Performance of a high resolution regional ocean–atmosphere coupled model over western North Pacific region: sensitivity to cumulus parameterizations

Abstract

In this study, we developed a high-resolution regional ocean–atmosphere coupled model, based on RegCM4 and a North Pacific Ocean model, through the OASIS3 coupler. The horizontal resolution of the atmospheric component (oceanic component) was set to 15 km (0.1°). With the motivation to customize the model over western North Pacific (WNP) region, the sensitivity to three cumulus parameterization schemes (MIT-Emanuel, Tiedtke, Kain–Fritsch) was investigated with the focus on the WNP summer monsoon of 2005. The results indicated that a simulation with the Tiedtke scheme exhibited the relatively best performance over this region in terms of sea surface temperature (SST), rainfall, and circulation. Fifty percent of the model grids had biases of SST (rainfall) within ± 0.5 °C (± 2 mm/day) over WNP. In addition, the simulation with the Tiedtke scheme reasonably captured the shape and magnitude of the monsoon trough rainfall, the northward movement of the rainband and the associated circulation changes over WNP. However, the apparent heat source (Q1) and moisture sink (Q2) in the simulation with the MIT-Emanuel (Kain–Fritsch) scheme were nearly half (double) those of the observed measurements, especially in the lower troposphere over the monsoon trough region. The too weak (too strong) diabatic heating favored a weaker (stronger) ascending motion, which led to the underestimation (overestimation) of vertical moisture advection over the monsoon trough, thus resulting in evident dry (wet) biases. Our analysis suggested that the improved performance using the Tiedtke scheme could be attributed to the improved simulated vertical profile of diabatic heating, the inclusion of the effects of large-scale forcing in the cumulus parameterization, and the reasonable simulated relationship of precipitation with low-level vertical velocity.