Thermodynamic and dynamic structure of atmosphere over the east coast of Peninsular Malaysia during the passage of a cold surge

Journal of Atmospheric and Solar-Terrestrial Physics 146 (2016) 58–68

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Thermodynamic and dynamic structure of atmosphere over the east coast of Peninsular Malaysia during the passage of a cold surge Azizan Abu Samah a,d, C.A. Babu a,b, Hamza Varikoden c, P.R. Jayakrishnan d,n, Ooi See Hai a a

National Antarctic Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Atmospheric Sciences, Cochin University of Science and Technology, Cochin 682016, India c Indian Institute of Tropical Meteorology, NCL, P. O., Pune 411008, India d Institute of Ocean and Earth Sciences, University of Malaya, 50603 Kuala Lumpur, Malaysia b

art ic l e i nf o

a b s t r a c t

Article history: Received 22 December 2015 Received in revised form 11 May 2016 Accepted 22 May 2016 Available online 24 May 2016

An intense field observation was carried out for a better understanding of cold surge features over Peninsular Malaysia during the winter monsoon season. The study utilizes vertical profiles of temperature, humidity and wind at high vertical and temporal resolution over Kota Bharu, situated in the east coast of Peninsular Malaysia. LCL were elevated during the passage of the cold surge as the relative humidity values decreased during the passage of cold surge. Level of Free Convection were below 800 hPa and equilibrium levels were close to the LFC in most of the cases. Convective available potential energy and convection inhibition energy values were small during most of the observations. Absence of local heating and instability mechanism are responsible for the peculiar thermodynamic structure during the passage of the cold surge. The wind in the lower atmosphere became northeasterly and was strong during the entire cold surge period. A slight increase in temperature near the surface and a drop in temperature just above the surface were marked by the passage of the cold surge. A remarkable increase in specific humidity was observed between 970 and 900 hPa during the cold surge period. Further, synoptic scale features were analyzed to identify the mechanism responsible for heavy rainfall. Low level convergence, upper level divergence and cyclonic vorticity prevailed over the region during the heavy rainfall event. Dynamic structure of the atmosphere as part of the organized convection associated with the winter monsoon was responsible for the vertical lifting and subsequent rainfall. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Cold surge Malaysian winter monsoon Thermodynamic structure Divergence and vorticity pattern

1. Introduction The Asian winter monsoon plays a vital role in all activities in the Maritime Continent. The winter monsoon over the Maritime Continent has a peculiar behavior associated with the formation of intermittent cold surges. Usually cold surges are formed during November to February with maximum intensity in January (Cheang, 1980). Chang et al. (2006) brought out an elaborate review on the role played by the cold surges on the East Asia winter monsoon. They described the formation of cold surges as a result of strengthening and southeastward movement of the Siberian–Mongolian High (SMH), which leads to an increase of northeasterly wind and a decrease of surface temperature to the east and south of the SMH. Outbreaks occur with an extension of the SMH to the southeast or a split of the high pressure area that moves to the southeast coast of China (Lim and Chang, 1981; Chan and Li, 2004; Chan, 2005). In early September, the Siberian–Mongolian High (SMH) builds up and becomes intense by n

Corresponding author. E-mail address: [email protected] (C.A. Babu).

http://dx.doi.org/10.1016/j.jastp.2016.05.011 1364-6826/& 2016 Elsevier Ltd. All rights reserved.

November. Cold air emanating from the SMH affects the South China and Indo-China in early October and the central South China Sea by late October. When the event progresses rapidly southward and affects the tropics, particularly in the vicinity of the South China Sea, the weather system is referred as a cold surge. The development of a cold surge starts with the building up and subsequent southeastward extension or split of the SMH pressure area. The center of the SMH either moves southeastward (Ding and Krishnamurti, 1987; Zhang et al., 1997) or remains nearly stationary but with packets of cold air propagates eastward in conjunction with small high pressure or anticyclonic centers (Wu and Chan, 1995; Chan and Li, 2004). Jeong et al. (2006) found a precursory signal in the stratospheric circulation prior to the formation of the cold surge in East Asia. They found strong stratospheric negative potential vorticity anomalies and rising of geopotential height over northern Eurasia about one week before the cold surge occurrence. Changes in upper tropospheric circulations over Siberia are favorable for the formation of cold surges. When northwesterly forms in the vicinity of Lake Baikal that is associated with an upper level wave, the situation causes the formation of a surge over southern coast of China by 1 to 2 days

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(Ramage, 1971; Lum, 1976; Chu, 1978; Boyle and Chen, 1987). Compo et al. (1999) showed that upper tropospheric northeasterly anomaly associated with a developing ridge over northern Siberia serves as a precursor of a cold surge several days ahead. The magnitude of the surface temperature drop diminishes over the equatorial South China Sea due to the modification by the warm sea surface (Chang et al., 1979; Johnson and Zimmerman, 1986). Approach of a cold surge over the South China Sea is indicated by strengthening of northerly wind, decrease in surface temperature and increase in surface pressure (Boyle and Chen, 1987; Compo et al., 1999; Thompson, 1951; Lau and Li, 1984; Chu, 1978; Chang et al., 1983; Tick and Samah, 2004). It is evident that the frequency and intensity of cold surges associated with the winter monsoon play vital role in the rainfall pattern over the Maritime Continent (Juneng et al., 2007). Cheang (1980) made an extensive study on the features of the Malaysian winter monsoon in association with cold surges. When the cold surges are in phase with the westward moving equatorial disturbance over the South China Sea, the disturbance can develop into a well defined vortex in the near equatorial trough followed by widespread torrential rainfall. Chang and Lau (1982) reported that about one day after the arrival of the surge at the northern South China Sea, enhanced deep tropical convection occurs downstream from the surge and accelerates both Hadley and Walker circulations through increased upper level divergence. Cheang (1987) brought out the coincidence of winter monsoon onset with (1) penetration of cold surge, (2) southward movement of monsoon trough into equatorial Southeast Asia and (3) reversal of wind from easterlies to westerlies at 200 hPa level over the southern China. When a short wave propagates into the quasi-stationary East Asian long wave trough near Japan, the intensification of the trough strengthens the northerly wind and almost simultaneously the cold surge reaches in the northern region of the South China Sea. A jet streak that is often associated with the short wave plays a vital role in forcing the SMH through subsidence in the jet entrance region (Wu and Chan, 1995; Chan and Li, 2004). Even though it originates from the middle latitude upper levels, the surge over the South China Sea is mostly confined in a shallow layer between the surface and 850 hPa (Ramage, 1971). The strong cold air advection associated with the upper level northerly wind component around Lake Baikal that precedes the cold surge must exceed the subsidence warming, so that air parcels descend the isentropic surface and reach the northern South China Sea as a cold surge (Chang et al., 2005). Lu et al. (2007) made a numerical case study of a cold surge during the passage at Taiwan. They found that the leading edge of the cold surge was maintained primarily by meridional thermal advection in comparison with that in the zonal and vertical. Thermodynamic and dynamic structure of the atmosphere was not properly understood during the passage of a cold surge over the east coast of Peninsular Malaysia. Hence an attempt was made to study the thermodynamic and dynamic characteristics during the passage of a cold surge over the coastal station, Kota Bharu (situated in the east coast of the Peninsular Malaysia) by arranging an intense field observation. This study brings out variation in thermodynamic and dynamic structure of the atmosphere (and rainfall characteristics) during the passage of a cold surge over Kota Bharu utilizing 3 to 6 hourly radiosonde temperature, humidity and wind profile data in addition to surface meteorological observations from Malaysian Meteorological Department, satellite derived products and reanalysis data sets. This is a step forward towards better understanding of cold surge related thermodynamics and dynamic processes in the tropical atmosphere. Thus the objectives are (1) to bring out variation of air temperature, wind and humidity at the surface and above during the passage of

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a cold surge, (2) to understand the depth of the atmosphere in which the cold surge influences on these parameters, (3) to study the thermodynamic structure of the atmosphere in association with the cold surge and (4) to study the dynamic structure associated with the synoptic circulation as part of the winter monsoon to identify the mechanism responsible for heavy rainfall during the period.

2. Data and methods The study was carried out during the winter monsoon season in 2008 utilizing Vaisalasonde observations of temperature, humidity and wind during the passage of a cold surge at Kota Bharu station (latitude: 6°10′ N, longitude: 102°18′ E and height above msl: 5 m) situated in the east coast of Peninsular Malaysia (Fig. 1). This station was selected for the study because it is located in the east coast facing the South China Sea in the gate way of cold surge and hence to meet the requirements for capturing signature of cold surge properly. The vertical profiles of temperature, humidity and wind were made available at a vertical resolution of about 6 hPa (by taking measurements at every 4 s after releasing the balloon). The sensor specification is given in Table 1. The intense observation period was chosen from 26th November to 1st December, 2008 with two days before and after the passage of a cold surge over Kota Bharu. 44 observations were taken during the period at an interval of 1–6 h. The time of observation is given along with thermodynamic parameters in Table 2. Since the raw data from the Vaisalasonde are not in regular intervals of pressure, we employed logarithmic interpolation scheme to obtain the data at 10 hPa interval in the vertical for all the profiles. We consider that this interpolation preserves the thermodynamic and dynamic characteristics of the individual soundings. In addition, data from National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalysis, Tropical Rain Measuring Mission (TRMM) rain rate, Infra Red (IR) satellite imageries and other surface meteorological observations were also utilized for a better understanding of the thermodynamic and dynamic structure of the atmosphere during the passage of the cold surge. In order to study the features of rainfall associated with passage of a cold surge, it is better to have rainfall data at high temporal resolution. We utilized TRMM rain rate available at a temporal resolution of 3 h and a spatial resolution of 0.25°  0.25° latitude– longitude grid (Kummerow et al., 1998) to bring out the rainfall features during the intense observation period.

Fig. 1. Location of Kota Bharu station in the east coast of Peninsular Malaysia.

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Table 1 Sensor specification of Vaisala Sonde (Vaisala RS 80-15 L). Parameter

Sensor type

Range

Pressure Capacitive aneroid Temperature Capacitive bead R. humidity HUMICAP thin film capacitor Wind speed Trimble Mini Omega

Resolution Accuracy

3–1060 hPa 0.1 hPa 90 to 60 °C 0.1 °C 0–100% 0.1% –

0.1 m s

0.5 hPa 0.2 °C 2% 1

1ms

1

Table 2 LFC, EL, CAPE and CINE values at Kota Bharu.

cold surge period. The link for the data is http://tropic.ssec.wisc. edu/. The thermodynamic structure of the atmosphere during the passage of the cold surge was investigated employing the parameters: Lifting Condensation Level (LCL), Level of Free Convection (LFC), Equilibrium Level (EL), Convective Available Potential Energy (CAPE), Convection Inhibition Energy (CINE), etc. These parameters were evaluated from the Vaisalasonde data as per the detailed methodology described in Babu (1996). CAPE and CINE were evaluated using the following equations

CAPE = −

PEL

∫P ( Tvp − Tve )⋅Rd⋅d ( ln P ) LFC

Obs No Date&Time YYMMDDHH

LFC (hPa) EL (hPa) CAPE (J/ kg)

CINE (J/kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

743 867 910 897 735 734 707 870 658 778 862 855 841 556 764 921 863 964 941 884 920 803 925 862 834 892 99 70 99 99 99 99 619 613 994 997 1000 993 961 99 881 844 941 820

108.9 19.7 17.3 18.4 82.5 98.7 125.3 39.8 111.3 105.1 38.9 25.9 36.7 212.9 71.1 18.7 26.6 1.5 .6 19.6 25.1 18.3 8.5 17.8 43.1 34.6 99.0 2992.3 99.0 99.0 99.0 99.0 320.1 431.1 22.2 790.9 .1 .3 1.9 99.0 29.0 17.9 11.9 53.6

08112600 08112602 08112605 08112608 08112611 08112614 08112617 08112620 08112623 08112703 08112705 08112709 08112711 08112714 08112717 08112720 08112721 08112723 08112803 08112806 08112808 08112811 08112814 08112817 08112820 08112823 08112902 08112903 08112906 08112907 08112909 08112911 08112912 08112914 08112917 08112920 08112923 08113002 08113008 08113012 08113014 08113016 08113020 08113023

738 257 909 897 728 709 663 820 491 403 835 246 827 543 448 920 850 159 921 877 710 795 714 557 297 852 99 70 99 99 99 99 80 80 976 222 150 901 956 99 824 838 936 796

.1 400.4 .0 .0 .3 2.2 4.4 4.8 51.0 141.4 .7 475.7 .3 .5 133.2 .0 .4 869.9 1.0 .1 28.5 .1 24.8 67.8 270.1 1.1 99.0 .0 99.0 99.0 99.0 99.0 266.4 271.5 3.0 402.4 836.7 24.5 .0 99.0 2.8 .1 .0 2.7

99 in the column for LFC/EL/CAPE/CINE indicates data missing. 70 in the column for LFC/EL indicates no LFC/EL. 80 in the column for EL indicates EL is beyond 200 hPa.

In addition, horizontal wind and vertical wind at different levels (available at a spatial resolution of 2.5°  2.5° latitude–longitude from NCEP–NCAR reanalysis product, Kalnay et al., 1996) were utilized for determining the dynamic structure of the atmosphere associated with the synoptic circulation during the period. Satellite derived cloud imageries in the IR band made available from the CIMSS METEOSAT satellite image archives over Malaysian region (location centered over the Southeast Asia) taken at 3 h interval during the intense observation period were also utilized for describing the time evolution of the clouds during the

CINE = −

∫P

PLFC SFC

( Tvp − Tve )⋅Rd⋅d ( ln P )

where PLFC is the Level of Free Convection for the air parcel raised from the surface, PEL is the Equilibrium Level for the parcel, Tve is the virtual temperature of the environment at pressure level P through which parcel rises, Tvp is the virtual temperature of the parcel and Rd is the specific gas constant for dry air.

3. Results and discussions 3.1. Characteristics of cold surge onset The evolution of a cold surge was confirmed as per the method suggested by Chen et al. (2002). They reported that the arrival of a cold surge is generally characterized by a steep rise of surface pressure, a sharp decrease in surface temperature and a strengthening of meridional wind at the surface of station near Taiwan region. To identify the cold surge onset features, we studied the time series of surface pressure, surface temperature and meridional wind at Taiwan. Fig. 2 describes time series of daily (a) surface pressure, (b) surface temperature and (c) meridional wind at Taiwan from 21st November to 10th December, 2008. The onset of cold surge over Taiwan region was marked with sudden increase of surface northeasterly wind, followed by the sharp increase of surface pressure and steep decrease of surface temperature. In association with the formation of the cold surge, the surface pressure was increased by more than 6 hPa from 26th to 28th November. The surface temperature was decreased to less than 17 °C on 28th November (i.e. from 21.5 °C within 2 days). The northerly wind at the surface over Taiwan was exceeded 10 m s 1 on 27th November. Thus the formation of a cold surge over the Taiwan region is evident on 27th November, indicated by a vertical line on the Fig. 2. Fig. 3 represents the temperature variation over Kota Bharu at different levels from surface to 900 hPa, where the signature of the cold surge is significant. We noticed a slight temperature drop at the surface and 1000 hPa around 06 and 12 UTC of 27th and temperature started to increase from 21 UTC of 27th to 06 UTC of 28th. It is striking to note that relatively higher temperature (more than 1.5 °C compared to that before and after the passage of the cold surge) was noticed at the surface and at 1000 hPa up to 06 UTC of 29th (until the departure of the cold surge from the station). Although this is in contrast to earlier results (Boyle and Chen, 1987 and others), higher temperature observed at the surface during passage of the cold surge can be attributed mainly on the modification on the system by the warm sea surface during its long travel over the South China Sea (Chang et al., 1979; Johnson and Zimmerman, 1986). Even though the average temperature near the surface during the cold surge period was high, it was less

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Fig. 2. Day-to-day variation of surface (a) pressure, (b) temperature and (c) meridional component of wind at Taiwan.

Fig. 3. Temporal variation of temperature over Kota Bharu (at different levels).

in the layer from 970 hPa to 900 hPa. Thus the modification on the temperature structure of the cold surge by the warm sea surface is effective below 970 hPa. A rapid fall of temperature was recorded immediately after the departure of the cold surge from the station (at 06 UTC of 29th near the surface),with a maximum drop at 1000 hPa level (4 °C). The variation of temperature near the surface was verified with the hourly surface observation and automatic weather station data made available for the station by the MMD. The temperature pattern during the passage of the cold surge just above the surface was different from surface. A temperature drop was registered in the layer 970–900 hPa from 15 UTC to 21 UTC of 27th by the passage of the cold surge. We examined how the temperature variation is felt in different levels during the passage of the cold surge. The temperature drop was remarkable in higher levels. Maximum temperature decrease (more than 2.5 °C) was recorded at 930 hPa level. The temperature drop occurred at different levels almost simultaneously by the approach of the cold surge. A gradual increase of temperature (an increasing trend) was noticed in the layer 970–900 hPa from 21 UTC of 27th to 03 UTC of 29th (during the presence of cold surge over the station) followed by the abrupt fall. Another steep temperature drop was also noticed in different levels at 06 UTC of 29th, immediately after the departure of cold surge from the station. Even though a slight increasing trend of temperature was found in the layer 970–900 hPa during the period of the cold surge, the average temperature in these levels during the cold surge period was less than that before the cold surge period. As explained earlier, the

average temperature at the surface and 1000 hPa was high during cold surge period. Thus the lapse rate of temperature in the layer just above the surface happened to be relatively high. Hence a shallow layer of super adiabatic lapse rate was formed over the station during the passage of the cold surge. The variation of temperature during the passage of the cold surge was noticed up to 850 hPa, confirming the earlier finding that the cold surge over the South China Sea is mostly confined in a shallow layer between the surface and 850 hPa (Ramage, 1971). The zonal and meridional wind components responded to the cold surge by changing the lower level wind direction from southwesterly to northeasterly (Fig. 4) during the passage of the cold surge from 18 UTC of 27th to 03 UTC of 29th November. Both zonal and meridional wind components strengthened with altitude. It is interesting to note that the northeasterly wind magnitude steadily increased by the approach of the cold surge. The entire zonal component was easterly and entire meridional component was northerly during the presence of cold surge at the station. Before and after the passage of the cold surge, the prominent wind direction in the lower layer was southwesterly. Maximum zonal component was noticed at 910 hPa (12 m s 1) while the maximum meridional component was at 970 hPa (14 m s 1). Both wind components reversed their direction around 06 UTC of 29th November as the cold surge was moved away from the station. Fig. 5(a) depicts the vertical time section of wind vectors (1000 hPa–100 hPa at 10 hPa interval) during the intense observation period. Easterly/Northeasterly wind dominates

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Fig. 4. Temporal variation of (a) zonal and (b) meridional components of wind.

Fig. 5. (a) Vertical time section of vector wind over Kota Bharu. (b) Vertical time section of specific humidity over Kota Bharu.

throughout the troposphere in most of the observation period. In the lower atmosphere, easterly wind strength was maximum around 900 hPa from 02 UTC of 26th to 17 UTC of 26th November. The wind core is found to elevate gradually to 850 hPa within 12 h (17 UTC of 26th to 05 UTC of 27th). Another easterly wind core (more than 25 m s 1) was observed at 650 hPa at 23 UTC of 26th November above the 900 hPa wind core. This is a peculiar wind

pattern with double easterly wind core within the lower troposphere. It was noticed that feeble wind prevailed in the lower atmosphere (up to 700 hPa) from 09 UTC to 20 UTC of 27th November. From 21 UTC of 27th to 03 UTC of 29th November, the northeasterly wind strength increased by the passage of the cold surge. The level of easterly wind and maximum wind core gradually increased during the period (maximum wind core shifted

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from 950 hPa at 23 UTC of 27th to 850 hPa at 03 UTC of 29th). It was noticed that in this period, the wind above 800 hPa was feeble up to 300 hPa. The lower atmospheric wind pattern was changed rapidly with feeble westerly from 06 UTC to 12 UTC of 29th. The lower regime was resumed systematically from 08 UTC of 30th and continued up to 23 UTC of 30th November (up to 850 hPa). The mid tropospheric wind pattern was strikingly strong (easterly) during 20 UTC of 26th to 21 UTC of 27th November in comparison with other periods. Moreover, the wind strength increased steadily beyond 900 hPa. The increase in wind strength progressed up to 21 UTC of 29th November. In certain occasions, the zonal wind strength at 900 hPa increased beyond 12 m s 1. Thus the change of wind direction in the lower layer and strengthening of wind with height can be attributed to the passage of the cold surge. The simultaneous signature of cold surge on the temperature and wind confirms the passage of the cold surge at the station between 18 UTC of 27th November and 03 UTC of 29th November. Thus the time required for reaching the cold surge from Taiwan region to Kota Bharu is less than a day. No remarkable variation was noticed on the pressure pattern at the surface or in the geopotential contour pattern in the upper level by the passage of the cold surge (figure is not included). Fig. 5(b) gives the vertical-time cross section of specific humidity at different levels during the cold surge experiment. On the day of cold surge onset, the specific humidity values are upto 8 g kg 1 even at 600 hPa. Then, throughout the period, high specific humidity values existed until 700 hPa, especially on 28th and 29th. Beyond 400 hPa, the humidity value is small and moisture content is feeble. All the moisture is mainly confined to be in the lower levels upto 850 hPa. The specific humidity values (g kg 1) at 1000, 970, 940 and 900 hPa during the intense observation period are given in Fig. 6. The specific humidity values at all levels show a rapid decrease and an increase within a short period when the cold surge approaches and moves away from the station. This variation of specific humidity can be attributed to rapid change of wind pattern during the arrival and departure of the cold surge at the station. A striking increase in the specific humidity values is noticed at different levels from 970 to 900 hPa during the passage of the cold surge over the station. An increase in specific humidity of about 2 g kg 1 was observed at the levels 970, 940 and 900 hPa between the arrival and departure of the cold surge at the station. This can be attributed to the intense moisture pumping caused by the strong northeasterly wind from the South China Sea associated with the cold surge. The increase in specific humidity during the passage of the cold surge was recorded up to 600 hPa. However,

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such variation in specific humidity was not observed at 1000 hPa. Moreover, surface relative humidity decreased for a short period from 03 UTC of 28th to 03 UTC of 29th November during the passage of cold surge due to increase in surface temperature (figure is not included). Widespread rainfall occurs in the Peninsular Malaysia during the passage of a cold surge (Juneng et al., 2007). Therefore, we made a detailed analysis of the rainfall pattern utilizing 3 hourly TRMM Rain Rate over the region from 26th November to 1st December (Fig. 7). Rainfall was occurred in the region associated with the passage of cold surge from 18 UTC of 27th to 03 UTC of 28th November. Except for a short period from 03 UTC to 18 UTC on 28th November, rainfall occurred over the region with three high rainfall events (more than 10 mm/h) during the intense observation period: one centered at 21 UTC of 26th November, the second at 12 UTC of 29th November and the third at 18 UTC of 30th November. We examined the reason for feeble rainfall for the short period and found that the dynamic structure as part of the organized convection was not favorable for heavy rainfall at Kota Bharu station. Three Hourly satellite cloud imageries from the METEOSAT were used to represent the spatial and temporal evolution of the clouds associated with the cold surge over the South China Sea. Fig. 8 represents the satellite cloud imageries at an interval of 6 hours from 26th to 30th November. The satellite imageries are helpful to infer the possibility for occurrence of rainfall over the station on the basis of movement of deep cloud clusters. In most of the cases, the presence of convective clouds as revealed by the satellite picture agree with the rainfall registered by the TRMM over the station, except at 12 UTC of 29th November in which high rainfall was reported at the station data as well as TRMM rain rate but thin or shallow cloud was seen in the satellite picture. We examined the cloud imageries to understand the reason for feeble rainfall from 03 to 18 UTC on 28th November. Deep cloud clusters were away from the station during the period and hence no remarkable rainfall was received in the station. In association with the passage of the cold surge over the station, the thermodynamic structure of the atmosphere behaves in a peculiar manner (Table 2). Kota Bharu station received good amount of rainfall from the beginning of November, 2008 as part of the winter monsoon (figure is not included) and hence the surface air was rich in moisture. The Lifting Condensation Level (LCL) was below 990 hPa in most of the observations (Fig. 9) due to high moisture content available near the surface. There were cases of exceptionally high values of LCL, close to the surface (below 1000 hPa) due to near saturated air at the surface. However, LCL

Fig. 6. Variation of specific humidity over Kota Bharu at different levels.

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Fig. 7. Variation of area averaged 3 hourly TRMM rain rate.

was above 960 hPa for a short period from 03 UTC of 28th to 02 UTC of 29th November. The relative humidity values at the surface decreased slightly during this short period due to the increase of temperature as explained earlier. This can be interpreted as the reason for elevating the LCL for the short period. The TRMM rain rate confirms non occurrence of rainfall during the period. Since LCL is the first level at which an air parcel lifted from the surface attains saturation, non occurrence of rainfall elevates the LCL due to insufficient moisture at the surface. The precipitable water content in three different layers of the atmosphere (surface to 100 hPa, surface to 500 hPa and surface to 700 hPa) are presented in Fig. 10 (the curves are not continuous due to non availability of data in certain profiles). The precipitable water values during the period were high due to moisture pumping from the South China Sea in association with the cold surge. The pattern of precipitable water in different layers behaves in a similar manner, since mechanism for the precipitable water for the different layers of the atmosphere is same. In all the observations, difference of precipitable water between the upper layers is small (6 kg m 2) compared to that in the lower layers (15 kg m 2). The moisture content of the top layer (500–100 hPa) is very small due to low temperature prevailed in the upper troposphere, as temperature determines water holding capacity of the air. In general, the amount of precipitable water in the lower layer (surface to 700 hPa) contributes significant portion (more than 40 kg m 2) in the total precipitable water of the atmosphere. Level of Free Convection (LFC) and Equilibrium Level (EL) values during the intense observation period were presented in the Table 2. The LFC values were more than 800 hPa in most of the cases, with exceptional small value of 556 hPa in one particular observation. In another case, LFC was not formed since the ambient air was warmer than the surface air parcel throughout in the troposphere. Even though high humidity was available at the surface, the LFC values were not close to the surface. Cloud cover/ intermittent rain (less insolation) can be attributed to the inhibition of instability near the surface and hence shooting up of the base of instability layer although abundant moisture is available in the lower atmosphere. The equilibrium level was formed just above the LFC in most of the cases, indicating that the depth of unstable layer is very shallow during the passage of the cold surge. Even when the equilibrium level was away from the LFC, the CAPE value was small as the difference between temperatures of the parcel and ambient air was very small. CAPE and CINE are important parameters for diagnosing thermodynamic structure of the atmosphere. The CAPE and CINE values pertaining to the intense observation period are also presented in Table 2. The CAPE values were small (highest CAPE value was less than 900 J kg 1) during the passage of the cold surge. As explained earlier, decrease in insolation (due to cloud cover/rain

associated with the cold surge) can be attributed as reason for reducing local heating and hence inhibiting instability near the surface. As per the thermodynamic structure revealed by the soundings, the surface air parcel was able to attain the LFC, but the thickness of the buoyant layer was shallow. Hence CAPE values were small in most of the cases. In other words, although thermodynamic structure was favorable for the surface air parcel to attain LFC, the surface air parcel was unable to rise much by its own in the absence of local heating (instability mechanism). Even if LFC is formed, EL is very close to the LFC, indicating shallow instability layer as reflected in the low values of CAPE. Further, the difference in temperature for the parcel and ambient air was small for most of the cases, leading to small CAPE values. The CINE values were also small during most of the observations (with an exceptionally high value of around 3000 J kg 1 due to no LFC, lowest stable layer was up to tropopause). The CINE values indicate the amount of energy required by the surface air parcel for crossing the stable layer above the surface for achieving free convection (by its own buoyancy force). In general, thermodynamic structure was not favorable for triggering local convection due to the absence of the local heating and instability. Since the environment was moisture rich, LCL was lowered and helped to achieve LFC at a relatively lower level as reflected in the low values of CINE. However, the unstable layer just above LFC was shallow as reflected in the values of the equilibrium level in most of the cases and hence the thermodynamic structure was not favorable for triggering local convection during the passage of the cold surge. We next present horizontal divergence, relative vorticity and vertical velocity prevailed during the intense observation period over Kota Bharu and adjoining areas for a better understanding of the role played by the dynamic structure in association with the organized convection in producing rainfall. A comparison of the dynamic structure on 28th and 29th November was also made to explain the reason for heavy rainfall on 29th in comparison with feeble rainfall on 28th. A horizontal convergence at 925 hPa level and a divergence at 200 hPa were noticed in most of the days (figure is not included). This low level convergence and upper level divergence obviously give rise to vertical motion to satisfy the mass continuity. We made a thorough study on relative vorticity pattern at 925 hPa. Cyclonic vorticity prevailed at 925 hPa during the entire intense observation period. Further, a comparison is made on the vorticity pattern on 28th and 29th November. Fig. 11(a) and (b) represents the vorticity pattern at 00 UTC and 12 UTC on 28th and Fig. 11(c) and (d) represents the vorticity pattern (  10 5 s 1) at 00 UTC and 12 UTC on 29th November respectively. Cyclonic vorticity prevailed over Kota Bharu and adjoining area in all the four cases. The cyclonic vorticity values are found to increase

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Fig. 8. METEOSAT color enhanced cloud imageries. The color code shows the cloud top temperatures corresponding to the clouds and thereby their heights. Deep intense clouds are with white color, then red and the lowest clouds are being represented by green color. The approximate location of the Kota Bharu observation site is marked in a very light black colored triangle in each panel. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

successively at Kota Bharu station from less than 1.5  10 5 s 1 at 00 UTC on 28th to more than 2.5  10 5 s 1 at 12 UTC on 29th. This increase in cyclonic vorticity and associated frictional convergence also make the vertical lifting strong. Intense low level convergence and strong cyclonic vorticity prevailed near the surface on 29th in comparison with that on 28th even in the presence of cold surge. Though the cyclonic vorticity value at 00 UTC on 28th was maximum (more than 5  10 5 s 1) in the central South China Sea, the value at the station was less. The location of maximum cyclonic vorticity pattern gradually shifted to the station by 12 UTC of 29th. The situation was responsible for the vertical

lifting and subsequent heavy rainfall on 29th as explained earlier. To confirm the vertical motion, we examined the vertical velocity (  10 1 Pa s 1, in the pressure coordinate) at 600 hPa at 00 UTC and 12 UTC on 28th and 29th November (Fig. 12(a)–(d)). An upward directed vertical velocity was noticed over Kota Bharu station as well as in the east coast of Peninsular Malaysia in all the four cases. The upward directed vertical velocity values at 00 UTC of 28th, 12 UTC of 28th, 00 UTC of 29th and 12 UTC of 29th were 0.70, 0.78, 1.41, 2.35 cm s 1 respectively (after converting into z coordinate). Thus the low level convergence, cyclonic vorticity and vertical velocity were more on 29th in comparison with that on

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Fig. 9. Variation of lifting condensation level.

28th, can be considered as the reason for more rainfall on 29th November. In addition, vertical time section of vertical velocity (vertical velocity  10 1 Pa s 1) centered over Kota Bharu (latitude: 6°10′ N, longitude: 102°18′ E) during the intense observation period (Fig. 13) indicates that upward directed vertical velocity prevailed over the station in a layer from 900 hPa to 200 hPa during the entire intense observation period with high values from 00 UTC to 12 UTC on 29th November coinciding with the heavy rainfall event. It is worth mentioning that an upward directed strong vertical motion prevailed in the entire troposphere for about 6 h accomplished as part of the organized convection, paving the way for vertical lifting and subsequent heavy rainfall as reported on 29th November. Thus strong upward directed vertical velocity embedded in the synoptic scale flow (derived from NCEP/NCAR data) supports the role played by the organized convection as part of the winter monsoon system. The strong moisture pumping from the South China Sea and vertical lifting associated with the low level convergence, frictional convergence and cyclonic vorticity can be considered as factors responsible for the heavy rainfall events during the period. Fig. 11. Relative vorticity pattern (  10 5 s 1) at (a) 00 UTC of 28th, (b) 12 UTC of 28th, (c) 00 UTC of 29th and (d) 12 UTC of 29th Nov., 2008.

4. Conclusion A study was carried out to bring out characteristics of weather elements, thermodynamic and dynamic structure during the passage of a cold surge over Kota Bharu utilizing Vaisalasonde measurements at high vertical and temporal resolution. We found that the cold surge took less than a day to reach from Taiwan to

Kota Bharu. The cold surge marked distinct signatures on temperature, wind and humidity at the surface and above during its passage over the station. Relatively higher temperature (more than 1.5 °C compared to that before and after the passage of the cold surge) was observed at the surface and 1000 hPa in association with the cold surge. But such rise in temperature was not felt at

Fig. 10. Variation of precipitable water.

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Fig. 12. Vertical velocity pattern (  10 1 Pa s 1) at (a) 00 UTC of 28th, (b) 12 UTC of 28th, (c) 00 UTC of 29th and (d) 12 UTC of 29th Nov., 2008.

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850 hPa (10 m s 1, easterly) and 650 hPa (25 m s 1, easterly). The specific humidity values were relatively high in the layer from 970 to 870 hPa during the cold surge period with rapid decrease of values during the onset and cessation of the cold surge. No specific variation was noticed on the pressure at Kota Bharu during the passage of the cold surge. The LCL was near surface during the intense observation period due to high relative humidity prevailed at the surface of the station. However, the LCL was elevated during a small period, when the surface relative humidity value was decreased. The precipitable water content of the atmosphere was high during the passage of the cold surge. During the intense observation period, the thermodynamic structure was not conducive for local convection due to the absence of local heating and subsequent instability. This is reflected in the values of CAPE, CINE, LFC and equilibrium level. We also studied dynamic structure of the atmosphere to identify mechanism responsible for heavy rainfall events and found that low level convergence, upper level divergence, cyclonic vorticity and upward directed vertical velocity were prevailed over the region during the intense observation period. The strong moisture pumping from the South China Sea and vertical lifting associated with the low level convergence, frictional convergence and cyclonic vorticity were factors responsible for the heavy rainfall during the period. In a comparison of the dynamic structure on 28th (feeble rainfall) and 29th (heavy rainfall), we found that intense low level convergence and strong cyclonic vorticity were prevailed over the station on 29th. Even though an area of high cyclonic vorticity was prevailed in the central South China Sea on 28th, it was away from the station. The location of maximum cyclonic vorticity pattern gradually shifted to the station by 29th. This situation was responsible for the intense vertical lifting as revealed by the vertical velocity pattern and subsequent heavy rainfall on 29th November.

Acknowledgment The first and fourth authors are thankful to the eScienceFund Project (No. 04-01-03-SF0410) and HICoE MOHE Project (Grant nos: IOES-2014 and IOES-2014B), for the financial support and all authors are grateful to the University of Malaya for providing the facilities. Fig. 13. Vertical time section of vertical velocity (0.1 Pa s

1

).

higher levels. Rather, a small temperature drop was experienced just above 1000 hPa (considerable decrease up to 900 hPa and feeble drop up to 850 hPa) during the passage of the cold surge. A rapid fall of temperature was registered at the surface and above during the onset and cessation of the cold surge. The temperature became normal immediately. The temperature drop was maximum at 930 hPa when the cold surge approached to the station and at the surface when the cold surge moved away from the station. A shallow layer of super adiabatic lapse rate formed over the station during the passage of the cold surge due to this peculiar temperature structure. No remarkable temperature variation was noticed above 850 hPa by the passage of the cold surge. The lower level wind (surface to 800 hPa) strengthened by the passage of the cold surge. Maximum zonal component (11 m s 1, easterly) was noticed at 910 hPa and maximum meridional component (14 m s 1, northerly) was at 970 hPa. The wind direction changed from southwesterly to northeasterly when the cold surge approached to the station. Northeasterly wind prevailed in the layer from surface to 800 hPa during the entire cold surge period and returned to southwesterly as the surge moved away from the station. We found a peculiar wind pattern of double wind core at

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