Classification and seasonal distribution of rain types based on surface and radar observations over a tropical coastal station

Abstract

Abstract An attempt has been made to classify stratiform and convective rain using an impact type disdrometer at a tropical coastal site based on (Testud et al., 2001) method. Using this classification scheme, all rain events were analysed and computed their duration (minutes) and accumulated water (mm) in convective, stratiform, transition and mixed rain types. For the first time, in this paper an atmospheric electric field mill was used for rain type classification. The classification results were confirmed with disdrometer, Micro Rain Radar (MRR), and atmospheric electric field mill (EFM) observations. In convective rain type, non-bright band and high electric field were correspondingly observed in MRR and EFM records. Whereas bright band and shallow electric fields were detected during stratiform rain in the above observations of instruments. Other supporting evidences were observed from disdrometer also in corresponding rain types. Seasonal variations in rain duration and associated accumulated water were investigated in the four seasons (winter, pre-monsoon, summer monsoon and post-monsoon) for different rain types. General and mixed rain events were identified depending on the presence of stratiform or convective type in them. Percentage of rain duration and events for the study period were analysed with respect to (a) mean monthly rain water availability and (b) the intensity pattern in the total rainfall encountered in each month. Among the four seasons, summer monsoon received maximum rainfall followed by post-monsoon. A bimodal variation in the rainfall with a primary peak in June and secondary peak in October was observed in the annual cycle. Rainfall was intense during the pre- and post-monsoon seasons and less intense during the summer monsoon season. Also, shorter duration rain events were repeated in summer monsoon and fewer rain events of longer duration are resulted during pre- and post-monsoon seasons. In this duration, mixed rain accounts second higher in all the seasons. It was more evident in summer monsoon and least in post-monsoon. Rainfall contribution from convective rain was dominant in all the seasons except in winter. In summer monsoon, mixed and transition rain types have contributed much water among the four seasons. In mixed and transition rain types, mixed rain contributed more in duration and transition contributed more to water. From day-night variation analysis, it was observed that there was day-high and night-low in pre-monsoon and its reversal in post-monsoon season, with almost same occurrence during the summer monsoon. In the case of number of rain events, general (mixed) events were less (more) during the summer monsoon. From statistical analyses, an increase was observed in the duration of rain events from winter to post-monsoon. About 20% of duration was aggregated by longer duration events in all the seasons except during the winter.