论文部分内容阅读
Downslope windstorm is a disastrous weather phenomenon,it may occur in stable atmospheric condition when airflow pass over the mountain of moderate height.Each year in Urumqi,at foot of Tianshan mountain,strong surface wind vary from 20 m/s to 40 m/s caused heavy destruction of the city facilities,while in Boulder,Colorado Rockies,the maximum downslope wind speed archived reached at 60 m/s.The spatial scale of downslope windstorm is from several to one hundred km,and its time scale is from one hour to one day,the phenomenon could be resolved in a meso-scale atmospheric model like WRF(Weather Research Forecast),but need parametrization in GCM(Global Circulation Model).The main object of this study is focus on downslope windstorm triggering mechanisms,which concerned 3 hypothesis: hydraulic jump,gravity wave breaking and lee-wave trapping,largely depend on the Froude number,Richardson number and Scorer parameter of the incident flow.The GCM gravity wave drag parametrization scheme using in the study is LM97 of LMDZ(GCM version of LMD laboratory).LM97 include the stress and drag effect due to the upstream flow blocking,downstream wave breaking and lee wave trapping at low atmosphere,involve the calculation of the Froude number,Richardson number and Scorer parameter.Although LM97 scheme was assessed by alleviating successfully the stationary biases in the stratosphere of GCM,none of evaluation of the parametrization has been effectuated on the low level downstream flow.There has no representation of the downslope windstorm triggering and intensity in the scheme.To find out the possible triggering mechanism,we use high resolution model WRF with grid-length of 1.5km forced by ERA40 reanalysis data to simulate the events presented in Boulder,and compare the simulation with that of LMDZ(2.5°x3.75°),which is zoomed and nudged in the same region during the same period.Further research is tend to build a windstorm triggering index in the GCM parametrization,and estimate the downslope windstorm intensity based on the law of conservation of energy,assuming that in the case of lee wave trapped and low level gravity wave broken,the sum of the kinetic and potential energy of the upstream and downstream remain constant.