Numerical Simulations And Mechanisms Of Severe Convective Wind Over China

Severe convective wind(SCW)is one of the severe convective systems in China.SCWs are mainly caused by strong downbursts relatedly with severe mesoscale convective storms,tornadoes,quasi-continuous straight-line winds produced by convective systems like bow echos and derechos,and the typhoon winds,casuing ships capsized,housed collapsed.SCW events also characterized by suddenness,locality,and rapid development.Owning to the minor time and space scale of SCW events,numerical weather model simulations/predictions are very chanllenging.Most of recent studies focused on SCWs are based on observations and idealized simulation and quantitative study based on real data model simulation is rather limited.The understanding of the dynamics and physical mechanism of SCWs is still much uncertain.The prediction of SCWs using operational numerical weather prediction models remains an international research hotspot.Two different types of SCWs(i.e.,downbursts and tornadoes),are simulated using real data at enough high model resolution and studied carefully in this dissertation.Based on dignose of vertical momentum equation,driving mechanism of downburst happened under special hot and humid environment in China is investigated quantitatively.Using "surrogates" such as updraft helicty(UH),the prediction skills on supercells and tornadoes of convection allowing ensemble model are also evaluated.A set of simulations of Funing EF4 tornado is conducted,with different numbers of two-way-interactive nesting levels(reaching a 49 m LES resolution when 5 levels are used).Resolution dependency of simulated tornadoes and associated wind hazards is studies and the choices of resolution for tornado forecasting balancing the resolution requirements to explicitly simulate tornadoes and computational costs as well as forecast timeliness are suggested.For downbursts,A downbursts-producing bow-echo event casing the capsizing of"Eastern Star" cruise ship over the Central China on 1 June 2015 is analyzed based on radar observations and a successful real-data ARPS intermittent assimilation and simulation at 0.5-km grid spacing.Strong surface wind over 34 m s-1 is also detected from the model simulation with the similar structure and distribution.Based on a back-tracking of the air parcels in the strong wind area,surface strong winds are believed to come from the shifted downdrafts from above 3km AGL in the bow echo system and change rapidly in time and space.The downdraft flow in the from above 3km rushes toward to the ground,slows down in the vertical direction as the flow approaches the ground,shifts horizontally to the front,descents again near the gust front area and generates surface wind hazard around Yangtze River channel area.Through budgets on vertical momentum budget,the hydrometeor loading effect of effective buoyancy is the most significant process to drive the downdraft flow.In addition,the blocking high-pressure system over the gust front area is also important for pushing part of the shifted downdraft accelerating down again and producing strong wind hazard near the ground.As for tornadoes,a severe EF4 tornado causing housed collapsed and 97 fatalities is simulated using WRF-ARW based convection allowing ensemble model.Ensemble members' results show poor skill on tornadic parental supercell storm and have very limited ability predicting supercell directly.By using suitable "surrogates" such as updraft helicty(UH),the ensembles get better prediction skills on supercells by detecting the existence and intensity of middle and low level mesocyclone.But skills on tornadoes are still very limited and RVORT1 is the most skillful indicator.A set of simulations of Funing EF4 tornado is conducted,with different numbers of two-way-interactive nesting levels,with up to five levels of nesting and down to 4000,1333,444,148,and 49 m grid spacing.All five experiments reproduce the general characteristics of observed supercell storm but tornado-like vortex does not develop until a 444 m grid spacing is used.EF1,EF2 and EF3 intensity is reached with 444,148 and 49 m grid spacings,respectively,based on diagnosed 10 m wind.The tornado vortex simulated on the 444 m grid trends to remain a one-cell structure with maximum near-surface vertical vorticity ?max found at the center of the tornado vortex while a ring of high-vorticity develops at the mature stage of tornado vortex on the 148 m grid,leading to a two-cell structure.With an even higher resolution on the 49 m grid,microscale vortices along outflow boundary that merge into and organize the main tornado vortex and multiple 'suction vortices' that develop along the high-vorticity ring within the main tornado vortex at the later stage are explicitly resolved,leading to multi-vortex tornadoes.The sub-vortices within the main tornado vortex create localized regions of most intense winds due to super-positioning of the sub-vortex circulation with the main vortex circulation and the system translation speed;the sub-vortices also create more irregular strong surface wind patterns,localized 'damage cores',and broaden the tornadic wind swath,results consistent with evidences seen in the damage survey of the Funing tornado.Balancing the trade-off between being able to explicitly forecast the tornadic vortices and damage intensity,and computational expense and forecast timeliness requirements,we recommend horizontal grid spacings of 400-500 m for realtime warn-on-forecast tornado forecast applications in order to capture at least tornado-like vortex,but grid spacings of?50 m for research studies on tornado dynamics,prediction and predictability and for realtime explicit tornado prediction when computer speed eventually catches up.