The Applicability And Improvement Of Convective Parameterization Scheme In Regions With Complex Terrain

Convective parameterization scheme has been the long-lasting bottleneck of global modeling and one of the edge-cutting problems in atmospheric sciences.The complicated influence of terrain on convection modifies its surrounding convection and precipitation,making the numerical modeling of convection in the area more difficult.The study focuses on the precipitation biases in complex terrain represented by the Tibetan Plateau(TP)and investigates the relationship between the biases and the convective parameterization scheme.Based on the relationships,this study attempts to modify the Tiedtke convective parameterization,making it more suited for modeling convection and precipitation in complex terrain regions.The main conclusions are as follows:(1)The evaluation of precipitation derived from global climate models in regions with complex terrain.The precipitation of 19 models participating in the CMIP6 High Resolution Model Intercomparison Project(High Res MIP)in the TP was evaluated against station-satellite merged precipitation data(CMPA),satellite precipitation(IMERG)and reanalysis data(ERA5).The results indicate that all models,including ERA5,overestimate the amount and frequency of precipitation in the TP and that the bias has been reduced with the increasing model resolution.The finer model resolution also improved the simulation of precipitation diurnal cycle.The differences in diurnal peak hours among different altitudes on the main body of TP and the nocturnal precipitation in the Sichuan Basin all have been more accurately described.The model with higher temporal resolution improves the simulation effect on the elevation dependence of hourly precipitation characteristics,such as the diurnal cycle and frequency-intensity structure.Yet,it is still challenging for global climate models to overcome the disadvantages including the wet bias,the advancement of diurnal peak hour and the frequent occurrence of weak precipitation.Further investigations of the moist parameterization schemes are needed to find new solutions.(2)The effect of topography on precipitation at various scales.The complex effect of the topography of the Tibetan Plateau on precipitation and convection results from the interaction of topography at various scales.The more accurate topography is one of the most significant elements in GRIST’s superior simulation performance of precipitation compared to High Res MIP.By evaluating the precipitation derived from High Res MIP models(spatial resolutions ranging from 0.27° to 1.5°)and(spatial resolution of 3.5 km in the study area),global climate models’ precipitation characteristics and biases with coarser and finer prescribed terrain are obtained.For improving precipitation in regions with complex terrain,especially for global climate models with coarse resolution,it is important to thoroughly consider the impact of different scales of topography on precipitation and convection.(3)The applicability of Tiedtke convective parameterization scheme in regions with complex terrain.Convective parameterization directly influences the formation of convection and its precipitation.Tiedtke scheme is widely used in numeric models and has a solid physic basis and high operational efficiency.As a grid column in GCM,single-column model can be run independently of other model components and is frequently used to test and evaluate physical parameterization schemes.The study digs into the Tiedtke scheme by using the single-column model of CAMS-CSM,which participated in CMIP6 DECK experiment and is one of the 19 High Res MIP models we evaluated.The relationship between the convective scheme and the model’s wet bias in the eastern periphery of the Tibetan Plateau was analyzed.In EPTP,the convective precipitation occurs infrequently,shows weak intensity,and thus produces little rainfall amount.As altitude increases,the frequency of deep convection increases,the depth of cloud reduces,and the proportion of convection that can produce precipitation falls;the frequency and intensity of convective precipitation also decrease.Further analysis of the temperature and humidity profiles on the grid reveals that the influence of thermal factors on convection at higher altitudes is more significant,while the column lacks moisture during that time.Thus,a large amount of dry convection will be initiated and transport water vapor vertically to moisturize the upper air and intensify the occurrence of large-scale precipitation with weak intensity.Though the influence of humidity on convection at lower altitudes is more significant,Tiedtke scheme shows low sensitivity to the environmental humidity in EPTP.It thus can’t produce precipitation with large intensity at lower altitudes either.(4)The improvement of Tiedtke convective parameterization scheme.Targeting the issues with the Tiedtke scheme in EPTP,several modifications of the scheme are presented,and the results are evaluated.Firstly,adjusting the depth threshold of convective precipitable clouds according to the altitude of the grid dynamically improves the problem that the original threshold is too strict for high-altitude areas to generate convective precipitation.Adjusting the threshold increases the convective precipitation amount and frequency,and the moisture distribution in convective and large-scale precipitation becomes more reasonable.Secondly,based on the wind direction on the grid and the altitude difference between the grid and/or sub-grid,the tendency of specific humidity flowing into the grid is decomposed orthogonally.The false over inflow of moisture owing to the inclined model layer results from large terrain fluctuation is decreased.The precipitation amount and frequency decreased with the intensity unchanged in areas with considerable altitude variations.Furthermore,lowering the convective adjustment time according to the grid’s altitude can release the instability energy in less time,which is more consistent with the characteristics of deep convection in TP.By doing this,the intensity of convective precipitation has grown without affecting the precipitation at lower altitudes.