Urban-Rural differences in characteristics for Shallow Clouds observed over Chennai, a tropical megacity in India

Shallow clouds significantly influence the Earth's radiative balance by modulating incoming and outgoing shortwave radiation. The formation and characteristics of shallow clouds are closely coupled with the land-atmosphere interactions and hence urbanization can have a distinct impact on the occurrence of shallow clouds. This study investigates the occurrence and radiative properties of shallow clouds using simultaneous observations of mini–Micro-Pulsed Lidar (MPL), pyranometers and a network of sky imager over Chennai. These observations span over four sites across Chennai, representing a mix of urban and semi-urban microenvironments, during the months of January and February. Largely, shallow clouds are having a mean cloud base height of 800-1000 m and a mean cloud thickness of 250-300 m. Mean cloud radiative forcing (CRF) of -200 W/m², confirms the strong cooling effect of shallow clouds. The diurnal variation of hourly averaged cloud fraction (CF) shows a peak of 0.3 around 10:00AM, followed by a gradual decline, consistent with the lifecycle of shallow clouds driven by local meteorological conditions, including temperature and humidity fluctuations. A strong negative correlation (r = -0.95) was observed between CF and CRF, highlighting enhanced cooling effects of shallow clouds with increasing CF. The detailed differences in CRF, CF and cloud albedo from the three additional sites spanning the urban to rural transect across the city will be discussed. Observational findings will provide valuable empirical data for refining cloud-climate interactions over urban environments in northeast monsoon region.