Weekly variation of summer monsoon rainfall in the vicinity of a tropical coastal metropolitan city: A study using observations from TRMM Precipitation Radar (PR) and Ozone Monitoring Instrument (OMI)

Occurrence of weekly cycle in meteorological and hydrological processes can be an indicator of the influence of anthropogenic activities on climatic processes. In the present study, we analyzed gridded hourly observations from TRMM Precipitation Radar for past 15 years (1998-2012) during summer monsoon season (June-to-September). Our main objective is to assess if there is a detectable weekly signal in monsoon rainfall over the Indian monsoon domain, and whether this signal is causally related to the aerosols in the urban vicinity. The rainfall indices such as number rainy days and mean rainfall were used for the analysis of weekly variation, while two statistical tests “chi-square” and “ball-in-box test” were used to define the significance of weekly variations. We observed the most interesting weekly variation of rainfall in the vicinity of a tropical coastal metropolitan city “Chennai” in South India (13.1 N, 80.2 E), in which most of the rainfall occurred during midweek over the metropolitan area, while the surrounding non-urban regions (rural and oceanic areas) received most rainfall during weekends. The pattern of the phase of weekly variation appears coherent and symmetric about the metropolitan center. Statistical significance tests indicate that the weekly cycle of rainfall is more prominent during the night time compared to the day time. Also, the nature of downstream amplification of rainfall during night time is very different from that during daytime, which could be attributed to the continental scale diurnal variation of upper-level monsoon circulation, and its interaction with the urban environment. Analysis of aerosol abrorption optical depth from Ozone Monitoring Instrument (OMI) indicates significant weekly variation of aerosols in the central region of Chennai metropolitan area, where the phase of the weekly variation of aerosol precedes that of rainfall by 1-2 days. Our analysis suggests that the urban aerosols may act to enhance the precipitation processes in a moist environment that is typically associated with tropical weather systems like Asian summer monsoon.