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The sensitivity to model resolution and domain size of the regional climate model RegCM4 is examined in simulating a deficit (1987), excess (1988), and normal (1989) Indian summer monsoon (ISM) rainfall. The initial and boundary conditions are prescribed by the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim reanalysis at 1.5 x 1.5 degrees (EIN15). The model simulated precipitation is compared with India Meteorological Department (IMD) gridded precipitation data (0.25 degrees x 0.25 degrees) and other parameters are compared with EIN15. In the present study, eight different horizontal resolutions such as 60, 55, 50, 45, 40, 36, 27 and 18 km are used whereas four different domain sizes (D01, D02, D03, and D04; largest to smallest) centered at the Indian landmass are considered to investigate the model performance.

Results illustrate that the RegCM4 has the capability to depict the important semi-permanent features of the ISM, however, skills vary with the resolutions and domain sizes. Experiments with different resolutions reveal that the lower (850 hPa) and upper (200 hPa) tropospheric circulations are stronger over the Arabian Sea and Bay of Bengal in the 36 and 40 km grid-spacing simulations and closer to EIN15, however, it is weaker in the high-resolution simulations at 27 and 18 km grid-spacing. The biases in vertically integrated moisture transport (VIMT) are positive in the model simulations with resolutions from 36 to 60 km, but, it is negative with 27 and 18 km grid resolution. The simulated precipitation intensity increases as one moves from coarse to higher resolution till grid spacing 36 km. The model efficiency reduces to simulate higher precipitation intensity over the maximum precipitation zones of India during summer monsoon season when grid spacing 27 km and 18 km are used. The overall performance is better with the use of 36-40 km grid spacing than other resolutions in simulating ISM and associated rainfall. The sensitivity of domain sizes is examined by confining the model resolution at 40 km. The model simulated meteorological parameters, as well as derived parameters (such as pattern and intensity of precipitation, geopotential height, relative humidity, air temperature, wind, and CAPE), are represented better in D02 than other domains. The present study proposes a Comprehensive Rating Matrices (CRM) to evaluate the model performance considering all experiments (horizontal resolution and domain size) and different homogeneous regions, including errors obtained using different statistical methods. The CRM and statistical based skill score (SS) suggest that the performance of RegCM4 is the best at 40 km horizontal resolution followed by 36 km and 45 km grid-spacing and the D02 domain is suitable for ISM simulation.