Importance of cumulus parameterization for precipitation simulation over East Asia in June

A regional climate model, NIES/CCSR RAMS, was used with the ECMWF objective analysis data as boundary conditions to reproduce the precipitation pattern over East Asia in June, including that accompanying the Baiu front. In the control experiment, the simulated precipitation pattern was unrealistic, mainly in two respects: precipitation over the Baiu front was too weak, and strong precipitation was found over the North Pacific subtropical high at around 20°N. According to the ECMWF data, strong potential instability is maintained over the subtropical high, under which condition, the cumulus parameterization used in the model predicts strong precipitation to stabilize the atmosphere. Because the lower free atmosphere is very dry over the subtropical high, it is conjectured that the development of deep cumulus convection is suppressed by this dry air in the real atmosphere. When the cumulus parameterization was modified to include this assumption, the intensity of precipitation over the Baiu front as well as the unrealistic precipitation pattern over the subtropical high was clearly ameliorated. The intensification of precipitation over the Baiu front is considered to be due to the increased water vapor transport to the Baiu front, and the decreased stability with respect to the moist convection around the Baiu front. In semi-prognostic experiments with CCSR/NIES AGCM, this modification was found to be also effective in many other parts of the globe. Because problems in precipitation distribution similar to that discussed in the present study are found in many other climate models, the modification of cumulus parameterization suggested here could also be effective in those models.