Using Radar Data to Calibrate a Stochastic Parametrization of Organized Convection

Stochastic parameterizations are increasingly becoming skillful in representing unresolved atmospheric processes for global climate models. The stochastic multicloud model, used to simulate the life cycle of the three most common cloud types (cumulus congestus, deep convective, and stratiform) in tropical convective systems, is one example. In this model, these clouds interact with each other and with their environment according to intuitive-probabilistic rules determined by a set of predictors, depending on the large-scale atmospheric state and a set of transition time scale parameters. Here we use a Bayesian statistical method to infer these parameters from radar data. The Bayesian approach is applied to precipitation data collected by the Shared Mobile Atmospheric Research and Teaching Radar truck-mounted C-band radar located in the Maldives archipelago, while the corresponding large-scale predictors were derived from meteorological soundings taken during the Dynamics of the Madden-Julian Oscillation field campaign. The transition time scales were inferred from three different phases of the Madden-Julian Oscillation (suppressed, initiation, and active) and compared with previous studies. The performance of the stochastic multicloud model is also assessed, in a stand-alone mode, where the cloud model is forced directly by the observed predictors without feedback into the environmental variables. The results showed a wide spread in the inferred parameter values due in part to the lack of the desired sensitivity of the model to the predictors and the shortness of the training periods that did not include both active and suppressed convection phases simultaneously. Nonetheless, the resemblance of the stand-alone simulated cloud fraction time series to the radar data is encouraging.