Monsoon rainfall simulations with the Kuo and Betts-Miller schemes

Abstract

Two numerical experiments are performed using a nested grid regional model to study the performance of the Kuo and the Betts-Miller cumulus parameterization schemes in simulating the rainfall during an active monsoon period. Results indicate that the monsoon circulation features, such as the Somali jet and monsoon depression are better simulated with the Kuo scheme. With the Kuo scheme, predicted intensity and associated rainfall of the monsoon depression are in good agreement with the observations. Uncertainty in the adjustment parameters in the Betts-Miller scheme appears to have lead to the poor prediction of rainfall. Also, the Betts-Miller scheme showed considerable sensitivity to the convergence in the lower troposphere in the initial conditions over the Arabian Sea, leading to a prediction of a spurious intense tropical cyclone. This cyclone replaced the normal heat-low over the desert region. Rainfall distribution and its maximum along the west coast of India were predicted better with the Kuo scheme. Area-averaged convective heating rates indicated that the cumulus convection is deeper and more intense with the Kuo scheme. Also, area averaged evaporation rates far exceeded the rainfall rates with the Betts-Miller scheme while with the Kuo scheme these rates are in balance after the spinup period. Forecast erros in the zonally averaged specific humidities indicate that the model atmosphere is more humid with the Betts-Miller scheme.