Abstract. In the present study, we have used the Weather Research and Forecasting (WRF) model to simulate the features associated with a severe thunderstorm observed over Gadanki (13.5 • N, 79.2 • E), over southeast India, on 21 May 2008 and examined its sensitivity to four different microphysical (MP) schemes (Thompson, Lin, WSM6 and Morrison). We have used the WRF model with three nested domains with the innermost domain of 2 km grid spacing with explicit convection. The model was integrated for 36 h with the GFS initial conditions of 00:00 UTC, 21 May 2008. For validating simulated features of the thunderstorm, we have considered the vertical wind measurements made by the Indian MST radar installed at Gadanki, reflectivity profiles by the Doppler Weather Radar at Chennai, and automatic weather station data at Gadanki.There are major differences in the simulations of the thunderstorm among the MP schemes, in spite of using the same initial and boundary conditions and model configuration. First of all, all the four schemes simulated severe convection over Gadanki almost an hour before the observed storm. The DWR data suggested passage of two convective cores over Gadanki on 21 May, which was simulated by the model in all the four MP schemes. Comparatively, the Thompson scheme simulated the observed features of the updraft/downdraft cores reasonably well. However, all the four schemes underestimated strength and vertical extend of the updraft cores. The MP schemes also showed problems in simulating the downdrafts associated with the storm. While the Thompson scheme simulated surface rainfall distribution closer to observations, the other three schemes overestimated observed rainfall. However, all the four MP schemes simulated the surface wind variations associated with the thunderstorm reasonably well. The model simulated reflectivityCorrespondence to: M. Rajeevan (rajeevan@narl.gov.in) profiles were consistent with the observed reflectivity profile, showing two convective cores. These features are consistent with the simulated condensate profiles, which peaked around 5-6 km. As the results are dependent on initial conditions, in simulations with different initial conditions, different schemes may become closer to observations. The present study suggests not only large sensitivity but also variability of the microphysical schemes in the simulations of the thunderstorm. The study also emphasizes the need for a comprehensive observational campaign using multi-observational platforms to improve the parameterization of the cloud microphysics and land surface processes over the Indian region.

[1] The gamma parameters have been derived on the ground with the disdrometer and aloft with VHF and UHF radar measurements made at Gadanki in the southwest monsoon season. They have been used to study the variability of the shape-slope (m -L) relation with the climatic regime and also as a function of height. The m -L relation obtained at Gadanki differs from that derived at Florida and Oklahoma indicating climatic differences in the relation, which could be due to the microphysical differences in the rain DSD at these two locations. However, these differences could also arise due to the use of different type of disdrometers at these locations. For the first time, an attempt has been made to study the variation of this relation with height, and the analysis clearly reveals a significant variation in the coefficients of the relation with height. The vertical variability of the relation has been ascribed to the microphysical processes occurring in the height region concerned in the present study. These results suggest that for accurate retrieval of drop size distribution from polarimetric measurements and also for studies on the microphysics of rain systems, the vertical variability of the relation needs to be accounted, in particular in an environment where the DSD variations are considerable. In addition, the reduction of the scatter in the m -L plot after filtering light rain events, suggests that the m -L relation may be pertinent to moderate to heavy rain corroborating some of the earlier reports. Citation: Narayana Rao, T., N. V. P. Kirankumar, B. Radhakrishna, and D. Narayana Rao (2006), On the variability of the shape-slope parameter relations of the gamma raindrop size distribution model, Geophys. Res. Lett., 33, L22809,

The spatial and seasonal variability of the vertical structure of precipitation has been studied using 15 years of Tropical Rainfall Measuring Mission's Precipitation Radar (TRMM PR) version 7 data over India and adjoining oceans. Special emphasis has been put on six different climatic rain regimes and on different types of precipitation including the virga rain. The distribution of reflectivity factor (Z) above the freezing level height is broader in northwest India (NWI) and narrower over the Arabian Sea and west coast of India (ASWC) than in other selected regions, due to dominance of deep and shallow convective rain, respectively, in those regions. The height variation of contours in normalized distributions for Z indicates that evaporation of raindrops (low-level hydrometeor growth) could be significant in NWI (ASWC and Bay of Bengal). All the above features show clear seasonal variation and are observed predominantly during the southwest monsoon. The occurrence of virga rain clearly shows land-ocean contrast (less over the oceans) and seasonal variation (preponderant during premonsoon). Among different rain categories, the stratiform (convective) rain had highest (lowest) fraction of virga rain of >15-30% (<10%) over land regions. 1. The storm height (SH) vertical distributions show a peak in the vicinity of bright band (BB) in all regions, except for those regions and seasons, where convective precipitation is dominant. The well-defined BB feature and SH exhibit significant seasonal and regional variations, which are linked to variations in the occurrence of stratiform rain and height of BB. The spatial and seasonal variations of mean SH and the occurrence of deep and overshooting convective rain show good correspondence with the spatial variation of convective available potential energy.

[1] Long-term measurements of raindrop size distribution (DSD) made with the JossWaldvogel disdrometers at two sites (Gadanki, an inland station, and Cuddalore, a coastal station) in southeast India are utilized to study the seasonal and spatial variations of DSD. The stratified DSD data (based on rain rate R) show significant seasonal variation at both sites. Smaller-drop concentration is higher in the northeast monsoon (NEM) than in the southwest monsoon (SWM) for the same R. Paucity of smaller drops in SWM increases the mass weighted mean diameter (D m ) considerably. The seasonal differences are pronounced at Gadanki. The seasonal differences are found to be a regular feature at these locations as they are observed in all the years. The DSD has also shown clear diurnal variation with large D m values in evening hours. The possible causative mechanisms for the observed spatial and seasonal DSD differences are investigated in detail using satellite and radiosonde observations. In particular, the research attempted to address the following question: Are the observed seasonal differences in DSD arising at the cloud formation level or related to the microphysical processes occurring in the evolution of DSD. The low-level wind pattern and cloud effective radius and D m distributions in these seasons reveal that the cloud systems in SWM and NEM are continental and maritime, respectively, in nature. However, the microphysical and dynamical processes related to evaporation and convection also seem to play an important role in modifying the DSD. These processes are found predominantly in SWM and are, primarily, responsible for the changes in DSD during their evolution.

[1] The raindrop size distributions (RSDs) measured with an impact-type disdrometer have been utilized to study the differences in cyclonic RSD (1) from southwest monsoon (SWM) to northeast monsoon (NEM), (2) from that of noncyclonic rain, and (3) from cyclonic rain elsewhere. The stratified (based on rainfall rate R) cyclonic RSD exhibits significant seasonal variation, with more large drops and fewer small drops in SWM than in NEM. The big drops are almost absent in cyclonic RSD, whereas the small and medium-sized drops are larger in number than they are in noncyclonic rain. The average cloud effective radius in cyclones is nearly equal in SWM and NEM, suggesting that the nature of the cyclonic cloud may be similar (oceanic) in both seasons. The cyclonic RSD in the Bay of Bengal is consistent qualitatively with that observed elsewhere, but there exist some differences in rainfall bulk parameters. Implications of the observed seasonal and cyclonic to noncyclonic differences in RSD on quantitative rainfall estimation and cloud-modeling studies are also discussed.Citation: Radhakrishna, B., and T. Narayana Rao (2010), Differences in cyclonic raindrop size distribution from southwest to northeast monsoon season and from that of noncyclonic rain,