Influences Of Sub-grid Shallow Convective And Boundary-layer Process On Cloud Forecast In GRAPES Global Model

The study adopts a new cloud scheme, which considering the impact of the detrainment progress of shallow convection and boundary-layer clouds on the prediction of cloud, in Global/Regional Assimilation and Prediction System(GRAPES) global model for the purpose of improving the precision of low cloud prediction. This new scheme can solve deficiency of low cloud fraction and hydrometeors water contents forecasting in middle and low latitudes. The influence of detrainment progress of shallow convection and boundary-layer clouds on cloud fraction and hydrometeors water(ice) contents are parameterized to the forecast equation of cloud scheme. Case and batch tests are conducted with this modified scheme. Based on comparison between experimental results and observational data, the improvement on low cloud prediction induced by new cloud scheme has been clearly showed.The results show that boundary-layer term mainly affects the thin layer of clouds near top of boundary layer and the influence thickness is less than 200 hPa. The magnitude of the impact of shallow convection detrainment process on cloud fraction and hydrometeors water contents is equivalent to the one caused by boundary-layer term, but its effect on cloud thickness is more widely, which can extend from surface to 700 hPa. The modified scheme considering both boundary-layer term and shallow convection detrainment process mainly make increase of low cloud fraction and liquid hydrometeor contents below 700 hPa, especially for tropics. By use of this scheme, the prediction of low cloud over the ocean at middle and low latitudes has been improved and more close to the observations, in particular for the four main areas of low cloud that is known as eastern Pacific Ocean to the west of California and Peru, Atlantic Ocean to the west of central Africa, Indian Ocean to the west of Australia. These improvements also indirectly bring about improvements of cloud radiative forcing. The conclusions above are further confirmed by batch tests, general progress of prediction of cloud fraction and hydrometeors water contents highlights the important role that has been played by the detrainment process of shallow convection and boundary-layer clouds term which are the significant physical process of global model cloud scheme.