Prediction of the 1979 summer monsoon onset with modified parameterization schemes

Abstract

This paper describes the impact on the tropical simulation in the ECMWF model of various changes to the treatment of physical processes, with particular emphasis on the onset of the Asian summer monsoon. A series of 10-day forecasts were carried out, each integration starting from 1200 UTC 11 June 1979 and covering the rapid intensification of the monsoon over the Arabian Sea and Southern India. The changes to the physical processes involved modifications to the radiation and Kuo penetrative convective schemes and the introduction of a shallow convection scheme. The changes to the treatment of convection, particularly the introduction of the shallow convection scheme, are found to have a large impact on the tropical circulation and precipitation. In addition to an overall beneficial effect on the simulated large-scale flow (e.g., tradewind boundary layer structure, tradewinds, ITCZ) there is a significant improvement of the monsoon simulation. It is found that only when the radiation changes are combined with the convection changes is there a marked improvement in the monsoon region. The intensification of the low level flow over the Arabian Sea is then much improved as are the onset of the rains over Southern India and the establishment of the upper level cross equatorial return Bow.