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

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.