An Improvement Of Shallow Convection Parameterization Scheme To GRAPES-Meso

Shallow cumuli,which play an important role in the earth’s radiation budget,are occurring over large areas of the world’s oceans and continents generally.By venting air from the surface mixed layer towards the free troposphere,shallow convection strongly influences boundary layer temperature,humidity,cloud cover and winds.Meanwhile,shallow Convection is an early developing period of deep convection in most circumstances.Therefore,it comes to be a significant issue to improve the description of shallow convection processes in numerical models.However,researches on processes and parameterizations of shallow convection are not so thorough and popular than deep convection.This research is focused on the improvement of shallow convection scheme in the numerical weather forecast model. An improvement on the trigger function of shallow convection scheme to GRAPES-Meso has been designed and tested referring the cumulus potential treatment suggested by Berg eta al. (2005、2012), in which the trigger function is related to the distribution of temperature and humidity in the convective boundary layer determined by the Joint Probability Density Functions (JPDF),replacing the single temperature and humidity trigger function used in KF eta scheme. In the thesis, a precipitation case in Jiangnan is reasonable simulated by the improvement scheme,thus a special zone is chose for further analysis on trigger frequencies of shallow convection,properties of shallow cumuli,feedback of temperature, moisture of shallow convection,precipitation and T2m. The verification to forecast results of 2-month experiment indicates main conclusions as follows:(1) The improvement scheme uses a number of parcels relating to JPDF by providing an array of perturbations regarding temperature and humidity.Increasing the number of JPDF bins while holding the bin size constant would offer more chances to trigger convection,but also increase the length of time needed to complete the simulations.The higher resolution JPDF would also offer more chances to trigger convection,but lead to greater computational expense.(2) The improvement scheme,which enhancing the trigger frequencies of shallow convection to GRAPES-Meso, could trigger an earlier and sustainable shallow convection.Accordingly, the improvement scheme could predict shallow cumuli earlier,which maintaining a longer time,and heights of cloud bottoms are simulated more accurately.(3) Feedback to the environment of the improvement scheme are reasonable. With more shallow convection could be triggered and occurred in an earlier simulation time than original one, a stronger feedback of temperature, moisture and water vapor condensate of cloud to model grid scale are correspondingly identified in lower layers from several hundred meters to about 2km above the ground.(4) With the improvement on shallow convection, there is a slight positive impact to the fraction of subgrid/grid scale precipitation to the total precipitation of GRAPES-Meso.On the one hand, more shallow convection could be triggered,more unstable energy are consumed,deep convection would be inhibited with less moist consuming for grid scale precipitation. On the other hand, feedback of water vapor condensate of cloud to grid scale are increasing.(5) The improvement on the trigger function of shallow convection scheme could result in the positive influence on 24h forecasts of accumulative precipitation and surface temperature in certain degree.Namely, there are slight improvement on the TS of light rain and precipitation characteristics.The positive impact of the improvement scheme appears relative clearly in the regions which shallow cumuli occurring,while it shows a weak positive influence in the statistic averaging,which might turns out to be a more value work for refined weather forecast.