| Based on the ground automatic weather station(AWS)observation data and sounding data data at Zhangjiakou(ZJK)station during two winters from 2016 to 2017,the diurnal variation characteristics of meteorological elements such as wind persistence,wind speed,temperature and humidity,as well as the horizontal distribution characteristics of local circulation were analyzed statistically by station type and region.In addition,WRF(Weather Research and Forecasting)mesoscale numerical simulation was carried out on a selected typical Mountain-Valley wind day,which was under a weak synoptic pattern to deepen the understanding and analysis of the vertical circulation characteristics in the complex terrain and the influence of local circulation on the temperature,humidity and wind structure of the boundary layer.Results from data analyzing show are as follows:1.The diurnal variation characteristics of wind persistence,wind speed,wind direction,temperature and humidity are significantly different in different topographic features.Wind persistence can be used as an index to distinguish the influence of different topographical features on the wind field near the surface.Based on the variation characteristics of wind variability,all automatic stations in the whole region can be divided into 10 different types,representing the different influence of different topography features.2.The characteristics of prevailing wind direction,conversion time,wind speed,temperature and humidity are different in different regions,and different scales of slopes,canyons,basins and plains can produce different scales of slope wind,valley wind and mountain-plain circulation.The influence of synoptic-scale cold advection is obvious on the top of higher elevation.The wind direction is parallel to the topographic axis in the mountain slope,transition zone,the gully and canyon with smaller scale.The diurnal temperature range is mainly determined by the topographic scale,the smaller the topographic scale is,the greater the diurnal temperature range is.The central basin is significantly affected by the topography,the volume effect of the basin and the subsidence warming of the air above it make the diurnal temperature range of the basin larger than that of the surrounding plain and mountain area.The significant thermal difference between the plain and the northwest mountainous area forms a large-scale mountain-plain circulation.3.Weak synoptic wind field of the actually observed wind reflects the characteristics of local circulation and the change of temperature humidity,the local wind isolated from theactual wind during the whole winter,regardless of the influence of large-scale synoptic background wind field and partial topographic dynamic action,can be regarded as an ideal local valley wind circulation generated only by thermally driving,and its diurnal variation characteristics are more obvious.The characteristics of local circulation variation can also be found in the high-altitude mountain areas where there is no obvious diurnal variation in the actual wind field and in the canyon basins which are greatly affected by topographic dynamics.4.The horizontal wind field obtained by the numerical model can better reflect the characteristics of mountain and valley wind circulation,and the simulated wind direction and valley wind conversion period have good consistency with the observation.The influence height of local circulation and the diurnal variation characteristics of wind profile simulated in vertical direction are in good agreement with the observation,and the error is within acceptable range.5.The diurnal variation characteristics of potential temperature,wind speed,temperature,humidity profile and vertical wind profile can reflect the influence of different topographic features on boundary layer height and the structure of wind temperature and humidity.According to the variation rule of boundary layer temperature,it can be divided into three types: a.Adjoining inversion type,there is only an adjoining inversion layer or a weak isothermal layer near the surface without upper air inversion,obviously near the top of the mountain and the mountain gully at a higher altitude.b.Upper air inversion type,there is only an upper air inversion,this type is more obvious in plain area,small valley area and deep valley area.c.Double-layer inversion type,the adjoining inversion layer and upper air inversion exist at the same time,obviously around the central basin.According to the variation rule of wind velocity profile at night,the representative sites can also be divided into three types: a.Low-level jet type,that is,the low-level jet exists at night,obviously in small canyons,front slopes,the middle area of the basin and the plain.b.Wind speed increasing gradually type,that is,the nocturnal wind speed increases with height,without a low-level jet,obviously in the deep valley,the top of the mountain and the gully of higher elevations.c.wind speed decreaseing gradually type.that is,the wind speed near the surface is larger and the low-level wind speed decreases with the height,especially at the transition area between the small canyon and the basin and at the entrance of the central basin.This study shows that the thermal and dynamic effects of complex topography have an important influence on the distribution characteristics of horizontal mountain-valley wind circulation and meteorological elements as well as the evolution characteristics of boundary layer wind,temperature and humidity structure under the wind field of weak synoptic wind field in winter.This study can provide support for understanding the dynamic action of the terrain-guided air flow and the thermal-dynamic action of the terrain-lift air,and provide a reference for the precise prediction of the local circulation and meteorological elements in the complex topography. |
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