Numerical Simulation of Orographic-Convective Rainfall with Kuo and Betts-Miller Cumulus Parameterization Schemes

Abstract

Two different cumulus parameterization schemes, one developed by Kuo and the other by Betts-Miller, are used to simulate the orographic-convective rainfall associated with the Western Ghats for two days during which monsoon rainfall was moderate to heavy. A ten-layer primitive equation limited area nested grid model is used to perform numerical simulations. It is found that predicted rainfall near the Western Ghats with the Kuo scheme agrees well with the observations. With the Betts-Miller scheme, model failed to predict rainfall over this region.
To find out uncertainties in the adjustment parameters used in the Betts-Miller scheme, five sensitivity experiments are performed. Different values are assigned to the two adjustment parameters, namely the relaxation time scale and the saturation pressure departure, in each of the sensitivity experiments. Results from these sensitivity studies indicate that specification of relaxation time scale depends on the model horizontal resolution. Relaxation time scale needs to be smaller as the model horizontal resolution increases. Also, rainfall predictions are less sensitive to different values of relaxation time scales than those for the saturation pressure departure. Variations in the prescribed thermodynamic reference profiles caused by small prescribed changes in the values of saturation pressure departure led to improvements in the rainfall predictions. It was also found that there exists a lower limit on the values of relaxation time scales and saturation pressure departures for the monsoon region beyond which predicted rainfall rates do not show further improvement.