The performance of land surface and cumulus convection scheme in the simulation of Indian Summer Monsoon using RegCM4

Abstract:

Indian Sumer Monsoon (ISM) is the most dominant tropical circulation process in the Atmospheric General Circulation System. It contributes about 70% of the total annual precipitation during June-September over India. Summer rainfall has substantial effect to the agriculture and economy of the country. ISM is dominated by convection and land surface processes also play important role through transfer of energy and moisture. These processes must be well represented in the numerical models for better simulation of ISM. Therefore it is important to investigate the performance of various land surface and cumulus schemes in simulating ISM. In this study, performance of the convection and land surface schemes in simulating ISM are investigated using RegCM 4. The performance of two land surface schemes [BATS, CLM 3.5] and five convection schemes [MIT, Kuo, Grell, Grell over Land and MIT over Ocean (GL_MO), Grell over Ocean and MIT over Land (GO_ML)] are tested. The model at 30 km resolution covering (30˚E-120˚E, 15˚S-45˚N) is integrated for the period of 1^st May-30^thSeptember during 2007, 2008 and 2009. The initial and boundary conditions are derived from NNRP1 data at 2.5˚×2.5˚ resolution. The Sea Surface Temperature is taken from NOAA Optimum Interpolation weekly mean data at 1˚×1˚ resolution.

Some important features of ISM simulated by the model; viz. Somaly jet, Tropical easterly jet (TEJ), Heat low, Tibetan high etc. are analyzed with NCEP reanalysis. Heat low is well produced by the model with MIT, GO_ML and GL_MO schemes. The Somaly jet is usually weak but better simulated with MIT scheme. Although the model is able to reproduce the TEJ and Subtropical westerly jet, the location and strength of the jets are varying scheme-wise and is better simulated by MIT scheme. The model is also able to simulate the stronger easterly jet in 2007 compared to 2009 with MIT scheme. The rainfall over Central and North West India is significantly under-predicted by the model with all these schemes. The monthly and seasonal precipitation is better simulated by the model with the combination of MIT convection scheme and CLM compared to the other combinations although it over-predicts rainfall over heavy precipitation zones. Considering overall performances, RegCM4 shows better skill in simulating ISM with MIT convection scheme using CLM.