Simulation of 2006 monsoon using T170L42 AGCM: sensitivity to convective parameterization schemes

Abstract

In this study we used the National Center for Environmental Prediction (NCEP) T170L42 Global Forecast System model for simulating the monsoon of 2006 using two different cumulus parameterization schemes viz. the Simple Arakawa-Schubert (SAS) scheme and the Relaxed Arakawa-Schubert (RAS) scheme. The NCEP Coupled Forecast System (CFS) monthly forecast sea surface temperatures (SST) were used for simulating the monsoon season. It is seen that both the schemes forecast the mean seasonal rainfall close to the observed. However, there were differences in the spatial and temporal distribution of the rainfall. The spatial distribution of the precipitation simulated by the RAS scheme was comparable to the observed. The SAS scheme simulated realistic distribution of the rainfall in the months of June and July. In the months of August and September, the SAS scheme could not simulate the convective regions over the south and central Bay of Bengal, Arabian Sea and the Indian landmass. The SAS scheme simulated withdrawal-like symptoms of the monsoon in the month of August itself. However, the same SAS scheme simulated more realistic rainfall distribution over the Bay of Bengal when the model was run with climatological SST and also when the model was run at a resolution of T62 and with CFS-forecast SST. The onset of the monsoon over the Kerala coast of India simulated with both the schemes was within one pentad of the observed date of onset.