雨滴谱谱形参数在云微物理参数化方案中的改进研究

The evolution of raindrop size distribution directly affects the formation of precipitation. In the bulk microphysics scheme, the shape parameter, which represents the relative dispersion of the hydrometeor size spectrum, plays an important role in the computation of sedimentation and instantaneous growth rates of evaporation, accretion and so on. As is well-known, there is abundant microphysical information in the raindrop measurements, which is one of the most essential way to understand the physical processes of cloud and precipitation. However, for the currently widely used two-moment bulk schemes, the shape parameter of raindrop size distribution is mostly set as zero which is greatly different from the observation. Therefore, research on the ways to take full advantage of the observations to make improvements for bulk schemes in describing the actual distribution characteristics of raindrop shape parameter should be crucially important...People's production and living is particularly sensitive to meteorological disasters in Jiangsu Province, which is an both economically advanced and densely populated area with dense observations including rich raindrop-size distribution data. In this study, all the available diagnostic relations would be taken full advantage of and information of the raindrop shape parameter extracted from the local disdrometer measurements should also be incorporated into the bulk microphysics scheme with the purpose of determining the most effective empirical formulas which to be optimized for maximum. on the basis of complementary advantages, a more reasonable parameterization method for the raindrop shape parameter, especially consistent with the local raindrop features, could be constructed, which is to be tested by the weather events in Jianghuai areas as well to verify it’s general applicability. As a result, improvements could be achieved in the microphysics schemes, which is beneficial to raise the numerical forecasting abilities over microphysical processes and increase the capability for rainfall forecasts.

雨滴谱的发展演变直接决定最终降水的形成。云微物理参数化方案中雨滴谱的谱形参数体现了雨滴尺度分布的离散程度，对影响雨滴发展演变的沉降、蒸发、碰并等过程起着至关重要的作用。而目前广泛应用的双参数方案中，雨滴谱谱形参数往往被设为固定常数的方式同实际观测存在较大差距。雨滴谱观测含有丰富的降水微物理特征信息，因而如何充分地利用我国的观测资料，在双参数方案中合理且真实地体现雨滴谱形参数的特征便十分地重要。江苏地区经济发达、人口稠密，对气象灾害尤为敏感。因此本研究将综合运用现有的雨滴谱谱形参数的各种经验公式，融入江苏地区丰富的实测地表雨滴谱的特征信息，确定有效的雨滴谱谱形参数的经验公式并加以优化，开展云微物理参数化方案的本地化研究，并检验其在江淮地区应用的普适性。通过优势互补构建出更为合理且符合本地雨滴谱特征的谱形参数的参数化方法，以期提升中尺度数值模式对我国云微物理过程的模拟能力和降水的预报水平。

现有云微物理参数化方案包含大量基于国外观测的参数化方法，模式在我国实际使用时预报结果普遍“退化”。因此了解我国云和降水的实测特征并由此展开方案的完善和本地化工作十分必要。.以江苏为例，项目首先对夏季不同时段不同类型降水宏微观特征进行统计归纳分析，获得雨滴谱特征数据集，建立天气背景条件与降水率和雨滴微观特征的可能物理联系：.1.较大尺度雨滴样本比例是分钟级对流极端降水频率、平均降水强度的主要决定因子。对流降水时，梅雨开始前和梅雨结束后在降水率、雨滴尺度等方面更突出；层云降水时梅雨期表现最为突出。.2.小尺度雨滴样本比例是样本雨滴数量的主要影响因子。东亚夏季风的推进导致对流和雨滴破碎等过程逐渐增强、小尺度雨滴样本比例增加；层云降水时，碰并作用充分导致小尺度雨滴样本比例容易偏低。.上述特征和规律利于对东亚夏季风推进不同阶段降水过程有更加立体和综合的认识；雨滴谱特征量及经验关系式，也为雷达、卫星反演产品算法的改进等工作提供可靠的观测依据和研究基础。..其次本项目基于雨滴谱谱形参数-斜率二项式关系（μ-Λ）的显著地域差异性，探索多种优化思路和技术方法，结合已有经验公式及本地雨滴谱特征信息优势，开展雨滴谱谱形参数的参数化方法的本地化研发工作：.1.通过改写和调试形成能够稳定运行的改进方案，获得更优的基于气柱模式经验公式，且24小时累积j降水TS评分总体均有提高。基于优选气柱模式经验公式同时结合近地面层的基于本地雨滴谱μ-Λ经验关系式的融合改进试验在小雨和中雨量级均获得提高，体现了气柱模式经验公式与本地观测特征优势互补的联合应用效果。.2.改进试验对不同量级降水的范围和强度产生影响。雨滴落速减慢及云系中停留时间增加等是降水发生变化的可能主要原因。改进试验对多数个例具有改进效果，具有一定的普适性特征。.上述工作对提高云物理过程参数化方案的本地应用水平有着重要的意义，也为方案的进一步完善和在其它地区的本地化研究提供参考依据。