Sensitivity of a regional ocean-atmosphere coupled model to convection parameterization over western North Pacific

With the motivation to improve the simulation of western North Pacific (WNP) summer monsoon, the regional climate model RegCM3 is coupled with the Princeton ocean model (POM2000) through the coupler OASIS3.0 (Ocean Atmosphere Sea Ice Soil 3.0). The performance of the regional ocean-atmosphere coupled model (hereafter ROAM) is assessed by doing case simulation of the 1998 summer monsoon. The cold bias of simulated sea surface temperature (SST) is evident as previous ROAM simulations over the Asian-Australian summer monsoon region. Five sets of sensitivity experiments with convection suppression criterion based on the averaged relative humidity from cloud base to cloud top are designed to improve the performance of ROAM. The results show that the column-averaged cloud fraction is reduced in convection suppression experiments. A reduction of column-averaged cloud cover, which is dominated by the decrease of convective cloud cover, increases the solar shortwave radiation reaching in sea surface, then warms the SST. A reduction of convective rainfall is followed by an increase of large-scale rainfall which results from increasing cloud water. When the critical value is set to 0.70, the rainfall is partly improved in terms of the spatial distribution and root-mean-square error. The percentage of convective rainfall over WNP is also improved. The authors show evidence that the SST cold biases, which are evident in many regional ocean-atmosphere coupled models in the Asian-Australian summer monsoon region, may partly stem from the overestimation of convection frequency by the atmospheric model.