天山北坡一次大雾天气的数值模拟分析

Numerical Simulation of a Heavy Fog Weather on the North Slope of the Tianshan Mountains

利用常规气象观测数据、NCEP再分析资料和WRF4.0中尺度数值模式,对2019年12月8—15日新疆天山北坡出现的一次持续性大雾天气的成因进行分析。结果表明:此次大雾天气出现在500 hPa新疆脊控制、850 hPa暖中心维持、地面蒙古冷高压影响的环流背景下。雾开始和维持阶段,地面1200 m存在逆温强度为0.9℃/100 m的强逆温层,为大雾的形成和维持提供了静力稳定条件;大雾一般出现在辐射降温最明显的傍晚前后;大雾天气出现后2 m气温和地面温度温差始终维持在5℃左右,地、气温差使地面积雪一直升华,为大雾天气持续提供了充足的水汽条件;近地层大气一直存在2.0 m/s以下的微风,形成的湍流维持了雾滴悬浮的平衡状态。当逆温层中上层出现6~10 m/s偏东风时,雾层厚度增加;中上层风速过大或地面~600 m风向一致时,雾减弱或消散。

In this paper,the ground meteorological observation,NCEP reanalysis data,and WRF4.0mesoscale numerical model were used to find the causes of persistent foggy weather that occurred on the north slope of the Tianshan Mountains in Xinjiang from December 8 to 15,2019.The results showed that this foggy period was dominated by Xinjiang ridge at 500 hPa,warm center at 850 hPa,and cold high pressure in western Mongolia on the ground.In the beginning and maintenance stage of this foggy period,the strong inversion layer with an inversion intensity above 0.9℃/100 m existed at the height of 1000~1200 m,which is conducive to the maintenance of heavy fog.Generally,heavy fog developed severely when the radiation cooled obviously.Besides,the temperature difference between the 2 m temperature and the ground surface was always at about 5℃,which benefited the sublimation of snow on the ground.It provided sufficient water vapor for this foggy period.In addition,the wind speed below 2.0 m/s causes turbulence which maintains the equilibrium state of droplet suspension,and the easterly wind of about 6~10 m/s occurs in the upper and middle layers of the temperature inversion layer,leading to increases in the thickness of the fog layer.Finally,the strong wind in the middle and upper layers or the wind direction remains consistent from ground level to 600 m,which would result in the fog weakening and dissipating.