一次冷锋过境时引发多种天气现象的成因分析

利用观测数据、雷达产品、ERA5再分析资料对2021年4月15日河北保定一次天气过程进行综合分析，结果表明：(1)天气背景高层干冷，低层有暖脊，整层大气较为干燥。冷锋过境引起地面风力加大，锋面触发干对流后，冷池出流进一步加大了地面风速，湍流加强并出现扬沙。(2)对流云内存在水平弱风区，其径向速度近于0 m/s，弱风区后侧的雷达回波强度呈增加趋势。雷达低仰角0径向速度区的上方有水凝物粒子迅速集中、生长，生长区内径向速度为5～10 m/s，降雹区域上空各仰角均为0径向速度。(3)下击暴流和动量下传引发了干对流大风，降水粒子的蒸发、拖曳作用进一步加剧了气流下沉，其中蒸发作用的贡献更显著。(4)干对流消亡后，锋面东移，锋后西北风减弱，扬沙天气结束，较小的近地层风通量有利于沙尘粒子缓慢下降，地面能见度好转但PM₁₀浓度依然较高。(5)夜间冷空气下沉，上游输送的沙尘粒子随之沉降，造成了浮尘天气。

Mechanism analysis of various weather phenomenon triggered by a cold front

Using observational data, radar products and the ERA5 reanalysis data, the mechanism analysis of a missing forecast occurred in Baoding, Hebei province on April 15, 2021, was studied. The results were as follows: (1)There was a dry and cold air at upper level overlapping warm air at low level with low relative humidity. Surface wind speed increased during the passing of the cold front, and triggered dry convection which formed the outflow of the cold pool, thus strengthened surface wind speed and turbulence accompanied by sand blowing. (2)There was weak radial wind zone inner convective cloud, and the radial velocity was close to zero. The echo intensity increased on the rear of the weak radial wind zone. Hydrometer gathered and grew above zero velocity at lower elevation angle while the radial velocity was from 5 to 10 m/s . Hail area was located in the zero velocity zone at each elevation angle. (3) The gust of dry convective was trigger by downburst and downward-transport momentum, enhanced by the evaporation and drag of rain droplets, and evaporation effect contributed more. (4) After the decay of dry convection, the northwesterly wind became weak while the cold front moved eastward, leading to less dust. The weak mass rate of air flow near-surface was conducive to falling slowly of dust particles, thus the visibility at surface improved while the PM10 concentration was still relatively high. (5) At night ,the cold air accompanied with the dust particles transported from the upwind regions gradually sinking, lead to dust weather.