Observations of the thermodynamic structure of marine atmospheric boundary layer over Bay of Bengal, Northern Indian Ocean and Arabian Sea during premonsoon period

Abstract

This paper presents the observations of the thermodynamic structure of the Marine Atmospheric Boundary Layer (MABL) over Bay of Bengal (BoB), Northern Indian Ocean (NIO) and Arabian Sea (AS) regions, using radiosonde observations carried out as a part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) during March–May 2006. Most of the profiles show the general structure of tropical MABL which consists of surface layer, sub-cloud layer (mixed layer), transition layer, cloud layer and trade wind inversion (TWI). However, in few soundings over BoB and AS, it was observed that the cloud layer is remarkably well-mixed, forming a double mixed layer (DML) structure. This feature is seen to be closely associated with the low level anticyclonic system over these sectors. The average profiles of vertical wind (corresponding to the region of observation) obtained from NCEP showed that there is an increase in the vertical velocity on the days when such a double mixed layer was observed and the altitude region of this high ω coincides with that of the DML. The study also reveals some important differences in the MABL structure during convectively active and weak periods. The spatial variation of the parameters like mixed layer (ML) depth, height of lifting condensation level (LCL), level of free convection (LFC), and trade wind inversion (TWI) are presented and discussed in detail. The TWI observations are first of their kind from the BoB region. The TWI top is found to be at a higher height compared to the observations over other oceanic regions, and the relationship between the TWI altitude and lapse rate of specific humidity is used to explain why the TWI was formed at a higher height. The observations of the MABL parameters are also compared with those reported from the previous campaigns over these regions.