The Convective-Atmospheric-Boundary-Layer Height and its dependence upon Meteorological Variables At a Tropical Coastal Station during Onshore and Offshore Flows

Abstract

The height of the atmospheric boundary layer (ABL) plays a crucial role in the vertical transport of energy, moisture, and pollutants from the surface. We investigate the development of the convective ABL (CABL) height over a tropical coastal station and quantify its variations with the shortwave radiative flux, near-surface air temperature (Tair), soil skin temperature, soil moisture content, lower tropospheric thermal structure, and virtual potential temperature lapse rate (VPLR) during onshore and offshore flows, based on multi-year (2012–2017) observations carried out using a microwave radiometer profiler and in situ probes at Thumba (8.5° N, 77° E), located in the south-west of Indian Peninsula. The maximum CABL height increases linearly with the VPLR at the rate of 140 to 200 m per °C km−1 (correlation coefficient of 0.82 to 0.92) during different seasons. The delayed onset of daytime onshore flow results in a greater CABL height as continental conditions persist longer, allowing more CABL growth, whereas the earlier arrival of the onshore flow leads to early development of a thermal internal boundary layer with a lower CABL height. When offshore flow prevails, the CABL develops like the continental CABL, with a peak CABL height greater than that during onshore flow by about 300 m. The onset of onshore flow lowers the daytime increase in Tair by about 2 °C. Such quantifications for distinct flow conditions are very sparse over tropical coastal regions and would be useful for understanding coastal air-pollution dispersal as well as validation and improvements in numerical modelling of the CABL under different wind conditions.