On the Influences of Urbanization on the Extreme Rainfall over Zhengzhou on 20 July 2021: A Convection-Permitting Ensemble Modeling Study

This study investigates the influences of urban land cover on the extreme rainfall event over the Zhengzhou city in central China on 20 July 2021 using the Weather Research and Forecasting model at a convection-permitting scale[1-km resolution in the innermost domain(d3)].Two ensembles of simulation(CTRL,NURB),each consisting of 11 members with a multi-layer urban canopy model and various combinations of physics schemes,were conducted using different land cover scenarios:(i)the real urban land cover,(ii)all cities in d3 being replaced with natural land cover.The results suggest that CTRL reasonably reproduces the spatiotemporal evolution of rainstorms and the 24-h rainfall accumulation over the key region,although the maximum hourly rainfall is underestimated and displaced to the west or southwest by most members.The ensemble mean 24-h rainfall accumulation over the key region of heavy rainfall is reduced by 13%,and the maximum hourly rainfall simulated by each member is reduced by 15–70 mm in CTRL relative to NURB.The reduction in the simulated rainfall by urbanization is closely associated with numerous cities/towns to the south,southeast,and east of Zhengzhou.Their heating effects jointly lead to formation of anomalous upward motions in and above the planetary boundary layer(PBL),which exaggerates the PBL drying effect due to reduced evapotranspiration and also enhances the wind stilling effect due to increased surface friction in urban areas.As a result,the lateral inflows of moisture and high-θe(equivalent potential temperature)air from south and east to Zhengzhou are reduced.

Application of Gaussian Weight to Improve Perturbation Features of Convection-Permitting Ensemble Forecast Based on Local Breeding of Growing Modes

Local breeding of growing modes(LBGM)is a method used to generate initial condition perturbation(ICP)for convection-permitting ensemble forecasts.Equal weights(EWs)are usually presumed in LBGM during the localization of ICP,without considering different contributions of the grid points within the local radius.To address this problem,Gaussian weights(GWs)are proposed in this study,which can accommodate the varied influences of the grids inside the local radius on the central grid through a Gaussian function.Specifically,two convection-permitting ensemble forecast experiments based on LBGM with GWs and EWs are compared and analyzed respectively for two squall line cases.The results showed that the use of the GWs intensified the local characteristics of the ICP and made the distribution of the ICP fields more flow-dependent.Kinetic energy spectrum of the ICP indicated that there could be more large-scale information in the ICP by using the GWs.In addition,mesoscale information also improved slightly.For forecast of nonprecipitation variables,GWs improved the relationship between the root-mean-square error and the spread and contributed to the forecasting accuracy of wind,temperature,geopotential height,and humidity.For the precipitation forecast,GWs simulated the precipitation structure successfully and provided better probability forecasting during the evolution of the two squall line processes than the EWs.

Formation and Transport of a Saharan Dust Plume in Early Summer

This research studies the capability of the Weather Research and Forecasting model coupled with the Chemistry/Aerosol module(WRF-Chem)with and without parametrization to reproduce a dust storm,which was held on 27th June 2018 over Sahara region.The authors use satellite observations and ground-based measurements to evaluate the WRF-Chem simulations.The sensitivities of WRF-Chem Model are tested on the replication of haboob features with a tuned GOCART aerosol module.Comparisons of simulations with satellite and ground-based observations show that WRF-Chem is able to reproduce the Aerosol Optical Depth(AOD)distribution and associated changes of haboob in the meteorological fields with temperature drops of about 9℃and wind gust 20 m·s–1.The WRF-Chem Convec-tion-permitting model(CPM)shows strong 10-meter winds induced a large dust emission along the leading edge of a convective cold pool(LECCP).The CPM indicates heavy dust transported over the West African coast(16°W-10°W;6°N-21°N)which has a potential for long-distance travel on 27th June between 1100 UTC and 1500 UTC.The daily precipitation is improved in the CPM with a spatial distribution similar to the GPM-IMERG precipitation and maxi-mum rainfall located at the right place.As well as raising a large amount of dust,the haboob caused considerable dam-age along its route.

Impacts of Topographic Complexity on Modeling Moisture Transport and Precipitation over the Tibetan Plateau in Summer

The non-hydrostatic global variable resolution model(MPAS-atmosphere)is used to conduct the simulations for the South Asian Summer monsoon season(June,July,and August)in 2015 with a refinement over the Tibetan Plateau(TP)at the convection-permitting scale(4 km).Two experiments with different topographical datasets,complex(4-km)and smooth(60-km)topography,are designed to investigate the impacts of topographical complexity on moisture transport and precipitation.Compared with the observations and reanalysis data,the simulation can successfully capture the general features of key meteorological fields over the TP despite slightly underestimating the inflow through the southern TP.The results indicate that the complex topography can decrease the inward and outward moisture transport,ultimately increasing the total net moisture transport into the TP by~11%.The impacts of complex topography on precipitation are negligible over the TP,but the spatial distributions of precipitation over the Himalayas are significantly modulated.With the inclusion of complex topography,the sharper southern slopes of the Himalayas shift the lifted airflow and hence precipitation northward compared to the smooth topography.In addition,more small-scale valleys are resolved by the inclusion of complex topography,which serve as channels for moisture transport across the Himalayas,further favoring a northward shift of precipitation.Overall,the difference between the two experiments with different topography datasets is mainly attributed to their differing representation of the degree of the southern slopes of the Himalayas and the extent to which the valleys are resolved.