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青藏高原东南一次典型降水性层状云的水凝物分类和过冷液态水含量分布特征

Distribution Properties of Hydrometeor Classification and Supercooled Liquid Water Content for a Typical Precipitating Stratiform Cloud on the Southeastern Tibet Plateau

  • 摘要: 云中水凝物和过冷液态水含量分布特征对于揭示云和降水形成的微物理机制,建立和检验数值模式云物理参数化方案均具有重要作用。基于第二次青藏高原科考在林芝获得的Ka波段云雷达观测数据,在进行质量控制的基础上,研究了2019年9月16~17日一次典型降水性层状云的水凝物分类和过冷液态水含量时空分布特征。 研究结果表明,基于 “K?领域频数法” 的数据去噪率达到1.5~5.0 %,补值率达到3.5~7.0 %。使用 “逐库订正法” 对雷达衰减订正前后的差值为0~5 dBZ。研究发现,该地区典型的层状云降水形成机制具有独特性,是由大尺度环流抬升形成的中高层云和地形抬升形成的低云上下耦合、合并后产生。在初始形成阶段,云顶海拔高度达到12 km,上下层云明显分离,无融化层亮带形成。云内水凝物分布比较均匀,中高层冷云区主要由冰晶、雪组成,存在较高过冷云水含量。在成熟阶段,上下层云实现合并,降水形成,云顶高度下降到10 km左右。云内不均匀性显著增加,形成了明显的融化层亮带和弱对流泡。云内以冰雪粒子为主,对流泡区也存在少量霰粒子,过冷云水主要分布在对流泡区,最大可达到0.5~0.6 g m-3。在衰减阶段,大尺度天气系统过境后造成高层冷云快速减弱,以低层地形云产生的弱暖性降水为主,融化层亮带消失。融化层以上存在很浅薄的冰雪层。

     

    Abstract: The distribution properties of hydrometeor classification and supercooled liquid water content in clouds have important roles in clarifying the microphysical formation mechanisms of clouds and precipitation, as well as establishing and validating cloud microphysical parameterization schemes in numerical models. On basis of Ka?band radar data collected during the Second Tibet Plateau Scientific Expedition in Nyingchi, southeast Tibet Plateau, the distribution properties of hydrometeor classification and supercooled liquid water content for a typical precipitating stratiform cloud on September 16?17, 2019 are investigated after data quality control procedures. The results show that the data denoising rate of the cloud radar based on the “K?nearest neighbor frequency method” is within the range of 1.5 -5.0 %, and the data gap filling rate is within the range of 3.5-7.0%. The difference between the attenuation?corrected radar data and raw data is between 0 and 5 dBZ by applying an iterative correction method. It is found that the precipitation formation mechanism for the typical stratiform cloud in the study region had some unique characteristics. The precipitation was formed by the merging of middle and high clouds induced by lifting of large-scale atmospheric circulation with low?level clouds formed by orographic lifting. In the initial stage, cloud top reached 12 km and the clouds at upper and lower levels were separated distinctly. There was no evident bright band at the melting layer. The particle distribution within the cloud was relatively homogeneous, and ice crystals and snow particles were dominate with a relatively high supercooled liquid water content in the middle and high clouds. In the mature stage, the middle and high clouds merged with the low clouds, leading to precipitation formation and cloud top decrease to be around 10 km. The clouds became inhomogeneous with evident bright band at the melting layer and weak embedded convective cells. The dominant hydrometeors were ice and snow particles, with a small amount of graupel in the embedded convective cells. The supercooled liquid water content was mainly distributed in the embedded convective cells with a maximum value of 0.5?0.6 g m-3. In the decaying stage, as the large?scale weather system passed over the study region, the middle and high cold clouds weakened rapidly, and weak warm rain generated by low?level orographic clouds became dominant, resulting in the disappearance of the bright band at the melting layer. A thin layer of ice and snow was present above the melting layer.

     

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