Impact Of Ensemble Cumulus Parameterization Closures On The Simulation Of Summer Diurnal Precipitation Over Southern China

Summer precipitation in the southern contiguous China has distinct diurnal variations with considerable regional features.Over the eastern periphery of the Tibet Plateau summer rainfall peaks around the midnight,and there is an eastward-delayed diurnal phase of precipitation down the Yangtze River Valley,which is characterized by double rainfall peaks in the middle and lower reaches with their primary peaks occurring in the early morning and late afternoon.Over the southeast coastal region,summer precipitation has a solid late-afternoon peak.Previous studies have pointed out that the simulation of the diurnal cycle precipitation has significant regional sensitivity to the selection of convective parameterization schemes,with the simulation deviations mainly related to the defects and regional dependence of the convective closures.However,these studies are mostly based on different models and physical processes,requiring a unified model framework to quantitatively and systematically assess the impact of convective closure.Therefore,this paper used the Ensemble cumulus parameterization(ECP)in the regional climate model CWRF to compare the impact of different closure assumptions on the simulation of diurnal variation of summer precipitation over southern China.The main goal was to understand the sources of regional sensitivity of different convective parameterization schemes,and attempt to improve the simulation of summer rainfall diurnal variation over Southern China.The specific conclusions are as follows:(1)Based on the 1998-2015 long-term integral model results of the regional climate model CWRF with a grid spacing of 30 kilometers,a systematic evaluation of 8 different convective parameterization schemes for the four key areas(A: the upper reaches of the Yangtze River Basin;B: the middle reaches of the Yangtze River Basin;C: the lower plains of the Yangtze River Basin;D: South China)was studied from the perspective of precipitation types,events and regional forcing mechanisms.The ECP control scheme and the NSAS scheme better produced the afternoon precipitation peaks in the lower reaches of the Yangtze River and South China.The Donner scheme and the Tiedtke scheme better produced the morning precipitation peaks in the upper and middle reaches of the Yangtze River.Only the ECP scheme captured the double peak structure in the morning and afternoon in the lower reach of the Yangtze River..(2)The ECP scheme incorporates five major types of cumulus closure assumptions,including the quasi-equilibrium closure(AS),the vertical velocity closure(W),the moisture convergence closure(MC),the CAPE-removal closure(KF),and the large-scale instability tendency closure(TD).The comparison of different closure assumptions showed that the W and MC closure coupled by the ECP?CTL scheme captured the double-peak structure of convective precipitation peaks in the lower reaches of the Yangtze River,and it captured the morning peak of stratiform cloud precipitation as well,which explained the characteristics of the ECP?CTL scheme;the AS closure used by the Donner scheme and the TD closure successfully restrained the formation of afternoon convective precipitation in the eastern part of the Tibet Plateau,and generated more stratiform cloud precipitation,thus better simulating the peak precipitation at night.(3)FFSQP(FORTRAN Feasible Sequential Quadratic Programming)was used to construct an ensemble model based on 16 algorithms in the five categories of closures.The optimization results showed that the ENS7 scheme significantly improved the precipitation overestimation in the upper reaches of the Yangtze River,better simulated the peak of night precipitation,and relatively delayed the early afternoon precipitation peak in the lower reaches of the Yangtze River,but underestimated the average precipitation in South China,and still can not simulate the morning precipitation peak in the middle reaches of the Yangtze River.At the same time,preliminary simulation tests on a higher resolution of 15 km grid scale found that although the overestimation of the ECP control scheme coupled with W and MC closures in the four regions has increased,it still captured the double-peak structure of the lower Yangtze River.While in the AS closure,the overestimation of precipitation in the upper reaches of the Yangtze River was improved.The TD closure successfully captured the peak of morning precipitation in the middle reaches of the Yangtze River,though the underestimated of the average precipitation in the lower reaches of the Yangtze River and South China was increased.Therefore,in higher resolution simulations in future,reducing the weights of the W and MC assumptions in the ECP scheme,and increasing the AS and TD assumptions can not only offset the overestimation of average precipitation,but also further improve the diurnal phase of precipitation in the upper and middle reaches of the Yangtze River.