A Three-Dimensional Planetary Boundary Layer Model for the Somali Jet

Abstract

This paper is an extension of an earlier study on the planetary boundary layer dynamics of the low level monsoonal flow over the Arabian Sea (Krishnamurti and Wong, 1979), where the long term steady state motion field for a boundary layer was determined using a zonally symmetric model with a prescribed pressure field. In that study we examined the balance of forces in the surface layer and the planetary boundary layer for regions across the equator, across and along the low-level Somali jet, and across an intertropical convergence zone. The important role of advective accelerations in the near-equatorial balance of forces was demonstrated. The important study is based on a three-dimensional model that removes the restriction of zonal symmetry. This mesoscale fine mesh model, with a horizontal resolution of ∼55 km and a vertical resolution of 200 m, is integrated to examine the evolution of three-dimensional planetary boundary layer flows for prescribed three-dimensional pressure patterns. The observations of the pressure field were obtained from the climatological analysis of Van De Boogaard (1977) and the MONSOON 77 and MONEX 1979 data sets. The results of these long term integrations and the balance of forces are discussed and compared with those inferred from cross-equatorial trajectories of constant level balloons.