霰粒子下落速度对云系及降水发展影响的数值研究

Numerical simulations of the influence of the graupel fall terminal velocity on cloud system and precipitation development.

  • 摘要: 云和降水的形成是动力过程与微物理过程相互作用的产物,云数值模式中的微物理过程参数化方案对云和降水发展过程有直接影响。在云数值模式中,粒子群体的下落速度都是用质量加权下落末速度公式来表达,而且不同的模式采用的公式存在差异,质量加权下落末速度中参数取值不同,引起的粒子下落末速度不同。为了了解粒子下落末速度变化对云系和降水发展的影响,对2004年8月12日一次冷锋降水过程,利用中尺度ARPS模式做模拟研究。在分析降水机制的基础上,对霰这一下落末速度较大的降水粒子,做下落末速度(Vg)的敏感性试验,从动力、热力、微物理的角度,通过数值模拟对比分析了霰下落末速度减小对降水分布和强度、云系的移动、云系的宏观热力和动力场的影响,并给出了影响的途径和机理。结果表明: V g变化对云的厚度和含水量有影响,下落末速度减小对冰晶、雪、霰的含水量垂直分布及分布随时间变化影响较大,其中,霰的含水量显著减少,雪的含水量增加,并调整了云中水质粒的空间分布; Vg减小对地面累积总降水量的分布影响较小,但对降水强度的分布影响较大。V g减小时,降水强度减小,降水时间延迟,因此,霰下落末速度变化将调整底层降水分布;对于云系的移动情况基本上没有影响,但对云中水质粒的空间分布有影响;霰下落末速度变化影响云中霰的融化和撞冻增长从而影响热力场。末速度减小时,霰和雪的融化量明显减小,导致非绝热冷却率的减小,引起下沉气流的减小。

     

    Abstract: Formation of cloud and precipitation is a result of the interactions between the dynamic processes and the cloud microphysical processes. The microphysical parameterization schemes in the cloud numerical models have a direct influence on the developing processes of the cloud and precipitation. In cloud numerical models, the fall velocity of hydrometeors is expressed as mass weighted terminal velocity, and there are different expressions with different parameter value selections in different models. Different coefficient values in even the same expression may result in different mass weighted terminal velocity values of hydrometeors. To further study the influence of the terminal velocity changes on cloud and precipitation development, a cold frontal rainfall event in North China on 12 August 2004 is simulated by using the ARPS (Advance Regional Prediction System) mesoscale model. On the basis of the precipitation mechanism, we performed sensitive experiments by changing the graupel fall terminal velocity ( V g) to investigate the impact of the Vg variation on the cloud system movement, precipitation distribution and intensity, and changes of thermodynamic and dynamic fields. The results show that the variation of Vg alters the cloud thickness and the water content of hydrometeors. The decease of Vg has a stronger influence on vertical distributions of the water content of ice, graupel and snow and time variations of their distributions. The graupel (snow) water content decreases (increases) evidently, and the space distributions of hydrometeor are modulated. The reduction of V g has a smaller impact on the distribution of surface accumulative rainfall, but a larger impact on rainfall intensity. When Vg deceases, the rainfall intensity also deceases and the precipitation initiation time is delayed. The Vg change modulates the precipitation distribution in the low levels, and it has almost no impact on the cloud system movement. On the other hand, Vg also has an influence on the thermodynamic field in clouds by modifying the graupel melting and the accretion growth. When Vg decreases, the melting amount of graupel and snow decreases, which results in a decrease of the diabatic heating rate, and thus the decrease of associated downdrafts.

     

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