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华北夏季一次积层混合云降水过程的飞机观测和模拟研究

Aircraft Observations and Numerical Simulations of Precipitation Processes in a Case of Summer Stratiform Clouds with Embedded Convection over North China

  • 摘要: 利用飞机探测资料结合WRF(Weather Research and Forecasting)模式耦合ISHMAEL(Ice-Spheroids Habit Model with Aspect-Ratio Evolution)微物理方案,对2017年8月27日在华北地区夏季一次积层混合云降水过程的微物理特征和垂直结构进行了探测资料分析和数值模拟研究。飞机探测显示,层云区过冷水含量不大,对流云区局部过冷水含量较大,峰值达到0.566 g m−3,总水含量在高层含量最大可达2.267 g m−3。在层状云的中上部的冰相层,云滴及冰雪晶数浓度存在跃增现象,冰晶形状主要为尺度较大的聚合体,冰晶增长过程主要为聚合和凝华过程。在中部0℃层附近,液水含量较上层明显增加,主要为雨滴及未完全融化的降水粒子。在中下部的暖层,云滴数浓度较中高层明显增多,云滴谱明显变窄。模拟结果显示,不同类型的冰晶含量和分布差别较大,板状冰晶含量最高,柱状冰晶含量最低,板状冰晶和聚合状冰晶分布高度较一致,高值中心主要分布在−10℃以上,柱状冰晶分布高度较低,主要形成于冰水混合层。本次层状云降水过程,在降水增强阶段存在“播种—供给”机制,高层冰雪晶以凝华和聚合增长为主,对中低层起到播撒效应,其下落到暖层后的融化过程对雨滴增长有较大贡献。在降水较弱阶段,播种云的作用不明显,降水主要是通过暖区的云水转化形成。

     

    Abstract: In this paper, aircraft observation data combined with WRF (Weather Research and Forecasting) model coupled with the ISHMAEL (Ice-Spheroids Habit Model with Aspect-Ratio Evolution) microphysical scheme are used to analyze the microphysical characteristics and vertical structure of summer stratiform clouds with embedded convection precipitation process in the North China area that occurred on 27 August 2017. The analysis of aircraft observations reveals that the supercooled water content in the stratus cloud region was not considerable, whereas it is locally considerable in the convective cloud region, with a peak value of 0.566 g m−3. The maximum total water content in the upper layer can reach 2.267 g m−3. In the upper and middle layers of stratiform clouds, a sudden increase is observed in the concentration of cloud droplets, ice, and snow crystals. The shape of ice crystals is mainly in the form of large aggregates, and the growth process of ice crystals mainly involves aggregation and condensation. Near the 0℃ layer in the middle of the layered clouds, the liquid water content considerably increases compared with that in the upper layer, mainly consisting of raindrops and partially melted precipitation particles. In the warm cloud layer, located in the middle and lower parts of the layered clouds, the number and concentration of cloud droplets substantially increase compared to those in the middle and upper layers. The cloud droplet spectrum is considerably narrower. The simulation results indicated that the contents and distributions of different types of ice crystals vary substantially. The content of ICE1 was the highest, while that of ICE2 was the lowest. The distributions of ICE1 and ICE3 demonstrated a high degree of consistency, with a predominant concentration at temperatures exceeding −10℃. Conversely, the distribution height of ICE2 was lower, primarily confined to the ice-water mixing layer. The results implied that during the process of stratiform cloud precipitation, a “seeding-supply” mechanism operates during the stage when precipitation is enhanced. At this phase, layers of snow and ice crystals primarily grow through deposition and coagulation, exerting a seeding effect on the lower layers. As these layers descend into the warm layer, the subsequent melting considerably contributes to the augmentation of raindrops. By contrast, during the weak precipitation stage, the role of seeding clouds was observed to be less pronounced, with precipitation formation mainly occurring through the conversion of clouds to rainwater in the warm region.

     

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