Global variable-resolution model simulation of rainfall diurnal cycle during boreal summer

Simulating diurnal cycle of rainfall is a difficult challenge for general circulation models. We developed a global unstructured mesh model, Global-to-Regional Integrated forecast SysTem (GRIST), targeting at unified weather-to-climate forecast. The performance of the model in simulating the summer precipitation over East Asia has been evaluated. Yet the performance from a global perspective remains less understood. In this study, we focus on the simulations of precipitation diurnal cycle during boreal summer, and examine four AMIP simulation results of the GRIST model. These configurations mainly differ in the horizontal resolution. Thus, they reflect the direct changes due to varying resolutions. By refining the resolution over East Asia (VR-EA) and North America (VR-NA) respectively, we analyze the similarities and differences in model behaviors in simulating diurnal cycle of precipitation over these two refinement regions. VR-EA well reproduces the nocturnal rainfall, while VR-NA fails in certain regions respectively. The underlying responses to resolution of these two models are similar. For regions dominated by nocturnal rainfall, the refined resolution significantly increases the composited precipitation intensity at night up to the magnitude of the observation but has little impact on the composite percentage. The percentage of peak rainfall within 00-06h in the model over the Southern Great Plains remains lower than the observation as the resolution refines. Given the much lower occurrence frequency, the contribution of the intense precipitation to the climatological nocturnal rainfall amounts is small in VR-NA. Over East Asia, since the precipitation frequency is comparable to the observation, VR-EA benefits from the increased precipitation intensity due to higher resolution. No apparent artificial features are observed in the transition zone of the variable-resolution mesh. The results suggest that the variable-resolution modeling is cost-effective for simulating the diurnal cycle of climatological summer precipitation.