Influences of sub-grid convective processes on cloud forecast in the GRAPES global model

The purpose of the research is to improve the prediction of the hydrometeors water content, cloud fraction and grid precipitation by the Global/Regional Assimilation and Prediction System (GRAPES) global model at 50 km spatial resolution for the low latitudes and equatorial regions. This study adopts a new scheme that considers the impact of this sub-grid convective process by adding source-sink terms in the prediction equations of cloud water/ice and cloud fraction. Distinct forecast results from the previous cloud scheme are obtained. The forecasts of cloud and precipitation in the low latitudes and equatorial regions have been significantly improved after consideration of the effect of the sub-grid convective process. Moreover, compared with the observational data from the Clouds and the Earth's Radiant Energy System (CERES) and Tropical Rainfall Measuring Mission (TRMM), it is confirmed that the forecasting capability of the GRAPES global model has been promoted especially for the cloud and precipitation in the tropics, with the proportion of grid-scale precipitation enhances from 5% to 25%. Further analysis reveals that the influence of the sub-scale convective process on gird-scale cloud and precipitation depends on the distribution and intensity of updraft mass flux, which are larger from the new scheme in the tropics, leading to notable increases of cloud and precipitation there. In addition, the maximum updraft mass flux resides in the 450-650 hPa layer, resulting in more pronounced improvement in forecast of middle-level clouds than high clouds and low-level clouds, and causes cloud top to rise with cloud bottom dropping downward.