A MULTI-THERMAL MODEL OF CUMULUS GLACIATION VIA THE HALLETT-MOSSOP PROCESS

Analysis of field observations has yielded the conclusion that the Hal lett-Mossop process (H-M) of secondary ice production plays a major ro le in the glaciation of summertime cumulus clouds over New Mexico. Oth er studies have revealed that these clouds possess a characteristic mu lti-thermal structure. In an effort to quantify more fully the role of H-M in such clouds, and to establish which of the salient dynamical a nd microphysical parameters play important roles in the glaciation pro cess, a model of ice-particle growth and splinter production in a simp le multi-thermal framework is developed. The characteristics of the mo del are prescribed with values that are based on the above mentioned f ield studies. The model cloud possesses four distinct regions: the mai n updraught, a quiescent (debris) region, the cloud top, and a downdra ught region. The trajectories of all primary ice particles introduced into the cloud at t = 0, together with those created as a consequence of the operation of H-M, are followed as they grow and are transported around the cloud. The sensitivities of these trajectories and a multi plication factor f to variations in parameters such as updraught speed , liquid water content, L, thermal depth, inter-thermal interval, and downdraught characteristics are examined. These tests reveal that f is particularly sensitive to the values of L in the distinct regions of the cloud. Basically, combinations of parameter values which produce r apid growth of graupel pellets, large number of thermals, and efficien t transport between cloud top and the Hallett-Mossop temperature band yield the most rapid ice-particle multiplication.