Response of atmospheric surface layer parameters to cloudiness over a tropical coastal station

Abstract

This study examines the response of surface layer parameters and ABL height (ABLH) to the change in cloudiness over Kochi. Outgoing long wave radiation (OLR) is considered as a proxy for the cloudiness. Temperature, horizontal wind speed, vertical velocity variance (VVV), sensible heat flux, momentum flux, turbulent kinetic energy (TKE), drag coefficient and ABLH corresponding to each OLR bin are evaluated during different seasons. It is observed that the temperature response curve is similar for the pre-monsoon and the monsoon. Increase in cloudiness is associated with decrease in temperature during these seasons and it is attributed to the presence of organized convections. Pre-monsoon consists of organized convections as well as local convections. During post monsoon, an increase in temperature is observed in association with local convections having the OLR values less than 180 Wm−2. Horizontal wind speed response is similar during the pre-monsoon and post monsoon seasons with minimum corresponding to the bin 180–199 W m−2 and an increase in wind speed to both side of the OLR bins (cloudy and clear sky conditions). The increase in wind speed during cloudy (clear sky) conditions can be connected with strong downdraft from convective clouds (strong sea breeze). However, during monsoon and winter, the surface wind strengthens with decrease in cloudiness. Reduction in cloudiness increases the surface fluxes, TKE and ABLH during all the seasons. A drastic decrease (increase) in temperature, sensible heat flux and ABLH (horizontal wind speed, momentum flux and TKE) is observed during the passage of a depression.