Structures of the Monsoon Low–Level Flow and the Monsoon Boundary Layer over the East Central Arabian Sea

Abstract

The monsoon boundary layer was subjected to aerial measurements by the National Center for Atmospheric Research (NCAR) Electra on 20 and 24 June 1979 as part of the Arabian Sea component of the Summer Monsoon Experiment (MONEX). Gust probe (also called fast response or turbulence) measurements of u (eastward), v (northward), w, T and ?v, (absolute humidity), available from 80 to 550 m are used to compute turbulent fluxes, while slow response (one-second) measurements of u, v, T and Td available from 80 to 20 m, are used to relate thew fluxes to the low-level flow. The turbulence Rights on 20 June were made under an extensive synoptic scale cloud system. Within this cloud system variations in the cloud type and convective activity occurred between the two areas where intensive boundary layer (RL) measurements were made. The southern area, which had lower ?v values, was dominated by multiple cloud layers, downward virtual heat flux, weak upward moisture flux in the BL, and a sharply defined jet stream. The northern area, only 400 km north northwest, was characterized by Cumulus (Cu) and Cumulus Congestus clouds, a BL with a conditionally unstable thermal stratification, strong upward moisture flux and a lower (in elevation) jet stream. On 24 June, flights were made under two distinct types of cloud systems. Measurements under the western fringe of a synoptic cloud system showed a BL dominated by downward virtual heat flux and a weak upward moisture flux. Farther westward the measurements revealed a BL with a conditionally unstable stratification associated with a dramatic increase in cumulus clouds. The results suggest that monsoon cloud conditions may strongly influence the stability characteristics and turbulent exchange processes of the monsoon BL.