An Evaluation of the Simulation of Monthly to Seasonal Summer Monsoon Rainfall over India with a Coupled Ocean Atmosphere General Circulation Model (GloSea)

Abstract

The performance of the UK Met Office’s coupled ocean-atmosphere General Circulation Model (GCM) is evaluated in simulation of summer monsoon rainfall over Indian monsoon region. The UK Met Office’s Global Seasonal (GloSea) forecasting model is initialized at 0000 UTC of 1st May and integrated for a period of 6 month with 15 ensemble members to generate the model forecast. These experiments have been conducted in similar approach from 1987 to 2002 (16 years) to have monthly as well as seasonal forecast of individual year. The model simulated rainfall is compared with the verification analysis (Xie-Arkin) during the monsoon season from June to September (JJAS). The monthly forecast climatology from June to September separately and the seasonal forecast climatology (June to September; JJAS) of rainfall are well simulated by the model with two maxima viz., one over the west coast of India and other over the head Bay of Bengal region. However, the rainfall magnitude over the west-coast of India is less in the model simulation for monthly as well as in seasonal simulation. The model has shown good skill in simulation of seasonal (JJAS) mean rainfall over the Indian monsoon region. However, a little overestimation in rainfall is noted (approximately 4%) when considered the Indian monsoon region covering the land region and surrounding oceanic regions. The pattern correlation during JJAS shows highly significant correlation coefficients (CCs) over the global tropics (0.91) and Indian monsoon region (0.82). Similarly the Root Mean Square Error (RMSE) during JJAS is found to be less (1.01) over the global tropics than the Indian monsoon region (1.68). The interannual variability of forecast ensemble mean rainfall over the Indian monsoon region shows similar behaviour with that of verification rainfall variability with Correlation Coefficient of about 0.43 during the 16 years period from 1987 to 2002. The Anomaly Correlation Coefficients (ACCs) between verification and simulated rainfall during 1987–2002 over the Indian monsoon region is quite significant (more than 0.6 during some years). Overall, it can be stated that the performance of the UK Met Office’s seasonal mean simulation is reasonably good.