The Effects Of Convection Representation Across The "Gray Zone" In Multigrid Nesting Simulations Of Extreme Precipitation Over Shanghai

In this paper,WRF model is used to simulate two extreme precipitation events during warm season and one Meiyu front precipitation event in Shanghai with different grid nesting and cumulus treatment.Based on the hourly precipitation data of automatic observation stations in the eastern part of China,the influence of cumulus treatment on extreme precipitation simulation at different grid scales(27,15,9,5,3,1 km)is systematically evaluated.The cumulus treatment used in this paper includes traditional Kain Fristch(KF),scale-aware Grell Freitas(GF)convection parameterization schemes,and explicit convection(EC)in grey zone(1-10km).At the same time,the traditional triple nesting(3:1 or 5:1)is compared with a new large ratio double nesting(9:1 or 15:1)configuration,and the impact of cumulus convective expressions on high-resolution grid precipitation simulation through different nesting methods is also studied.Based on various statistical methods(Thread score,Bias score,Correlation coefficient,Root mean square error)to evaluate the daily extreme precipitation and its diurnal cycle variations.The simulation results show that the double nesting with large ratio(9:1 or 15:1)is more advantageous for extreme precipitation simulation than the traditional triple nesting(3:1 or 5:1).Using double nesting with large nesting ratio can not only save computing resources,but also effectively avoid a series of problems caused by the uncertainty of cumulus convection expression in the process of crossing the "gray zone".In addition,the in-depth physical mechanism analysis of simulation results is also carried out in the aspects from the contribution rate of convective precipitation,large-scale forcing fields,thermal and dynamic structure of convective development.The conclusions are as follows:(1)In the D1 grids(> 10 km),at 27-km grid,using KF systematically underestimated the range and intensity of extreme precipitation,while using GF systematically overestimated the range of extreme precipitation and resulted a fake precipitation peak in the afternoon;At 15-km grid,the systematic deviation from KF and GF has both been improved.As the resolution increased to 9 km,the grey zone grid,using KF has more advantages than GF and EC in forecasting the range,intensity and diurnal cycle variations of extreme precipitation.Using GF scheme performed better on simulating the time of primary hourly precipitation peak,while EC has poor prediction on intensity of hourly precipitation rate.Therefore,in "grey zone" 9 km grid,using convective parameterization is still needed for the extreme precipitation prediction,KF and GF schemes both have their own advantages.(2)In the D2 grids(5 km & 3 km),using KF or GF has advantages over EC for the prediction of extreme precipitation range,intensity and its diurnal cycle variations on the 5-km grid;However,on the 3 km grid,the prediction results are related to the selection of convection parameterization schemes.It is found that KF scheme has advantages over EC scheme in both intensity and hourly variations of extreme precipitation forecasting.Using GF is better than EC in diurnal cycle forecasting for extreme rainfall,but worse than EC in forecasting the intensity of extreme precipitation.It shows that the expression of cumulus convection on 3 km grid in "grey zone" is still uncertain.(3)In the 1 km grid(D3),it is more advantageous to use the double nesting with large ratio(15:1 or 9:1)than the traditional triple nesting for extreme precipitation simulations.The results of extreme precipitation prediction in the 1-km grid are not significantly improved,or even reduced in most cases,when the intermediate nesting(5km and 3km)is added between the parent grid(15km or 9km)and the inner grid(1km).Using 9KF-1 is more advantageous to simulate the range,intensity and hourly variations of extreme precipitation than using 9GF-1 or 9EC-1.The simulation advantage of KF scheme may be related to the closure hypothesis based on the release of CAPE,and its convective triggering mechanism considering the vertical velocity and the effect of large-scale convergence of the cloud bottom due to the cumulus entrainment.The variations of water vapor convergence and vertical upward motion simulated by KF scheme in the parent grid(15 km or 9 km)are consistent with the hourly variations of extreme precipitation,in which the contribution of convective precipitation is larger.In KF scheme,the hourly precipitation intensity in 1-km grid is affected by the regulation and organization of mesoscale convection activity in D1 grids which are related to heavy precipitation.However,in scale-aware GF scheme,the convective precipitation of 15 km to 9 km in D1 grid is significantly reduced,and the regulation for the precipitation in 1-km grid is weak,which resulted in its primary rainfall peak was underestimated.In addition,in this paper we also try to improve the simulation results from the perspective of the boundary layer scheme.It is found that the local MYNN boundary layer scheme is easy to form an unstable top of boundary layer due to the weak vertical mixing of heat and momentum in the boundary layer,which leads to a strong vertical upward movement from the top of the boundary layer to the middle and upper levels of the free atmosphere and produced stronger precipitation.The local MYNN scheme has more advantages in capturing extreme precipitation than the non-local YSU scheme.Therefore,based on the large ration double nesting grid,we proposed that using MYNN in the 9KF-1 or 15KF-1 configuration produced most realistic intensity and hourly variations of extreme rainfall.