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The impact of modified fractional conversion parameter (from cloud condensate to precipitation) in the revised simplified Arakawa-Schubert (RSAS) convection scheme in Climate Forecast System version 2 on the simulation of Indian summer monsoon (ISM) is examined. While the default fractional conversion parameter is constant in RSAS, the modified parameter has the form of an exponential function of temperature above the freezing level, whereas below the freezing, level it is kept constant. The model simulation indicates RSAS with modified conversion parameter (RSAS_mod) shows a better fidelity in capturing the mean monsoon features over the ISM region. The spatial distribution of precipitation shows notable improvement over the ISM region. Most of the global general circulation models has a tendency to grossly overestimate (underestimate) the convective (large-scale) rainfall over the ISM region, which has somewhat improved in RSAS_mod simulation. It is suggested that reduced rate of conversion of cloud condensate to convective precipitation above the freezing level leads to suppression of convective precipitation, which further increases the detrained moisture from the upper-level, resulting enhancement in large-scale precipitation. Further, improvement has been noted in outgoing longwave radiation, wind circulation, total cloud fraction, and dynamical and thermodynamical processes in RSAS_mod simulation. The modified conversion parameter helps in improving the feedback between moisture and convective processes through better lower tropospheric moistening. In addition to mean summer monsoon, the RSAS_mod indicates its potential in predicting an extreme rainfall event over Mumbai in high-resolution global forecast system at T1534 horizontal resolution. However, its fidelity needs to be further tested for more number of heavy rainfall events.

Plain Language Summary It has been reported by recent literatures that Climate Forecast System version 2 has prominent bias in the rainfall simulation over the global land and oceanic region. It is also noted that major contribution to the rainfall is from the convective rainfall of the model and relatively lesser fraction is contributed by the large-scale rainfall. Keeping this in view, the impact of modified fractional conversion parameter (from cloud condensate to precipitation) in the revised simplified Arakawa-Schubert (RSAS) convection scheme in Climate Forecast System version 2 on the simulation of Indian summer monsoon is examined. It is found that reduced conversion of cloud condensate to convective precipitation above the freezing level leads to suppression of convective precipitation, which further increases the detrained moisture from the upper level, resulting enhancement in large-scale precipitation. As such this has improved the outgoing longwave radiation, wind circulation, total cloud fraction, and dynamical and thermodynamical processes in RSAS_mod simulation and particularly the large-scale rainfall distribution and propagation during Indian summer monsoon.