Hail in an Axisymmetric Cloud Model

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  • 1 National Center for Atmospheric Research, Boulder, Colo, 80303
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Abstract

Hail development was studied in a two-dimensional, time-dependent, axisymmetric cloud model with detailed microphysics. A strong relationship between the dynamics and the microphysics of the model appears to govern hail formation through three different stages.

When the cloud is developing, ice crystals which are nucleated on ice nuclei near the cloud top are carried down along the cloud boundary by downdrafts and are reintroduced into the major updraft region by inflow near the cloud base.

When riming of relatively large recycled ice crystals occurs, graupel is formed. The graupel falls along the cloud boundary and the downdraft is intensified. Below the melting level, graupel pellets melt and large drops break up. The resulting small drops are carried into major updraft areas and grow larger by collection processes.

When the updraft in the cloud becomes weaker, because of the accumulation of precipitation elements and the propagation of downward momentum from cloud boundary to cloud center, graupel and small hail near the cloud top fall through the major updraft column. During their fall they capture large drops and relatively large hail is thus formed. Drag forces due to the hail accelerate the downdraft, the cloud then dissipates, and small hail follows. The lifetime of the cloud is approximately 40 min.

Abstract

Hail development was studied in a two-dimensional, time-dependent, axisymmetric cloud model with detailed microphysics. A strong relationship between the dynamics and the microphysics of the model appears to govern hail formation through three different stages.

When the cloud is developing, ice crystals which are nucleated on ice nuclei near the cloud top are carried down along the cloud boundary by downdrafts and are reintroduced into the major updraft region by inflow near the cloud base.

When riming of relatively large recycled ice crystals occurs, graupel is formed. The graupel falls along the cloud boundary and the downdraft is intensified. Below the melting level, graupel pellets melt and large drops break up. The resulting small drops are carried into major updraft areas and grow larger by collection processes.

When the updraft in the cloud becomes weaker, because of the accumulation of precipitation elements and the propagation of downward momentum from cloud boundary to cloud center, graupel and small hail near the cloud top fall through the major updraft column. During their fall they capture large drops and relatively large hail is thus formed. Drag forces due to the hail accelerate the downdraft, the cloud then dissipates, and small hail follows. The lifetime of the cloud is approximately 40 min.

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