Cloud Morphology and Microphysics of Precipitation Events during Interseasonal Phases of Monsoon over Mumbai, India

Mumbai city situated on the west coast of Indian subcontinent is considered to be one of the most populous city of the world. The city typically experiences heavy rainfall spells during the pre-monsoon and monsoon periods from the cloud systems originating over the eastern and western part of the region respectively. In this study, the vertical structure of clouds and microphysical characteristics of precipitation during the pre-monsoon and monsoon period for 2018 are studied over Mumbai. As per the report of India Meteorological Department (IMD) of 2018, the onset date of monsoon over Mumbai is June 09, 2018. As such, the period from March 01 – June 08 are considered to be pre-monsoon period whereas June 09- September 30 are considered to be the monsoon months. A Joss-Waldvogel Disdrometer was set up at IMD campus in Santacruz (Western Mumbai). These measurements were considered with the reflectivity data from S-band Doppler Weather Radar placed at Colaba in southern Mumbai. The wind direction and corresponding rainfall observation over Santacruz shows that Mumbai receives rain primarily from easterly winds during the pre-monsoon time which then shifts to the south-westerly winds during the monsoon period. There is an interesting difference in characteristics found for the pre-monsoon and monsoon rainfall. In particular, a distinct diurnal variation with three peaks was noted for the pre-monsoon period. The dominance of urban convective environment in the pre-monsoon period and the impact of moisture supply from the marine sources over the city during the monsoon months are considered to be contributing factors for the contrasting diurnal pattern of rainfall for these inter-seasonal phases of monsoon. The corresponding vertical profile of radar reflectivity also shows that the rainfall peaks are complimented with clouds and hydrometeors yielding higher reflectivity during pre-monsoon season. The above observations has been portrayed in Fig. 1 which shows the diurnal variation of rainfall, raindrop size distribution, vertical profile of radar reflectivity and the occurrences of lightning over Santacruz during the pre-monsoon and monsoon period. The figure clearly signifies that the mentioned three peaks of rainfall during the pre-monsoon period are associated with the larger raindrop - the highest being in the range of 1- 4.5 mm during 20:00 - 22:00 hrs. The lightning scenario around 50 km centering the region of study also complemented the above observations with their strong presence during 18:00 -22:00 hrs. All these features clearly signifies that the strong convective environment persists over Mumbai during the pre-monsoon months while such distinct features of clouds and precipitation are not visible for the monsoon period. The microphysical characteristics of rainfall shows, raindrops of larger diameter dominate the total pre-monsoon months in comparison to the monsoon period. The role of urban convection appears to be an important contributor for a relatively higher localized CAPE and convective rainfall dominance (48% in pre-monsoon versus 23% in monsoon). These higher CAPE provide a conductive environment for more vigorous updraft-downdraft leading to smaller drops aloft and larger drops to precipitate locally. A detailed case study of two similar rainfall events during the contrasting inter-seasonal period has been undertaken and will be presented.