北京冬季降水粒子谱及其下落速度的分布特征

Distributions of particle size and fall velocities of winter precipitation in Beijing

中文关键词:  冬季降水  粒子下落速度  粒子形态

英文关键词:Winter precipitation  Particle velocity  Particle shape

中文摘要:

      为了深入探讨北京冬季云降水的微物理特征，提高雷达反演冬季固态降水的精度和冬季降水的预报水平，利用PARSIVEL（Particle Size and Velocity）降水粒子谱仪所观测的冬季降水粒子谱，结合地面显微镜粒子图像和云雷达数据，对比分析了北京海坨山地区冬季过冷雨滴、霰粒、雪花、混合态降水的粒子谱和下落速度特征，得到主要结论如下：（1）霰粒降水过程的云顶最高，整层的含水量最大，低层的退偏振比（LDR）最小，粒子更接近于球形；降雪过程的云顶最低，云中含水量最少，低层的退偏振比较大；混合态降水过程的雷达回波强度和高度特征介于两者之间，但低层的退偏振比最大；（2）在云中上升或下沉气流及湍流的影响下，过冷雨滴、霰粒和雪的下落速度均对称分布于各自理论下落末速度曲线的两侧。因此可根据粒子浓度相对于其直径和速度分布的中轴线位置，判断出该段降水过程中的主要粒子形态；（3）冬季雪花、霰粒和混合态降水粒子下落速度分布的散度较雨滴更大，其原因是由于冷云降水过程的粒子形态复杂，且固态粒子下落过程中更容易受破碎、聚并和凇附等微物理过程影响；（4）在4种降水类型中，雪的平均直径和离散度最大，雨滴最小；混合态降水粒子的总数浓度最大，雨滴的总数浓度最低，并且4种降水类型的粒子数浓度、平均直径和离散度均随降水强度的增大而增大。

英文摘要:

      Joint size and fall velocity distributions of particles were measured with a Particle Size and Velocity (PARSIVEL) precipitation disdrometer in a field experiment conducted during winter at Haituo Mountain in Beijing. The microscope camera and cloud radar were also used during the same period. Microphysical properties and fall velocities of super cooled liquid rain, graupel, snow and mixed-phase precipitation particles are compared. The major results are as follows. (1) The cloud of graupel precipitation contains more liquid water and the cloud top is the highest; LDR (linear depolarization ratio) is the smallest in the bottom of cloud among the three types of precipitation,which means that particles are nearly spherical. The cloud of snow precipitation contains less liquid water and the top is the lowest. The reflectivity and LDR values of mixed-phase precipitation are between that of the other two types of precipitation. (2) Examination of the particle fall velocity reveals that the liquid raindrop, snow crystal and graupel fall velocities are mainly determined by theoretical terminal velocities, and distribute symmetrically along the theoretical terminal velocity lines due to updrafts, downdrafts and turbulences in clouds. Based on this result, we can distinguish the particle type for a period. (3) The spread of particle velocity is larger for solid particles than for liquid particles. The microphysics processes are complex in cold clouds that produce many types of particle shape with different terminal velocities. Moreover, the turbulence, riming, breakup and coalescence likely cause the large spread of particles velocity. (4) The mean diameter and dispersion are the largest in snow and the smallest in rain. The concentration is the largest in mixed-phase precipitation and the smallest in rain. The concentration, mean diameter and dispersion of the four types of precipitation all increase with precipitation rate.