This paper describes the results of monthly latitudinal (8Â°-30Â° N) and latitude belts (8Â°-10Â°, 10Â°-15Â°, 15Â°-20Â°, 20Â°-25Â°, and 25Â°-30Â°N) averaged seasonal thunderstorm activity over India by using monthly data from a large number of Indian stations from 1970 to 1980. The latitudinal variation in the premonsoon (March-April-May) and monsoon season (June-September) months is described and the results are discussed. An examination of the seasonal thunderstorm day activity in the first four belts indicated systematic changes in their signals of semiannual oscillation. These changes are noted to be a function of latitude and season and appear to be consistent with the seasonal migration of the Intertropical Convergence Zone and solar heating of the Indian landmass. We compare the thunderstorm day activity with the monthly mean maximum values of the surface wet-bulb (Tw) temperatures in the five latitude belts over the Indian region. By using rainfall data for the same period of study, the relationship between seasonal rainfall and number of thunderstorm days over the 11 year period is examined. The results of variation of the ratio of monthly rainfall to thunderstorm days (RTR) during different phases of the southwest monsoon are also presented. Results of the monthly mean electrical conditions of mesoscale and isolated deep convective storms at Pune are summarized. It is noted that the electrification of the premonsoon season thunderstorms dominated by a factor of 3-4 over the monsoon ones. We have examined at length the possible influence of the El Nino on the occurrence and electrification of thunderstorms over the Indian region.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JD02592

Thunderstorm activity over India and the Indian southwest monsoon

Annual variations of mixed-layer characteristics at New Delhi, India have been studied for a weak monsoon (1987) and a strong monsoon (1988) year. In the weak monsoon year (1987), the maximum mixing depthhmax was found to have a value of around 3000 m during the pre-monsoon, less than 2000 m during the summer monsoon, around 2000 m during the post-monsoon, and less than 1000 m in the winter season. For the strong monsoon year (1988),hmax values were less than 1987 values for comparable periods throughout the year. The seasonal and yearly differences ofhmax were explained by the surface energy balance and potential temperature gradient γ at a time close to sunrise. According to the spatial patterns of γ obtained by an objective analysis of the 850 to 700 hPa layers. mixed-layer characteristics obtained at New Delhi are representative of the north and central regions of India.

Mixed-layer characteristics as related to the monsoon climate of New Delhi, India

Based on the latest data (I. M. D., 1999) of monthly number of station thunderstorm days (Thn) and mean maximum surface air temperatures (Tmax) of 276 Indian stations, an important and a long pending issue of east- west contrast in the frequencies of occurrence of thunderstorms over India is resolved in substantial details in this study. On the premise of nearly equal land areas, and density of the station network, and location of ITCZ; India is divided in two regions: Eastern Region (ER) and Western Region (WR) across the 79° E longitude line over India. Results pertaining to the contrasting features of Thn over ER and WR are presented. Our analysis showed that the annual total Thn over the ER are 4763, and over the WR are 3194 and the prominent difference in Thn is associated with monsoon season months over the ER and WR. The temperature sensitivity of occurrence of Thn showed that thunderstorms respond exponentially to modest increment in Tmax on the semi-annual and annual time-scales in both the regions but with clear contrast in their sensitivity over ER. This result is in good agreement with the results cited in many studies, where relationship between global tropical surface air temperatures and GEC parameters are investigated. Our analysis suggests that the hot and humid extensive land region of the ITCZ of the ER is more suitable and responsible for the development of thunderstorms. Further, results showing higher values of CAPE and moisture field over the ER are discussed to explain the E-W pronounced contrast in the thunderstorm activity. It is believed that the results presented in this study will be useful.

Climatology of thunderstorm activity over the Indian region : A study of east -west contrast

The present study analyses thermodynamic indices variation over three sites of eastern Indian region: Bhubaneswar, Kolkata and Ranchi, associated with pre-monsoon thunderstorms for 20-year period (1987–2006) for Bhubaneswar and Kolkata and 15 years (1996–2010) for Ranchi. All three sites are showing a rise in humidity over the period, unveiling the climate change over the region. We evaluated the threshold values of various thermodynamic indices for periods of 5-year intervals at each site based on skill score analysis. The indices associated with potential, convective, latent instability and moisture are showing varying threshold values over all the sites, and some of the indices are showing a definite increase/decrease in these threshold values. All three sites are showing a decrease in thunderstorm frequency over the study period. The work identifies the thermodynamic indices, which tend to capture the global warming impact in the threshold values by either showing an increase or decrease with the time at each site. The results advocate that for a long-term analysis of thermodynamic indices, the threshold values may change from one period to another.

Evaluating the impact of climate change in threshold values of thermodynamic indices during pre-monsoon thunderstorm season over Eastern India

Rainfall mechanism over the rain-shadow region of north peninsular India during the summer monsoon season has been investigated using dynamic, thermodynamic, cloud microphysics and cloud dynamic (CMCD) forcings. Daily rainfall data has been used to understand rainfall variability. Daily ECMWF wind data for the period 2009–2011 have been used to study the wind divergence, shear and vertical velocity profiles. Daily thermodynamic parameters from upper air soundings of Hyderabad (17.448°N, 78.381°E), have been examined. Aircraft data have been used to study CMCD parameters. The divergence is found between surfaces to 850 hPa level whereas the convergence is at 850 hPa, which comes down to surface level during presence of low pressure systems. The divergence is observed at low (700 hPa), mid (600–300 hPa) and upper (250–150 hPa) tropospheric levels. In consequence of these divergence structures, three types of cloud systems viz. shallow, congestus and deep are developed with bases just above 850 hPa and tops at corresponding three divergent levels. The vertical profiles of relative humidity observed by radiosonde data have been analyzed to get the frequency distribution of shallow, congestus and deep clouds. The highest frequency observed is that of congestus clouds. The thermodynamic structure shows dry surface level and warm and moist middle troposphere with tongues of dry air and multilevel inversions which have been attributed to advection of aerosol-rich dry air. The aircraft observations showed high aerosol concentrations from surface to 5 km, polluted clouds with cloud droplet effective radius smaller than that required for collision–coalescence process. The factors which are responsible for causing low rainfall over the rain-shadow area have been identified.

Rainfall mechanism over the rain-shadow region of north peninsular India

The differences in the thermodynamic structure of the atmosphere over India and the adjoining areas between active and weak monsoon (southwest) periods in the various meteorological sub-divisions are presented.
     The mean dry bulb and dew point remperatures and equivalent potentioal temperatures during active and weak monsoon were calculated at all the radiosonde stations in India and neighbourhood and studied. The results show that whatever be the monsoon activity, there is no significant change in the dry bulb temperature at any level and the moisture content in the lower tropospheric levels remains high without any appreciable variation. The main change is in the moisture content in the mid-tropospherica levels. The implication of these results in the vertical circulation associated with active and weak monsoon and in the radiation budget are brought out, since the vigorous ascending air in the areas of strong monsoon ultimately subsides slowly over the regions of weak monsoon. The region of the seasonal monsoon trough has been indentified as the area of maximum total energy both in active and weak monsoon is least. This is also the area where convective instability is present upto greater heights than elsewhere. The estimated heights of cb tops are highest in the region of the seasonal monsoon trough. It is also shown that there is no reversal of virtual temperature gradient over the northwest India in the midtropospheric levels during the SW monsoon season.

V. Sadasivan

Papers

Thermodynamic structure of the atmosphere over India during southwest monsoon season