积层混合云淞附过程的观测和数值模拟研究

Stratiform clouds with embedded convection, as important precipitating clouds over northern China, are the primary objects in operational weather-modification activities. Riming is significant to both ice particle growth and precipitation formation. In most bulk microphysical parameterizations, the threshold and ratio of graupel produced from rimed snow are generally ad hoc, which is difficult to show the gradual increase of particle mass by riming. Using data from the new-generation airborne cloud probe observations and the WRF model, riming process and ice water distributions in stratiform clouds with embedded convection over northern China will be studied in the proposal. The temperature levels where riming predominant and corresponding particle habits will be investigated using particle spectra and image data from aircraft measurements. Based on mass-dimension and projected-area-dimension relationships, as well as adjustment of classification of ice particle types and calculation of particle fall speeds, the parameterization of the riming process implemented in the microphysics of the WRF model will be improved. The riming degree in different precipitation stages for typical clouds will be studied. Sensitivity experiments will be conducted to study the influence of such parameters as cloud water content and particle fall speed on riming rate. In addition, the Bergeron and melting processes as well as latent heat release will also be considered to investigate the influence of key ice microphysics on intra-cloud mass transfer and surface precipitation.

积层混合云是我国北方地区重要的降水系统和人工影响天气作业的主要对象，其中淞附过程对云内冰相粒子的增长和降水的发动起着至关重要的作用。现有的云物理谱型参数化模型中，一般人为划分淞附雪向霰自动转化的阀值和比例，难以准确反映雪向霰的逐渐转化过程。本项目拟利用新一代机载云微物理探测数据和中尺度WRF模式，研究我国北方地区积层混合云淞附过程和冰水含量分布；利用飞机探测的粒子谱和二维图像资料，研究淞附过程占主导作用的温度层次和典型大粒子形态；基于质量-直径关系和投影面积-直径关系，通过对冰相粒子分类方式和下落末速计算方法的调整，改进WRF模式云微物理模块中冰相粒子淞附过程的参数化方案，研究典型云系在不同降水阶段的淞附程度；通过数值敏感性试验对比研究云水含量、粒子下落末速等基本物理参量对淞附增长率的影响，结合贝吉龙和融化过程及相应的潜热释放揭示关键冰相微物理过程对云内质量输送和地面降水的影响规律。

淞附过程是影响积层混合云内冰相粒子增长和降水发动的关键微物理过程，现在的云微物理参数化模型中，对淞附过程的描述仍存在不足之处。针对淞附过程，本项目利用机载云微物理探测数据开展了云粒子形态和增长机制的分析，在此基础上利用中尺度WRF模式和数值敏感性试验研究了淞附过程对微物理过程和地面降水分布的影响。结果表明，对于积层混合云，聚并与淞附过程同时存在，是形成较大尺度冰雪晶的重要方式。对于研究所选取的降水过程，有无淞附过程对降水分布的影响较大，但对总降水量的影响很小。无淞附过程时，由于不同微物理方案中冰相粒子电容和淞附过程中潜热释放计算方法的不同，导致上升气流的变化程度不同。WRF模式中考虑淞附过程连续性的P3微物理方案比双参数化Morrison方案模拟的降水有所延迟并且更接近地面观测。由于P3方案能体现冰相粒子淞附过程中质量-加权降落末速和密度的逐渐转化过程，因而可以更好地再现层状云降水过程。对凝华潜热进行的数值敏感性试验表明，凝华潜热加热对云内上升气流发展和地面累积降水形成有重要影响。本研究对深入理解WRF模式中不同微物理方案对我国积层混合云系的可适用性具有一定意义，可为改进数值模式微物理模型和科学开发空中水资源提供理论指导。