Numerical Simulation Of Local Circulation Of Complex Topography On The Gaoligong Mountains

The Hengduan mountains,which is north-to-south direction,is very significant for the climate in southwest China.Long and narrow mountains.deep valleys and three parallel rivers form a unique geographical environment and mountain range,make the eastern and western area of the Hengduan mountains are affected by monsoons from the Pacific Ocean and the Indian Ocean respectively.Due to southwest monsoon in the west,the western side of the mountain is usually covered with topographic rain,while the leeward slope in the east is relatively dry.And the humidity decreases from east to west.The Gaoligong mountains is the westernmost mountain in the Hengduan mountain range,and its surrounding regional climate constitutes the climatic characteristics of the Hengduan mountain range.The study on the regional effect of the Gaoligong mountains also provides a reference for further studies on the climate in southwest China.In this paper,the mesoscale model WRF-ARW(V3.9)was used to simulated mountain-valley circulation over the region of south Gaoligong mountains on December 4,2016(winter monsoon period)and September 28,2016(summer monsoon period),respectively.Adding a sub-grid topography parameterization,simulation performance of wind field near surface layer had been improved.Applicability of five boundary layer parameterization schemes YSU,MYJ,MYNN3,ACM2 and BouLac under complex underlying surface of the Gaoligong mountains are compared.This paper aims to explore the local wind in the southern section of Gaoligong mountains under the influence of different background winds using suitable parameterization schemes,and analyze circulation structure and evolution,so as to understand the cause of formation and difference mechanism of local microclimate.The major conclusions of this study are listed as following:(1)In complex terrain of Gaoligong mountains,rough terrain processing will lead to the deviation of wind speed simulation.Simulations of WRF without sub-grid topography parameterization showed that frequency of low wind speed is small,while frequency of high wind speed is large.Sub-grid terrain parameterization scheme can reduce bias of wind speed simulation by increasing surface drag.Ability of temperature simulation was not improved greatly by using sub-grid terrain parameterization.For wind speed,both the negative deviation of low wind speed simulation and the positive deviation of high wind speed simulation were significantly improved because of the large value of surface drag Ct.After using the sub-grid parameterization,the dominant wind direction is more consistent with the observed wind direction.(2)Five boundary layer parameterization schemes(YSU,MYJ,MYNN3,ACM2 and BouLac)were used to simulate near-surface and high-altitude meteorological elements in Tengchong area.The YSU scheme has the best effect on temperature simulation.The mean absolute error of wind speed in ACM2 simulation is the smallest,and the wind speed sample structure of MYJ scheme and the ACM2 scheme is better.The absolute error of wind direction simulated by the MYNN3 scheme is the smallest,and the diurnal variation trend simulated by YSU and MYJ scheme is more consistent with the observation.The YSU scheme and the BouLac scheme are more suitable for the simulation of vertical distribution of temperature and wind speed at high altitude.(3)The dry season of Yunnan is dry and southwest airflow from the southern branch of the plateau.The wet season has southwest monsoon from the Bay of Bengal and southwest monsoon from west Pacific.Tengchong is mainly affected by the southwest monsoon from the Bay of Bengal.Analysis of 2016 observation data shows that Tengchong has lower temperature,higher humidity and more abundant precipitation than Baoshan.Based on the difference of meteorological elements between the two stations,local circulation characteristics are analyzed.In the southern Gaoligong mountains,valley wind circulation appeared at 9:00 am and turned to mountain wind circulation at 19:00 pm.The local circulation is dominated by valley wind and mountain wind circulation,it is mostly southerly wind in the daytime,and northerly and westerly wind at night.During the day the wind speed is higher than that at night.In the dry season,the west wind is weak,which is conducive to the development of local circulation in the lower troposphere.Due to the temperature gradient in the valley,easterly wind forms,and the airflow bends and sinks,forming vortex,which is also conducive to the development of convective boundary layer.When the cold air moves to the south,the valley wind on the west side of Gaoligong mountains and the mountain wind on the east side weaken,the east and west wind meet at a high altitude,and the east wind is controlled below 5km in the lower layer.In the wet season,the wind is southerly throughout the day.The southwest monsoon of Bay of Bengal is strong and the system is strong.The strong southwest wind meets Gaoligong mountain,sinks in the west slope and forms a vortex,the turbulent mixing on the west side is sufficient,and the boundary layer is high.Under the influence of typhoon peripheral airflow,the lower layer also turns to the east wind,which makes the valley wind on the east side and mountain wind on the west side of Gaoligong mountain strengthen.The development of local circulation is restricted and the height of boundary layer is lower in wet season than that in dry season.