Introducing And Influence Testing Of The New Cloud Fraction Scheme In The GRAPES

As one of the key characteristics of weather and climate, cloud is not only thecomprehensive pattern of monentum, heat and water transportation but also plays a importantrole in energy balance between atmosphere and surface. The factors of cloud’s patterns, scales,structures and its evolution fiecely influence atmosphere radiation and temperature. Therefore,it has significance to exactly simulate cloud in weather models.Firstly, investigate the current cloud fraction schemes in international advanced synopticmodels(WRF,ECMWF) and analyse the theory of those schemes.Then EC cloud fractionscheme and WRF cloud fraction scheme are introduced into the Global/Regional Assimilationand Prediction System(GRAPES),which are comparing with the original cloud fractionscheme and the simple scheme by numerical experiment, satellite data and observational dataare used to evaluate the porperty of those schemes. Finally, based on the compare result andthe cloud feature in China, a new cloud fraction scheme are designed. And numericalexperiments are conducted to examine the simulating effect of the new scheme.The resultsshow that：(1) All schemes are able to simulate cloud coverage in a accurata manner,but thesimulation from EC scheme and simple scheme are more accurate；The total cloudinesssimulation in simple scheme is more than observation,that in GRAPES scheme and WRFscheme is less. And that in EC scheme is closer to observation；By comprehensive analysis,The3D cloud structure simulation in EC scheme is closer to observation by contrast. Then thethree dimensional cloud’s impact to radiation and surface temperature has been examined, andits results show that the total shortwave radiation amount per day and surface temperaturesimulated by EC scheme are more exact.(2) According the analysis and results, the reason of the fact that less cloud fractionsimulated by GRAPES is turned out only considering large scale cloud without subscale’s.Generally, cumulus of the scales that can not resolved by NWP models, which are frequentlydeveloped in East Asia. Thus a module of EC scheme combined with the scheme of GRAPESas a new cloud fraction scheme has been tested. The model results display that cloudcross-section, low cloud fraction, height of the cloud bottom, the total shortwave radiationamount and surface temperature are more exact simulated by new cloud fraction scheme.(3) In summery, the impact of the new cloud fraction scheme to three dimenssional cloudfraction and low atmosphere characteristic involed is positive. Thus this study has completedthe parameterization of cloud fraction and improved GRAPES prediction.