
2
14
1.6
0.5
1
3.7
2.9
0.4
2
7.7
1.4
1.6
2
21 December 2001 12 February 2002 15 April 2005 30 November 2007 9 March 2009 8 April 2010 30 April 2010 8 February 2012 15 April 2012 12 March 2014
imagery (Fig. 5). Tubi and Dayan (2014) noted that there was a good agreement between shape and orientation of TP and q at the pressure levels 600, 500, 400 and 300 hPa over middle-east. However over the Indian sector, the TP shape and orientation almost match with q only at 300–200 hPa. This indicates the transport of moisture from tropics to extratropics. In order to see whether all the PV intrusions lead to the formation of TP, a detailed statistics is presented on the characteristics of TP associated with all the PV intrusion events at 13.5°N over Indian sector (50°E– 90°E) occurred during the years 2000–2014 (Fig 7a–b). There is no PV intrusion at 13.5°N in the years 2000, 2003, 2008 and 2013 (Fig. 7a). None of the PV intrusion event leads to the generation of TP in 2004, 2006 and 2011. However in 2007, out of two PV intrusions, one leads to TP. There are three PV intrusions in the years 2001, 2005, 2010 and 2012. Out of these PV intrusions, two in 2001, 2010 and 2012 and one in 2005 lead to TP. In 2009, out of four PV intrusions only one leads to TP. Though there is only one PV intrusion in the years 2002 and 2014, both of them lead to TP. Fig 7b shows seasonal distribution of TP over Indian sector. There is no PV intrusion at 13.5°N during June–October
when there is tropical easterly jet (TEJ) over Indian sector. None of the PV intrusion leads to TP in January. In February and November, all the PV intrusions lead to TP. In March, out of 5 PV intrusions, four of them lead to TP and in April, out of 7 PV intrusions, only two lead to TP. In May and December, out of 4 PV intrusions, only one leads to TP. These results reveal that all the PV intrusions do not lead to the occurrence of TP and there are large seasonal and interannual variations in the occurrence of TP. Frohlich et al. (2013) studied the global climatology of TP for the years 1983–2006. They divided all globe into eight sectors and found the frequency of occurrence in each sector during boreal winter (October– March) and boreal summer (April–September). They noted that the frequency of TP was more during boreal winter at all sectors except at North Indian sector (0°–50°N, 30°E–100°E), where the TP frequency is more during boreal summer. During this time, most of the Indian sector is influenced by monsoon clouds. However the present study deals with the climatology of tropical plumes associated with PV intrusion at 13.5°N. Our result shows that TP occurrence frequency is more during February–April, when there are more intrusions. The cloudiness at
Fig. 8. (a–b) Latitude–longitude cross-section of PV at 350 K isentropic level for the days 4 May 2006 and 27 April 2009 and (c–d) latitude–longitude cross-section of merged brightness temperature for the same days.
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Table 2 Longitudinal band of PV intrusion (1.4 PVU) crossing 15°N (see text for details) and the pressure level up to which the PV intrusion (1.4 PVU) penetrated down. Date of PV intrusion
Longitudinal band (longitudinal width given in bracket)
Pressure level up to which the PV intrusion (1.4 PVU) penetrated down at 15°N
Remarks
1 March 2001 14 December 2001 21 December 2001 12 February 2002 3 March 2004 8 March 2004 8 March 2005 15 April 2005 6 December 2005 4 January 2006 4 May 2006 26 April 2007 30 November 2007 22 January 2009 9 March 2009 27 April 2009 7 May 2009 11 March 2010 8 April 2010 29 April 2010 10 April 2011 17 April 2011 16 May 2011 7 February 2012 31 March 2012 15 April 2012 12 March 2014
55.5–67.5 °E (12°) 58.5–64.5 °E (6°) 60–67.5 °E (7.5°) 73.5–78 °E (4.5°) 49.5–54 °E (4.5°) 78–82.5 °E (4.5°) 52.5–70.5 °E (18°) 48–55.5 °E (7.5°) 43.5–52.5 °E (9°) 49.5–60 °E (10.5°) 60–63 °E (3°) 49.5–54 °E (4.5°) 43.5–54 °E (10.5°) 46.5–60 °E (13.5°) 63–67.5 °E (4.5°) 66–69 °E (3°) 60–66 °E (6°) 72–73.5 °E (1.5°) 48–58.5 °E (10.5°) 61.5–66 °E (4.5°) 82.5–84 °E (1.5°) 69–73.5 °E (4.5°) 67.5–69 °E (1.5°) 46.5–58.5 °E (12°) 79.5–85.5 °E (6°) 43.5–55.5 °E (12°) 70.5–78 °E (7.5°)
250 hPa 200 hPa 200 hPa 300 hPa 400 hPa 400 hPa 225 hPa 225 hPa 200 hPa 200 hPa 250 hPa 225 hPa 200 hPa 225 hPa 300 hPa 300 hPa 250 hPa 225 hPa 200 hPa 250 hPa 200 hPa 250 hPa 300 hPa 200 hPa 300 hPa 200 hPa 250 hPa
Plume No plume Plume Plume No plume No plume No plume Plume No plume No plume No plume No plume Plume No plume Plume No plume No plume No plume Plume Plume No plume No plume No plume Plume No Plume Plume Plume
10°N–15°N initiated by upper tropospheric troughs in the tropical central eastern Pacific will last for 5–10 days (Liebmann and Hartmann, 1984, Liebmann, 1987). Frohlich et al. (2013) observed that the average life time of TP is nearly 1.6 days. Consistent with Frohlich et al. (2013), the present study also shows that the life time of plume is only 1–2 days over Indian sector. However, the TP on 21 December 2001 lasts for five days, when the PV intrusion event also lasts for five days from 21 to 25 December 2001. All the TP structures are oriented southwest–northeast in the northern hemisphere over Indian sector. Tubi and Dayan (2014) noted that all tropical plumes are not precipitative over Middle East. In the present study, the precipitation averaged over the region 0°–25°N,
50°E–90°E, where majority of tropical plumes are observed over Indian sector and the same averaged over the region where TP are present are calculated from TRMM daily mean precipitation data. It is observed that the average precipitation over plume area during March 2001, February 2002, February 2012 and April 2012 is less than that over Indian sector indicating that these TP events are non precipitative in these cases. However all the other cases seem to be precipitative. The details of tropical plumes associated with PV intrusion from 2000 to 2014, including its spatial extent, average precipitation and life time are listed in Table 1. Blackwell (2000) noted that in the absence of tropical convergent force the planetary wave trough is broad and the TP structures are absent over TCEP and when the tropical convergent force is present, the trough contracts further results in to the TP. In the present study we have checked the width of PV trough from the longitude band, where PV having value greater than or equal to 1.4 PVU crosses 15°N. Fig. 8a–b shows the two PV intrusion events on 4 May 2006 and 27 April 2009 respectively. The trough width is only ~ 3° longitude band on both days. However it can be inferred (Fig. 8c–d) that there is no TP structure during these days. Apart from this the trough width for all the PV intrusion cases from 2000 to 2014 are calculated and listed in Table 2. It is noted that one PV intrusion event, each in 2001, 2007 and 2010 and two cases in 2012 leads to the generation of TP, though the trough width is greater than 10°. In 2001, 2005 and 2012 PV intrusion with trough width 7.5° and in 2002, 2009 and 2010, intrusion with trough width 4.5° leads to TP structures. However in contrary to Blackwell (2000), it is noted that no PV intrusion with trough width less than 3° leads to the formation of TP structures over Indian sector. Three PV intrusion events, each in 2005, 2006 and 2009 with trough width greater than 10° do not lead to TP. However for all other PV intrusions which do not lead to TP, the trough width is below 6°. It is also observed that the TP do not have any relation with vertical penetration of PV trough (Table 2). There are cases in which TP occurs even when the PV intrusion is limited to 250 hPa (for instance on 1 March 2001 and 30 November 2007) and it does not occur, even when PV intrusion is penetrated down to 400 hPa (3 March 2004 and 8 March 2004). Recently, Sandhya et al. (2015) noted increase in the upper tropospheric humidity associated with all PV intrusion over Indian sector. They suggested that the poleward advection ahead of PV intrusion brings moisture from lower latitudes to higher latitudes, irrespective of whether the PV intrusion triggers convection or not. The latitude longitude cross section of PV at 350 K along with wind vector at same level for the PV intrusion days (9 March 2009, 8 April 2010, 30 April 2010 and
Fig. 9. Latitude–longitude cross section of PV at 350 K along with wind vector at same level for the PV intrusion days (9 March 2009, 8 April 2010, 30 April 2010 and 12 March 2014).
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Fig. 10. The latitude–longitude cross section of PV (PV greater than 1.4 PVU (black contour lines)) along with horizontal wind at 350 K for (a–b) two TP days (9 March 2009, 8 April 2010) and (c–d) two non TP days (4 May 2006 and 27 April 2009).
12 March 2014) shown in Fig.9 shows that wind is from lower latitude to higher latitude, ahead of PV intrusion. As both convection and poleward advection are normally associated with the PV intrusion to the eastern side of intrusion (Kiladis, 1998; Mathews and Kiladis, 1999; Funatsu and Waugh, 2008), it is suggested that the occurrence of TP may be due to the poleward advection associated with the PV intrusion. The horizontal wind vector at 350 K isentropic level for the two PV intrusion days (09 March 2009, 08 April 2010), which lead to TP generation and another two events (4 May 2006, 27 April 2009), which do not lead to TP structures along with PV greater than 1.4 PVU are shown (thick contour lines) in Fig. 10. The horizontal wind greater than 20 m/s is shown in the figure. The wind varies from 30 m/s– 50 m/s, to the eastern side of intrusion on 09 March 2009 and 08 April 2010, when there are TP structures, associated with PV intrusion events. However on 4 May 2006, wind is less than 20 m/s from 10°N–16.5°N and 20 m/s–40 m/s from 16.5°N–30°N. On 27 April 2009, wind is less than 20 m/s from 10°N–20°N and varies from 20 m/s to 40 m/s over 20°N–30°N. It can be inferred from the figure that, PV intrusion associated with strong poleward advection, ahead leads to the generation of TP structures, whereas, those events with weak poleward advection do not lead to the generation of TP structures. 4. Conclusion Four cases of PV intrusion events leading to the formation of tropical plumes over Indian sector are presented. The occurrences of tropical plumes are confirmed from the combination of NCEP/CPC merged IR brightness temperature (Tir) values less than 270 K and cloud bands in Meteosat IR imagery. A detailed statistics of all tropical plume events associated with PV intrusion at 13.5°N over Indian sector is presented. The occurrence of TP shows a large interannual variation and the TP occurrence frequency is found to be maximum during February–March. Unlike reported earlier, the PV intrusions having broad trough are also leading to TP over Indian sector, whereas the PV intrusions having narrow trough (less than 3° longitude band) do not lead to TP. References Anderson, R., Oliver, V., 1970. Some examples of the use of synchronous satellite pictures for studying changes in tropical cloudiness. Proc. Symposium on Tropical Meteorology, Honolulu, Amer. Meteor. Soc., E XII-1-E XII-6. Berrisford, P., Dee, D., Fielding, K., Fuentes, M., Kallberg, P., Kobayashi, S., Uppala, S., 2009. The ERA-Interim Archive, European Centre for Medium Range Weather Forecasts, Shinfield Park, Reading, Berkshire RG2 9AX, United Kingdom. pp. 1–16.
Blackwell, K.G., 2000. Tropical plumes in a barotropic model: a product of Rossby wave generation in the tropical upper troposphere. Mon. Weather Rev. 128, 2288–2302. Davis, N.E., 1981. Meteosat looks at general circulation: III. Tropical–extratropical interaction. Weather 36, 168–173. Fink, A.H., Knippertz, P., 2003. An extreme precipitation event in southern Morocco in spring 2002 and some hydrological implication. Weather 58, 377–387. Frohlich, L., Fink, A.H., Knippertz, P., Hohberger, E., 2013. An objective climatology of tropical plumes. J. Clim. 26 (14), 5044–5060. Funatsu, B.M., Waugh, D.W., 2008. Connections between potential vorticity intrusions and convection in the eastern tropical Pacific. J. Atmos. Sci. 65, 987–1002. Hill, H.W., 1969. A synoptic study of a large scale meridional trough in the Tasman Sea—New Zealand area. N. Z. J. Sci. 12, 576–593. Kentarchos, A.S., Roelofs, G.J., Lelieveld, J., 1999. Model study of a stratospheric intrusion event at lower mid-latitudes associated with the development of a cut-offlow. J. Geophys. Res. 1717–1727. Kiladis, G.N., 1998. Observations of Rossby waves linked to convection over the eastern tropical Pacific. J. Atmos. Sci. 55, 321–339. Kininmonth, W.R., 1983. Variability of rainfall over northern Australia. Variations in the Global Water Budget, D. Riedel, pp. 265–272. Knippertz, P., 2007. Tropical–extratropical interactions related to upper-level troughs at low latitudes. Dyn. Atmos. Oceans 43, 36–62. Knippertz, P., Martin, J.E., 2005. Tropical plumes and extreme precipitation in subtropical and tropical west Africa. Q. J. R. Meteorol. Soc. 131, 2337–2365. Knippertz, P., Martin, J.E., 2007. A Pacific moisture conveyor belt and its relationship to a significant precipitation event in the semiarid southwestern United States. Weather Forecast. 22, 125–144. Liebmann, B., 1987. Observed relationships between large-scale tropical convection and the tropical circulation on subseasonal time scales during northern winter. J. Atmos. Sci. 44, 2543–2561. Liebmann, B., Hartmann, D.L., 1984. An observational study of tropical midlatitude interaction on intraseasonal time scales during winter. J. Atmos. Sci. 41, 3333–3350. Manney, G.L., Zurek, R.W., Froidevaux, L., Waters, J.W., Neill, A.O., Swinbank, R., 1995. Lagrangian transport calculations using UARS Data. Part II: Ozone. J. Atmos. Sci. 3069–3081. Mathews, A.J., Kiladis, G.N., 1999. Interactions between ENSO, transient circulation and tropical convection over the Pacific. J. Clim. 12, 3062–3086. McGuirk, J.P., Ulsh, D.J., 1990. Evolution of tropical plumes in VAS water vapour imagery. Mon. Weather Rev. McGuirk, J.P., Thompson, A.H., Smith, N.R., 1987. Moisture bursts over the tropical Pacific Ocean. Mon. Weather Rev. 115, 788–798. McGuirk, J.P., Thompson, A.H., Schaefer, J.R., 1988. An eastern Pacific tropical plume. Mon. Weather Rev. 116, 2505–2521. Mohankumar, K., 2008. Stratosphere Troposphere Interactions—An Introduction. Springer, p. 416. Roca, R., Ramanathan, V., 2000. Scale dependence of monsoonal convective systems over the Indian Ocean. J. Clim. 13, 1286–1298. Sandhya, M., Sridharan, S., Indira Devi, M., 2015. Tropical upper tropospheric humidity variations due to potential vorticity intrusions. Ann. Geophys. 33, 1081–1089. Sandhya, M., Sridharan, S., 2014. Observational relations between potential vorticity intrusions and premonsoon rainfall over Indian sector. Atmos. Res. 137, 80–90. Schroeder, T.A., 1983. The subtropical jet stream and severe local storms—a view from the tropics, Preprints. 13th Conf. On Severe Local Storms, Tulsa, Amer. Meteorol. Soc., pp. 161–162 Thepenier, R.M., Cruette, D., 1981. Formation of cloud bands associated with the American subtropical jet stream and their interaction with midlatitude synoptic disturbances reaching Europe. Mon. Weather Rev. 109, 2209–2220.
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Tubi, A., Dayan, U., 2014. Tropical plumes over the middle east: climatology and synoptic conditions. Atmos. Res. 145-146, 168–181. Waugh, D.W., Polvani, L.M., 2000. Climatology of intrusions into the tropical upper troposphere. Geophys. Res. Lett. 27 (23), 3857–3860. Webster, P.J., Holton, J.R., 1982. Cross-equatorial response to middle-latitude forcing in a zonally varying basic state. J. Atmos. Sci. 39, 722–733. Wernli, H., Sprenger, M., 2007. Identification and ERA-15 climatology of potential vorticity streamers and cutoffs near the extratropical tropopause. J. Atmos. Sci. http://dx. doi.org/10.1175/JAS3912.1.
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Tropical plumes due to potential vorticity intrusions over Indian sector M. Sandhya a,b, S. Sridharan a,⁎, M. Indira Devi b a b
National Atmospheric Research Laboratory, Pakala Mandal Chittoor District, Andhra Pradesh, India Department of Physics, Andhra University, Visakhapatnam, Andhra Pradesh, India
a r t i c l e
i n f o
Article history: Received 1 May 2015 Received in revised form 18 December 2015 Accepted 30 December 2015 Available online 7 January 2016 Keywords: Tropical plumes Potential vorticity Specific humidity Tropical–extratropical interactions
a b s t r a c t Four cases of potential vorticity (PV) intrusion events over Indian sector (March 2009, April 2010, April 2010 and March 2014) which lead to the generation of tropical plumes (TP) are presented. The PV intrusions are identified from the threshold value of ERA (European Centre for Medium Range Weather Forecasting (ECMWF) Reanalysis)-interim PV at 350 K isentropic level greater than 1.4 potential vorticity unit (PVU) (1 PVU = 10−6 Km2kg−1 s−1) at 13.5°N. These PV intrusions trigger convection to the east, which is identified from the low (b270 K) infrared brightness temperature. It is noted that the spatial extent of convection is large in these cases and is similar to the TP structures commonly present over eastern Pacific and Atlantic sectors during northern winter. The Meteosat IR imagery also confirms the occurrence of tropical plumes over Indian sector. The TPs play a major role in the transport of moisture from lower latitudes to higher latitudes. The ERA-interim specific humidity averaged for 200–300 hPa shows large scale moisture transport from lower to higher latitudes tracking the plume structure. Apart from these, interannual and seasonal variations of the occurrence of TP in connection with the PV intrusion events over Indian sector for the years 2000–2014 are presented. It is found that the number of occurrence of TP is more during February–April and all the PV intrusions do not lead to the TP structures. The life time of majority of TP over Indian sector is found to be 1–2 days and all the TP are not precipitative. Unlike reported earlier, the PV intrusions having broad trough are also leading to TP over Indian sector, whereas the PV intrusions having narrow trough (less than 3° longitude band) do not lead to TP. Besides, the occurrence of TP does not relate to even the depth of penetration of PV trough. It is demonstrated that the occurrence of TP is due to the poleward advection associated with the PV intrusion. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tropical plumes (TP) are continuous bands of upper tropospheric clouds crossing 15°N and extending at least 2000 km in length (McGuirk and Ulsh, 1990). These upper level disturbances are important in the poleward transport of momentum and kinetic energy from low latitudes and play an important role in atmospheric general circulation (Knippertz, 2007). They are the indicators for tropical– extratropical interactions with large moisture transport and these structures affect local and global radiation budget (Tubi and Dayan, 2014, Frohlich et al., 2013). There are studies, which reported extreme precipitation events in the outer tropics or subtropics during the poleward moisture transport associated with the TP structures (Yoneyama and Parsons, 1999, Fink and Knippertz, 2003, Knippertz and Martin, 2005, 2007 to state a few). The number of TP occurrences is more in the central and eastern Pacific sectors followed by Atlantic sector (McGuirk et al., 1987) during northern hemispheric winter, when upper level westerlies are prevailing (Webster and Holton, 1982). McGuirk and Ulsh (1990) studied the occurrence of TP during October 1983–April 1984 ⁎ Corresponding author. Tel.: +91 8585 272124; fax: +91 8585 272018. E-mail address: [email protected] (S. Sridharan).
http://dx.doi.org/10.1016/j.atmosres.2015.12.020 0169-8095/© 2016 Elsevier B.V. All rights reserved.
and they observed approximately ten plume structures over northern Pacific during northern winter. Thepenier and Cruette (1981) noted approximately 145 TP structures stretching from the east of subtropical Pacific Ocean to Western Europe from 1976 to 1978 with maximum frequency of occurrence in December. An average of 30–40 TP structures occur per year over North Africa and subtropical North Atlantic (Zohdy, 1991). Frohlich et al. (2013) obtained the first global climatology of TP structures using 10.8 μm gridded brightness temperature data for the period 1983–2006. They have noted that the TP occurrence is largely confined to Pacific and Atlantic sectors during boreal winter. The distribution of TP structures is similar during boreal summer, however with lower frequencies, except for monsoon influenced regions. They also noted that TP occurrence frequencies are higher during boreal winter all over the globe except over North Indian Ocean (30°E–110°E) and North West Pacific (110°E–180°E), where the TP frequencies are more in boreal summer due to Indian summer monsoon. The TPs often develop when the downstream of extratropical upper-level troughs propagates into low latitudes, particularly over the wintertime eastern north Pacific and north Atlantic (Frohlich et al., 2013). The penetration of Rossby waves from midlatitude to tropical central eastern Pacific (TCEP) (PV intrusion) is an important source for the TP (Blackwell, 2000). However, Blackwell (2000) could simulate the TP
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structures over TCEP region by using a dry barotropic model and concluded that moist convective processes were not important for the generation of TP structures. They also noted that the plumes were completely absent in the absence of tropical convergent forcing and cyclonic Rossby wave sources. There are a few observational studies, which postulate that advection of PV ahead of low latitude upper level troughs, forces ascent and thereby generates spatially extended cloud band (McGuirk et al., 1988, Kiladis, 1998). However the exact physical mechanism for the TP structure generation is still not well explored. There are a few studies which suggest the relation between extension of mid-latitude upper level troughs and the occurrence of TP (Anderson and Oliver, 1970, Thepenier and Cruette, 1981). Knippertz (2007) suggested that the relation between TP and upper level disturbances appear to vary between different parts of TP, at different times of the evolution and from case to case. Extratropical stratospheric air with high PV can enter into the tropical troposphere through the tropopause fold at the western side of the upper level trough associated with jet streams (Kentarchos et al., 1999). These PV intrusions are common in the eastern Pacific and Atlantic sectors (Waugh and Polvani, 2000) during northern winter. Recently, Sandhya and Sridharan (2014) noted more PV intrusion during pre-monsoon months (March–May) over Indian sector. They found that though frequency of occurrence of PV intrusion is less compared to eastern Pacific and Atlantic sectors, the intrusions play a major role in triggering convection over Indian sector during premonsoon months. There are many studies over Pacific and Atlantic sectors which noted TP structures associated with PV intrusion (Wernli and Sprenger, 2007, McGuirk et al., 1987). However, TP structures associated with the PV intrusions have not been reported over Indian sector. The criteria for identifying TP is the occurrence of continuous middle to upper tropospheric clouds, which cross 15°N from either north or south with length ~ 2000 km (McGuirk et al., 1987). According to the brightness temperature value in IR (Tir) and water vapour (Twv) channels, there are four cloud categories (Roca and Ramanathan, 2000). The clouds are referred as deep convective if Tir ≤ 260 K, middle clouds if 260 K b Tir ≤ 270 K, low clouds (or clear sky) if Tir N 270 K and Twv N 246 K and cirrus if Tir N 270 K and Twv ≤ 246 K. In the present study, TP are identified over the Indian sector from the combination of merged IR brightness temperature value Tir ≤ 270 K and Meteosat-IR image (Meteosat images are available from June 2005 only). The cloud bands having minimum extension of 1500 km in length and crossing 15°N over Indian sector are considered as TP in the present study. A detailed statistics on TP over Indian sector is presented and their
29
characteristics are described. Their links with the shape of the PV intrusion troughs are presented. 2. Data sets 2.1. NCEP/CPC 4 km global-merged IR brightness temperature data The 4 km merged (60°N–60°S) IR brightness temperature data, from all available geostationary satellites GOES-8/10, METEOSAT-7/5 & GMS are taken from the site http://mirador.gsfc.nasa.gov for the present study. The availability of METEOSAT-7/5 provides unique opportunity of global coverage. The data provided are corrected for zenith angle dependence, since IR temperature is affected by the geometric effects and radiometric path extension. The hourly data are available from 7 February 2000 to present. The data at 1200 UTC are taken for the present study. 2.2. ERA-interim data sets European Centre for Medium Range Weather Forecasting (ECMWF) reanalysis (ERA) interim data sets are results from analysis conducted at 6-h intervals available for a 1.5° × 1.5° latitude–longitude grid and are prepared by ECMWF using their variational data assimilation system (Berrisford et al., 2009). These data sets are currently available in the website data-portal.ecmwf.int/data/d/interim_daily/for 15 isentropic levels and 37 pressure levels. In the present study, potential vorticity at 350 K isentropic level and specific humidity averaged for the pressure levels 200–300 hPa are used. The 350 K isentropic level has traditionally been used to identify the occurrence of PV intrusions into the tropical troposphere (Waugh and Polvani, 2000). The PV intrusion events are normally identified by fixing a threshold value in the range of 1–3 potential vorticity unit (PVU) (1 PVU = 10−6 Km2kg−1 s−1) at 350 K isentropic level (~12 km height). Though there is no universally accepted threshold value, different authors suggested different values, namely 1.4 PVU (Manney et al., 1995), 1.5 PVU (Mohankumar, 2008) and 2.0 PVU (Waugh and Polvani, 2000) and in the present study, we keep the minimum threshold value of 350 K isentropic level PV at 13.5°N as 1.4 PVU to identify the intrusion. 3. Results and discussion Fig. 1a–c shows latitude–longitude cross-section of PV at 350 K isentropic level for the days 7–9 March 2009. The PV intrusion event
Fig. 1. Latitude–longitude cross-section of PV at 350 K isentropic level and merged brightness temperature (Tir) for the days 7–9 March 2009.
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Fig. 2. Latitude–longitude cross-section of PV at 350 K isentropic level and merged brightness temperature (Tir) for the days 7–9 April 2010.
can be identified from the tongue of high PV greater than 1.4 PVU on these days. On 7 March 2009, the high PV tongue reaches 12°N in the longitude band 63°E–66°E. On subsequent days (8–9 March 2009), it reaches further to 10.5°N. Normally, convection gets triggered ahead (eastward side) of the PV intrusion as a result of decreased static stability and enhanced vertical motion (Funatsu and Waugh, 2008). Fig. 1d–f shows latitude–longitude cross-section of merged infrared brightness temperature (Tir) for the above mentioned PV intrusion days. On 7 March 2009, Tir is ~300 K over 0°–30°N and 50°E–80°E, indicating no convection. On the same day, convection can be inferred from low value of Tir (b230 K) over 0°–7°N, 80°E–90°E. However, it does not appear to be due to the PV intrusion, as the intrusion is at 12°N and far west to the localised convection region. On 8 March 2009, convective regime shifts to higher latitude (~11°N) over 80°E–82°E and on 9 March 2009, it is (Fig. 1f) over the region 4°N–25°N, 77°E–85°E with Tir b 240 K indicating the existence of spatially extended deep convective clouds. However Tir value less than 270 K shows that the spatially extended clouds from low to high latitudes are middle or upper level clouds (Roca and Ramanathan, 2000). These large spatially extended cloud
bands are similar to the TP which are common in the central and eastern Pacific and relatively less common in Atlantic Ocean (McGuirk et al., 1987) during northern winter. Though localised convection events are present on 7–8 March 2009, they do not match with the criteria to be identified as TP. Fig. 2 shows latitude–longitude cross section of PV and Tir for the days 7–9 April 2010. On 7 April 2010, PV with value greater than 1.4 PVU reaches 10.5°N over 55°E–60°E. On subsequent days, the width of the PV trough gets narrowed at lower latitude. The Tir on 7 April 2010 is less than 240 K over 13°N–20°N and 67°E–74°E which is east of the PV intrusion region. However as the trough gets narrowed on subsequent days, the convection extends over a large spatial area of 0°–25°N and 0°–80°E on 8 April 2010 and further to 90°E on 9 April 2010. The low Tir value (b 240 K) shows that the spatially extended clouds from low to high latitudes are middle clouds. These large spatially extended cloud bands are similar to the TP. However the localised convection event on 7 April 2010 does not meet the criteria to be identified as TP. Fig. 3 shows latitude–longitude cross-section of PV and Tir for the days 28–30 April 2010. On 28 April 2010, there is no intrusion of high
Fig. 3. Latitude–longitude cross-section of PV at 350 K isentropic level and merged brightness temperature (Tir) for the days 28–30 April 2010.
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Fig. 4. Latitude–longitude cross-section of PV at 350 K isentropic level and merged brightness temperature (Tir) for the days 11–13 March 2014.
Fig. 5. Meteosat-IR imagery for the day (a) 9 March 2009 (b) 8 April 2010 (c) 30 April 2010 and (d) 12 March 2014.
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Fig. 6. Latitude–longitude cross-section of specific humidity averaged over 200–300 hPa for the day (a) 9 March 2009 (b) 8 April 2010 (c) 30 April 2010 and (d) 12 March 2014.
PV to lower latitudes. However, on subsequent days (29–30 April 2010), the PV with values greater than 1.4 PVU intrudes to lower latitudes 15°N and 12°N respectively. There are no TP structures or spatially extended convection on 28 April 2010, which can be inferred from high Tir (Tir N 300 K), though there is a localised convection (inferred from Tir b 200 K) over Indian subcontinent. On the day of PV intrusion (29 April 2010), convection can be inferred from low Tir (Tir b 230 K) (Fig. 3e) to the east of high PV tongue. However, the spatial extent of convection is larger from 0° to 25°N and 57°E to 90°E on 30 April 2010 (Fig. 3f) indicates generation of TP structures, consistent with the penetration of high PV tongue to lower latitudes and when the tongue width gets narrowed. Fig 4 shows latitude–longitude cross-section of PV and Tir for the days 11–13 March 2014. The high PV (PV N 1.4 PVU) tongue reaches 15°N on 12 March 2014. However, the penetration of high PV tongue to lower latitude is not clear on 11 March 2014 and 13 March 2014. The convection can be inferred over a large spatial area covering 0°–25°N and 53°E–87°E on 12 March 2014 to the east of PV intrusion region (Fig. 4e). The T ir is in the range 240–270 K indicating the presence of middle clouds. This large spatial extent of convection is similar to the TP structures present in the central and eastern Pacific and Atlantic sectors during northern winter. However, the TP structure
is completely absent when there is no PV intrusion on 11 March 2014 and 13 March 2014. McGuirk et al. (1987) defined TP based on visual inspection of IR satellite imagery. The TP are visible in IR satellite imagery as extended cloud bands from tropics to subtropics. Fig. 5a–b shows meteosat-IR images for the days 9 March 2009, 8 April 2010, 30 April 2010 and 12 March 2014 at 12 UTC. All the figures show TP like structures extending to 1500–2000 km over Indian sector. The TP plays a major role in the synoptic scale interaction between tropics and mid-latitudes (McGuirk et al., 1987, 1988). The role of tropical plumes in tropical–mid latitude interactions is studied by Davis (1981); Kininmonth (1983); Schroeder (1983) and Hill (1969). The plumes are important for the transport of moisture at upper troposphere (UT) over large latitudinal extent (Tubi and Dayan, 2014). The specific humidity (q) averaged at 200–300 hPa for these days are shown in Fig. 6a–d.The q is higher than 2 × 10−4 kg/kg over the region (0°–15°N, 70°–95°E) on 9 March 2009, (0°–20°N, 65°–85°E) on 8 April 2010, (0°–25°N, 65°–90°E) on 30 April 2010 and (0°–20°N, 70°–90°E) on 12 March 2014, which suggests that these TP events are associated with large scale moisture transport. The large spatial extent and the spatial structure almost track the cloud structure in the Meteosat-IR
Fig. 7. (a) Interannual variations of PV intrusion events at 13.5°N and 50°E–90°E and the occurrence of TP from 2004 to 2013. (b) Seasonal variations of the PV intrusions and the TP occurrence for the years 2004–2013.
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Table 1 Characteristics of PV intrusions from 2000 to 2014 leading to TP structures over Indian sector. Date
Spatial extent
Length to width ratio
Averaged precipitation (mm/day) (0oN–25oN 50oE–90oE)
Averaged precipitation (mm/day) (over plume area)
Life time (days)
1 March 2001
13°N–27°N 60°E–84°E 2°N–25°N 75°E–90°E 12°N–30°N 80°E–90°E 12°N–25°N 75°E–88°E 0°N–22°N 50°E–85°E 4°N–25°N 77°E–85°E 0°N–25°N 60°E–80°E 0°N–25°N 57°E–90°E 9°N–30°N 52°E–90°E 5°N–20°N 50°E–68°E 0°N–25°N 53°E–87°E
2.8
0.3
0.2
1
4
4.1
10.7
5
5.3
0.73
0.32
2
6
0.58
1.2
2
3.4
2
2.2
2
4
2.3
9.5
2
3.8
3.5
5.4
2
2.3
4.9
5.8
2
14
1.6
0.5
1
3.7
2.9
0.4
2
7.7
1.4
1.6
2
21 December 2001 12 February 2002 15 April 2005 30 November 2007 9 March 2009 8 April 2010 30 April 2010 8 February 2012 15 April 2012 12 March 2014
imagery (Fig. 5). Tubi and Dayan (2014) noted that there was a good agreement between shape and orientation of TP and q at the pressure levels 600, 500, 400 and 300 hPa over middle-east. However over the Indian sector, the TP shape and orientation almost match with q only at 300–200 hPa. This indicates the transport of moisture from tropics to extratropics. In order to see whether all the PV intrusions lead to the formation of TP, a detailed statistics is presented on the characteristics of TP associated with all the PV intrusion events at 13.5°N over Indian sector (50°E– 90°E) occurred during the years 2000–2014 (Fig 7a–b). There is no PV intrusion at 13.5°N in the years 2000, 2003, 2008 and 2013 (Fig. 7a). None of the PV intrusion event leads to the generation of TP in 2004, 2006 and 2011. However in 2007, out of two PV intrusions, one leads to TP. There are three PV intrusions in the years 2001, 2005, 2010 and 2012. Out of these PV intrusions, two in 2001, 2010 and 2012 and one in 2005 lead to TP. In 2009, out of four PV intrusions only one leads to TP. Though there is only one PV intrusion in the years 2002 and 2014, both of them lead to TP. Fig 7b shows seasonal distribution of TP over Indian sector. There is no PV intrusion at 13.5°N during June–October
when there is tropical easterly jet (TEJ) over Indian sector. None of the PV intrusion leads to TP in January. In February and November, all the PV intrusions lead to TP. In March, out of 5 PV intrusions, four of them lead to TP and in April, out of 7 PV intrusions, only two lead to TP. In May and December, out of 4 PV intrusions, only one leads to TP. These results reveal that all the PV intrusions do not lead to the occurrence of TP and there are large seasonal and interannual variations in the occurrence of TP. Frohlich et al. (2013) studied the global climatology of TP for the years 1983–2006. They divided all globe into eight sectors and found the frequency of occurrence in each sector during boreal winter (October– March) and boreal summer (April–September). They noted that the frequency of TP was more during boreal winter at all sectors except at North Indian sector (0°–50°N, 30°E–100°E), where the TP frequency is more during boreal summer. During this time, most of the Indian sector is influenced by monsoon clouds. However the present study deals with the climatology of tropical plumes associated with PV intrusion at 13.5°N. Our result shows that TP occurrence frequency is more during February–April, when there are more intrusions. The cloudiness at
Fig. 8. (a–b) Latitude–longitude cross-section of PV at 350 K isentropic level for the days 4 May 2006 and 27 April 2009 and (c–d) latitude–longitude cross-section of merged brightness temperature for the same days.
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Table 2 Longitudinal band of PV intrusion (1.4 PVU) crossing 15°N (see text for details) and the pressure level up to which the PV intrusion (1.4 PVU) penetrated down. Date of PV intrusion
Longitudinal band (longitudinal width given in bracket)
Pressure level up to which the PV intrusion (1.4 PVU) penetrated down at 15°N
Remarks
1 March 2001 14 December 2001 21 December 2001 12 February 2002 3 March 2004 8 March 2004 8 March 2005 15 April 2005 6 December 2005 4 January 2006 4 May 2006 26 April 2007 30 November 2007 22 January 2009 9 March 2009 27 April 2009 7 May 2009 11 March 2010 8 April 2010 29 April 2010 10 April 2011 17 April 2011 16 May 2011 7 February 2012 31 March 2012 15 April 2012 12 March 2014
55.5–67.5 °E (12°) 58.5–64.5 °E (6°) 60–67.5 °E (7.5°) 73.5–78 °E (4.5°) 49.5–54 °E (4.5°) 78–82.5 °E (4.5°) 52.5–70.5 °E (18°) 48–55.5 °E (7.5°) 43.5–52.5 °E (9°) 49.5–60 °E (10.5°) 60–63 °E (3°) 49.5–54 °E (4.5°) 43.5–54 °E (10.5°) 46.5–60 °E (13.5°) 63–67.5 °E (4.5°) 66–69 °E (3°) 60–66 °E (6°) 72–73.5 °E (1.5°) 48–58.5 °E (10.5°) 61.5–66 °E (4.5°) 82.5–84 °E (1.5°) 69–73.5 °E (4.5°) 67.5–69 °E (1.5°) 46.5–58.5 °E (12°) 79.5–85.5 °E (6°) 43.5–55.5 °E (12°) 70.5–78 °E (7.5°)
250 hPa 200 hPa 200 hPa 300 hPa 400 hPa 400 hPa 225 hPa 225 hPa 200 hPa 200 hPa 250 hPa 225 hPa 200 hPa 225 hPa 300 hPa 300 hPa 250 hPa 225 hPa 200 hPa 250 hPa 200 hPa 250 hPa 300 hPa 200 hPa 300 hPa 200 hPa 250 hPa
Plume No plume Plume Plume No plume No plume No plume Plume No plume No plume No plume No plume Plume No plume Plume No plume No plume No plume Plume Plume No plume No plume No plume Plume No Plume Plume Plume
10°N–15°N initiated by upper tropospheric troughs in the tropical central eastern Pacific will last for 5–10 days (Liebmann and Hartmann, 1984, Liebmann, 1987). Frohlich et al. (2013) observed that the average life time of TP is nearly 1.6 days. Consistent with Frohlich et al. (2013), the present study also shows that the life time of plume is only 1–2 days over Indian sector. However, the TP on 21 December 2001 lasts for five days, when the PV intrusion event also lasts for five days from 21 to 25 December 2001. All the TP structures are oriented southwest–northeast in the northern hemisphere over Indian sector. Tubi and Dayan (2014) noted that all tropical plumes are not precipitative over Middle East. In the present study, the precipitation averaged over the region 0°–25°N,
50°E–90°E, where majority of tropical plumes are observed over Indian sector and the same averaged over the region where TP are present are calculated from TRMM daily mean precipitation data. It is observed that the average precipitation over plume area during March 2001, February 2002, February 2012 and April 2012 is less than that over Indian sector indicating that these TP events are non precipitative in these cases. However all the other cases seem to be precipitative. The details of tropical plumes associated with PV intrusion from 2000 to 2014, including its spatial extent, average precipitation and life time are listed in Table 1. Blackwell (2000) noted that in the absence of tropical convergent force the planetary wave trough is broad and the TP structures are absent over TCEP and when the tropical convergent force is present, the trough contracts further results in to the TP. In the present study we have checked the width of PV trough from the longitude band, where PV having value greater than or equal to 1.4 PVU crosses 15°N. Fig. 8a–b shows the two PV intrusion events on 4 May 2006 and 27 April 2009 respectively. The trough width is only ~ 3° longitude band on both days. However it can be inferred (Fig. 8c–d) that there is no TP structure during these days. Apart from this the trough width for all the PV intrusion cases from 2000 to 2014 are calculated and listed in Table 2. It is noted that one PV intrusion event, each in 2001, 2007 and 2010 and two cases in 2012 leads to the generation of TP, though the trough width is greater than 10°. In 2001, 2005 and 2012 PV intrusion with trough width 7.5° and in 2002, 2009 and 2010, intrusion with trough width 4.5° leads to TP structures. However in contrary to Blackwell (2000), it is noted that no PV intrusion with trough width less than 3° leads to the formation of TP structures over Indian sector. Three PV intrusion events, each in 2005, 2006 and 2009 with trough width greater than 10° do not lead to TP. However for all other PV intrusions which do not lead to TP, the trough width is below 6°. It is also observed that the TP do not have any relation with vertical penetration of PV trough (Table 2). There are cases in which TP occurs even when the PV intrusion is limited to 250 hPa (for instance on 1 March 2001 and 30 November 2007) and it does not occur, even when PV intrusion is penetrated down to 400 hPa (3 March 2004 and 8 March 2004). Recently, Sandhya et al. (2015) noted increase in the upper tropospheric humidity associated with all PV intrusion over Indian sector. They suggested that the poleward advection ahead of PV intrusion brings moisture from lower latitudes to higher latitudes, irrespective of whether the PV intrusion triggers convection or not. The latitude longitude cross section of PV at 350 K along with wind vector at same level for the PV intrusion days (9 March 2009, 8 April 2010, 30 April 2010 and
Fig. 9. Latitude–longitude cross section of PV at 350 K along with wind vector at same level for the PV intrusion days (9 March 2009, 8 April 2010, 30 April 2010 and 12 March 2014).
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Fig. 10. The latitude–longitude cross section of PV (PV greater than 1.4 PVU (black contour lines)) along with horizontal wind at 350 K for (a–b) two TP days (9 March 2009, 8 April 2010) and (c–d) two non TP days (4 May 2006 and 27 April 2009).
12 March 2014) shown in Fig.9 shows that wind is from lower latitude to higher latitude, ahead of PV intrusion. As both convection and poleward advection are normally associated with the PV intrusion to the eastern side of intrusion (Kiladis, 1998; Mathews and Kiladis, 1999; Funatsu and Waugh, 2008), it is suggested that the occurrence of TP may be due to the poleward advection associated with the PV intrusion. The horizontal wind vector at 350 K isentropic level for the two PV intrusion days (09 March 2009, 08 April 2010), which lead to TP generation and another two events (4 May 2006, 27 April 2009), which do not lead to TP structures along with PV greater than 1.4 PVU are shown (thick contour lines) in Fig. 10. The horizontal wind greater than 20 m/s is shown in the figure. The wind varies from 30 m/s– 50 m/s, to the eastern side of intrusion on 09 March 2009 and 08 April 2010, when there are TP structures, associated with PV intrusion events. However on 4 May 2006, wind is less than 20 m/s from 10°N–16.5°N and 20 m/s–40 m/s from 16.5°N–30°N. On 27 April 2009, wind is less than 20 m/s from 10°N–20°N and varies from 20 m/s to 40 m/s over 20°N–30°N. It can be inferred from the figure that, PV intrusion associated with strong poleward advection, ahead leads to the generation of TP structures, whereas, those events with weak poleward advection do not lead to the generation of TP structures. 4. Conclusion Four cases of PV intrusion events leading to the formation of tropical plumes over Indian sector are presented. The occurrences of tropical plumes are confirmed from the combination of NCEP/CPC merged IR brightness temperature (Tir) values less than 270 K and cloud bands in Meteosat IR imagery. A detailed statistics of all tropical plume events associated with PV intrusion at 13.5°N over Indian sector is presented. The occurrence of TP shows a large interannual variation and the TP occurrence frequency is found to be maximum during February–March. Unlike reported earlier, the PV intrusions having broad trough are also leading to TP over Indian sector, whereas the PV intrusions having narrow trough (less than 3° longitude band) do not lead to TP. References Anderson, R., Oliver, V., 1970. Some examples of the use of synchronous satellite pictures for studying changes in tropical cloudiness. Proc. Symposium on Tropical Meteorology, Honolulu, Amer. Meteor. Soc., E XII-1-E XII-6. Berrisford, P., Dee, D., Fielding, K., Fuentes, M., Kallberg, P., Kobayashi, S., Uppala, S., 2009. The ERA-Interim Archive, European Centre for Medium Range Weather Forecasts, Shinfield Park, Reading, Berkshire RG2 9AX, United Kingdom. pp. 1–16.
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