Analysis of stability parameters in relation to precipitation associated with pre-monsoon thunderstorms over Kolkata, India H P Nayak and M Mandal∗ Centre for Oceans, Rivers, Atmosphere and Land Sciences, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India ∗Corresponding author. e-mail: [email protected] The upper air RS/RW (Radio Sonde/Radio Wind) observations at Kolkata (22.65N, 88.45E) during pre- monsoon season March–May, 2005–2012 is used to compute some important dynamic/thermodynamic parameters and are analysed in relation to the precipitation associated with the thunderstorms over Kolkata, India. For this purpose, the pre-monsoon thunderstorms are classified as light precipitation (LP), moderate precipitation (MP) and heavy precipitation (HP) thunderstorms based on the magnitude of associated precipitation. Richardson number in non-uniformly saturated (Ri*) and saturated atmosphere (Ri); vertical shear of horizontal wind in 0–3, 0–6 and 3–7 km atmospheric layers; energy-helicity index (EHI) and vorticity generation parameter (VGP) are considered for the analysis. The instability measured ∗ in terms of Richardson number in non-uniformly saturated atmosphere (Ri ) well indicate the occurrence of thunderstorms about 2 hours in advance. Moderate vertical wind shear in lower troposphere (0–3 km) and weak shear in middle troposphere (3–7 km) leads to heavy precipitation thunderstorms. The wind shear in 3–7 km atmospheric layers, EHI and VGP are good predictors of precipitation associated with thunderstorm. Lower tropospheric wind shear and Richardson number is a poor discriminator of the three classified thunderstorms. 1. Introduction high socio-economic impact, thunderstorms are of serious concern to researchers and meteorologists. Thunderstorm is a severe weather phenomenon Extensive research has been carried out in the accompanied by strong winds, lightning, thun- last nine decades, to investigate various aspects der, rainfall and sometime hail. The Gangetic of thunderstorms (Normand 1921; Sohoni 1928; West Bengal gets affected by thunderstorms dur- Koteswaram and Srinivasan 1958;Koteswaram ing the pre-monsoon season (March–May) every and De 1959;Srinivasanet al. 1973; Chatterjee year. These thunderstorms cause damage to prop- et al. 1995; Sadhukhan and De 1998;Sadhukhan erty and crops; human and animal fatalities, and et al. 2000; Mukhopadhyay et al. 2005;Ghoshet al. also aviation hazards. The precipitation associated 2008). Several studies have been conducted to with the thunderstorms is very useful as it provides investigate the efficiency of various stability para- temporary relief from the summer heat and sup- meters and indices in representing the convec- plies much needed water to crops. Because of their tive environment leading to the occurrence of Keywords. Atmospheric instability; stability indices; Richardson number; vertical wind shear; energy-helicity index; vorticity generation parameter. J. Earth Syst. Sci. 123, No. 4, June 2014, pp. 689–703 c Indian Academy of Sciences 689 690 H P Nayak and M Mandal thunderstorms (Schultz 1989;Raviet al. 1999; Schultz (1989) tested seven stability indices for Kunz 2007;Dhawanet al. 2008; Tyagi et al. 2011). convective development over northeast Colorado A probabilistic model has also been developed and inferred that severe weather threat (SWEAT) to predict lightning over southeast India using index is the best indicator of severe weather. It dynamical and thermo-dynamical parameters as may be mentioned that among those seven indices predictors (Rajeevan et al. 2012). The model is SWEAT index was the only index which takes based on perfect prognostic method and has rea- wind shear into account. Rasmussen and Blanchard sonable skill in predicting lightning events over (1998) showed that energy helicity index (EHI) and the reason. But limited studies are conducted on vorticity generation parameter (VGP) can well dis- the analysis of stability parameters in relation to the criminate ordinary, super-cell and tornadic thun- precipitation associated with the thunderstorms. derstorms. Savvidou et al. (2010), in their study In the present study, several stability parameters of extreme and non-extreme thunderstorms over have been analysed in relation to the precipitation Cyprus illustrated that VGP can be used as a good associated with the pre-monsoon thunderstorms predictor for classified thunderstorms. over Kolkata during the period 2005–2012. In the present study, analysis of Richardson Richardson number (Ri) is a useful index in esti- number, vertical wind shear, EHI and VGP is con- mating the dynamic instability of the atmosphere ducted in relation to the precipitation associated that combines thermal instability with vertical with thunderstorms over Kolkata. General synop- wind shear (Shou et al. 2003). It helps in diagnos- tic conditions over the study area are explained in ing the genesis and development of severe weather section 2. The upper air observations, radar phenomena, and constructing different stability imageries and information regarding the occur- criteria (Gao and Cao 2007). Zhao et al. (2010) rence of thunderstorm used in the study are introduced a new Richardson number (Ri*) for discussed in section 3. Mathematical relation non-uniformly saturated atmosphere in which local used in computation of dynamic/thermodynamic Brunt Vaisala frequency was computed using gene- parameters and classification of pre-monsoon ralised potential temperature instead of potential thunderstorm is discussed in section 4.Resultsare temperature or equivalent potential temperature. discussed in section 5 and summary of the study is Cao et al. (2011) investigated atmospheric stabi- giveninsection6. lity associated with torrential rain events and heat wave events over north China and concluded that generalised potential temperature (θ∗) has greater 2. Synoptic situation over the Kolkata utility in representing dynamic instability in moist region during pre-monsoon period weather. In computation of local Brunt Vaisala fre- quency, equivalent potential temperature (θe)is The Gangetic West Bengal, is frequently affected appropriate for completely saturated atmosphere by thunderstorms in the pre-monsoon season. whereas potential temperature (θ) is appropriate Although thunderstorm is a mesoscale pheno- for dry atmosphere. Since real atmosphere is menon, its occurrence depends on the large scale neither absolutely dry nor entirely saturated, flow and the prevailing synoptic situations. The Ri* for non-uniformly saturated atmosphere is Gangetic West Bengal is surrounded by the Bay expected to provide a better measure of instability of Bengal in the south and Chota Nagpur plateau in real atmosphere. Observations suggest that con- in the west. During the pre-monsoon season, low densation might occur even when relative humidity pressure prevails over Chota Nagpur plateau and is over 78%. the adjoining areas, which gets intensified and Several previous studies (Malkus 1952;Newton extended in the afternoon (Srinivasan et al. 1973). and Newton 1959;Asai1964; Robe and Emanuel The well marked seasonal high pressure system 2001) showed that vertical shear of horizontal forms over the Bay of Bengal (Weston 1972). The wind plays an important role in convective deve- large scale circulation over the region is charac- lopment. Weisman and Klemp (1982) suggested terised by ascending motion over the landmass that in a given buoyant environment, weak ver- and descending motion over the adjoining sea. As tical wind shear generates short-lived single-cell a result, moisture incursion takes place from the thunderstorms, weak to moderate shear gene- Bay of Bengal along the eastern coast. This is rates multi-cell thunderstorms and moderate to reflected in the large scale circulation pattern with strong shear generates super-cell thunderstorms. a shallow layer of southerly/southwesterly flow over Fankhauser (1971, 1988) showed that strong ver- the Gangetic West Bengal and the adjoining area tical wind shear favours strong mean tropospheric near the surface and dry westerly aloft. The sta- wind and hence faster movement of storms. The ble transition layer between the two air streams study also suggested that strong wind shear leads in which moisture decreases rapidly with height to reduction of precipitation efficiency of the storm. plays an important role in the formation of severe Analysis of stability parameters in relation to precipitation 691 thunderstorms. Its presence prevents penetration Table 1. Classification of LP, MP and HP thunderstorms. of convection into the layer above and hence No. of Precipitation Thunderstorm the moisture and warmth in the layer increases. sounding (mm) classification This favours the outbreak of severe thunderstorms (Joseph et al. 2005). The thunderstorms formed 48 0–10 Light precipitation (LP) under this condition are known as Nor’westers 10 11–39 Moderate precipitation (MP) as they come from the northwest. Besides 6 ≥40 Heavy precipitation (HP) Nor’westers, another type of less intense thunder- storm also occurs over this region, which is termed for analysis. The accumulated precipitation asso- as sea breeze generated or southerly thunderstorm ciated with these thunderstorms is shown in (Lohar and Pal 1995; Dalal et al. 2012). figure 1. The observed upper air profile is verti- cally interpolated to generate a profile at every 3. Data used 25 hPa. The Richardson number for entirely sat- urated atmosphere and non-uniformly saturated The 12 UTC upper air RS/RW (Radio