| The lower atmosphere is the layer that affected by the complexity of Earth's surface most remarkably.It is also the channel for the exchange of material and energy between the Earth's surface and the atmosphere and it is closely related to human beings.By utilizing the observational data from the Gobi area adjacent to Bosten Lake in 2013,2016 and 2017,the temporal and spatial variations of near-surface meteorological elements,near-surface layer micro-meteorological characteristics and atmospheric boundary layer structure in a region of 14km×13km were analyzed and some conclusions have been reached in the study of local land-atmosphere interactions.In addition,the diurnal evolution of the local flow field was simulated and analyzed by using WRF mesoscale numerical model.Large eddy simulation of near-surface wind field under realistic conditions was performed by using 6-fold grid-nesting for dynamical downscaling purpose.Sensitivity analysis of land surface impacting factors was also carried out for wind field simulation.The main conclusions are as follows:(1)Because of the thermal effect of Bosten Lake and its influence on water transport,the study area shows obvious microclimate characteristics.In spring and summer,the temperature gradient in the north-south direction is very obvious,and the highest temperature gradient is close to 0.5?/km while the daytime difference of temperature between north and south is almost disappeared in autumn.After freezing and covered with snow in winter,Bosten Lake becomes an absolute source of cold.Under the effect of land-lake thermal difference and topography,the near-surface flow field shows a diurnal alternation all year round in the observation area that is the north wind from the lake dominates during the day,while the southeast wind dominates at night.Lake-breeze is strongest in the summer and obvious in spring while in the autumn,the lake-breeze duration is the shortest due to the decrease of the lake-land thermal difference.In winter,with the recovery of thermal differences between lake and land,the lake-breeze partially restored its strength during the daytime.In spring,summer and winter,the wind-speed in the observation area is highly consistent within 6km offshore,and the wind-speed near the land surface shows an obvious diurnal variation.Generally speaking,the wind-speed in daytime is higher than that at night,and the wind-speed is smaller during the wind-direction transition period,but there is no obvious diurnal variation pattern of wind-speed in autumn.The influence of Bosten Lake on the water transport in the adjacent Gobi area runs through the whole year,and is the strongest in summer that the influence range is over 8.5km and the average daily specific humidity difference in the north-south direction of the observation area is 2.5g·kg-1.The variation of atmospheric specific humidity along the Lakeshore during the daytime was significantly different from that in the adjacent Gobi area.The atmospheric humidity influence of Bosten Lake is the lowest in winter and the influence duration is the shortest.(2)The structure and evolution of the atmospheric boundary layer in the study area reflect the basic characteristics of the arid Gobi region of Northwest China.The stability of the near-surface layer shows obvious seasonal variation,and the turbulent motion is most intense in summer.In spring,the wind shear contribution to turbulence is the highest in the whole year.In autumn,the work done by buoyancy and shear stress on turbulence shows a downward trend.In winter,turbulence is suppressed for most of the day.The thermodynamic structure of the boundary layer shows obvious diurnal variation,the potential temperature profile has obvious stratified structure,and the stable boundary layer and convective boundary layer are clearly visible.The average height of the convective boundary layer in summer is about 1800 m.Under clear weather conditions,an extra-thick boundary layer with a depth of more than 4300 m can be seen,while a stable boundary layer with a depth of more than 1700 m appeared earlier.Due to the influence of water vapor advection,aloft atmospheric humidity inversion will occur in spring,summer and autumn,while touching ground humidity inversion will occur in winter.In the daytime,the specific humidity in the convective boundary layer decreases upward and the iso-humidity distribution and humidity inversion may also occur.The surface evaporation is the main contribution of the water vapor in the nocturnal boundary layer.The wind-speed profiles in the near-surface layer vary greatly in different seasons,but there is an obvious turning point in the wind-speed profiles at 70 m above ground in most of the year and the gradient of wind-speed in the upper part is larger than that in the lower part.(3)Because of the influence of the heterogeneous underlying surface of the lake and land,there are also obvious "cold island" effects in the observation area.At night,the boundary layer temperature in the lakeshore area is lower than that in the adjacent Gobi area.During the day,a "cross phenomenon" similar to heat island appears,that is,the temperature in the lower part of the lakeshore boundary layer is low while the temperature in other parts is high compared with the adjacent Gobi area.The water vapor content in the boundary layer of the whole day is higher than that in the adjacent Gobi,and the humidity difference in the local boundary layer is more obvious than that in the oasis cold island study.The stable boundary layer on the lakeshore appears earlier and lasts longer.The atmospheric turbulent mixing is weak in the lakeshore area during the daytime,and there is an obvious difference in the shape of the convective boundary layer potential temperature profiles compared with the adjacent Gobi,and no obvious mixing layer and entrainment zone appeared.In addition,there are obvious local circulations in the boundary layer in summer,and the average height of the lake-breeze circulation is over 2000 m in summer afternoon.(4)The results of mesoscale simulation can clearly show the diurnal evolution of the atmospheric flow field in the Bosten Lake area.At night the divergence zone of flow field was formed along the peak of the southern Kuruketag Mountains,and the convergence center of flow field was formed on the surface of Bosten Lake.During the daytime,the lake surface changes into divergence center,and a strong convergence zone is formed along the peak of the Kuruketag Mountains,accompanied by an intense upward motion of airflow,while the lake surface is dominated by subsident airflow.In the afternoon,with the strength increase of the lake-breeze,an obvious lake-breeze circulation is formed over the study area.Kuruketag Mountains create favorable conditions for the diurnal evolution of the local flow field.On the one hand,the topographic circulation caused by the uneven heating of the mountain body is consistent with the circulation formed by the thermal difference between lake and land,which makes the local circulation last longer and have a larger influencing range.On the other hand,the east-west trending mountain body blocks the influence of the north-south background wind field effectively,making the circulation develops to a higher level.(5)Using the high-resolution WRF-LES to simulate the near-surface wind field can improve the fluctuation and intermittence of the mesoscale simulated results,makes it more consistent with the actual atmospheric characteristics,but increases the uncertainty of the simulation results in the meantime.In addition,when WRF-LES is applied to complex terrain area,using terrain data matching with grid scale is very important to improve the accuracy of wind field simulation near the land surface.Soil moisture and roughness length are also the main land surface impacting factors.Increasing soil moisture will reduce near-surface wind speed,while surface roughness will affect momentum downward transport in the planetary boundary layer.In addition,the influence of surface roughness on near-surface wind speed is related somehow to atmospheric stability. |
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