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基于雷达观测的福建转折性降雪垂直特征分析

Analysis of Vertical Characteristics of a Transitional Snow Event Based on Weather Radars in Fujian

  • 摘要: 2022年2月19~23日福建地区出现转折性降雪天气,环流背景是中层暖层和低层冷层并存,地面有雨、雨夹雪和纯雪等多种天气现象,预报难度大。基于天气雷达等资料,采用基于粒子谱数据的降水类型分类方法对各站点降纯雪和雨夹雪时段进行划分,结合天气形势和雷达参量垂直特征,重点分析了环境温度演变对转折性降雪的影响以及转折性降雪的垂直演变特征。结果表明:(1)地面气温和地形分布较为均匀时,可根据反射率因子垂直廓线(VPR)和环境温度特征判断雨夹雪和纯雪之间的转换,即纯雪阶段VPR随高度降低单调递增,雨夹雪阶段廓线中层存在融化层亮带特征;分析时应特别关注雷达对低层降水的观测能力,当雷达受观测环境限制,无法反映近地层温度差异造成的降水特征变化时,即使在高空VPR相似的情况下,地面降水类型也可能显著不同;(2)降雪粒子的双偏振雷达参量垂直特征表明,转折性降雪高层和融化层高度以上的粒子增长机制与一般纯雪过程类似,不同点在于受中层暖层和低层冷层影响,粒子经历了复杂的融化和冻结过程。粒子下落到暖层后出现融化层亮带特征,融化层以下则视低层冷层强度和地面气温不同,粒子完全或部分冻结,地面可能出现纯雪或雨夹雪;雷达参量垂直结构特征(例如融化层亮带的强度变化)能实时反映环境温度变化,有助于判断地面降水粒子相态,可一定程度上弥补模式预报不够精准和单点探空垂直观测欠缺的问题。

     

    Abstract: A transitional snow event occurred in Fujian Province from 19 to 23 February 2022 and was characterized by the coexistence of warm and cold layers in the middle and lower tropospheres, respectively. Multiple precipitation types occurred on the ground, including rain, snow, and wet snow, increasing forecasting difficulty. Based on data from weather radars and other sources, a classification method for precipitation types is first used to identify the periods of snow and wet snow at each weather station; on this basis, the influence of environmental temperature evolution on transitional snow events and the vertical characteristics of transitional snow events are investigated by analyzing the synoptic circulation and vertical features of radar variables. Results demonstrate the following points. (1) The transition between snow and wet snow could be clearly distinguished according to the characteristics of vertical profiles of reflectivity (VPR) and environmental temperature information for areas with uniform topography and surface temperature, i.e., VPR monotonously increases with decreasing height during a snow stage, while a considerable bright band appears in the middle layer of VPR during a wet snow stage. Great Considerable emphasis should be placed on the capability of weather radars to observe precipitation in the lower level. If radar variables could not capture the change in precipitation characteristics caused by the temperature difference in the near-surface layer due to the limitation of the observation environment, the ground precipitation types could be markedly different although the VPR features resemble each other in higher layers. (2) The vertical characteristics of polarimetric radar variables indicate that the growth mechanism of hydrometeors in the upper layer and above the melting layer is similar to that of general dry snow events. The major differences in transitional snow events are the complicated melting and refreezing processes of particles caused by the warm and cold layers. A bright band signature appears when hydrometeors fall through the warm layer; then, melted particles are fully or partially frozen, reaching the ground as snow or wet snow, depending on the intensity of the cold layer beneath the warm layer and the surface temperature. The vertical features of radar variables (e.g., the change in the intensity of the bright band) can timely reflect the change in environmental temperature, which is conducive to distinguishing precipitation types and can compensate for the inaccurate forecasting of numerical models and the absence of atmospheric profiles to some extent.

     

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