A multi‐thermal model of cumulus glaciation via the hallett‐Mossop process

Abstract

AbstractAnalysis of field observations has yielded the conclusion that the Hallett‐Mossop process (H‐M) of secondary ice production plays a major role in the glaciation of summertime cumulus clouds over New Mexico. Other studies have revealed that these clouds possess a characteristic multi‐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 and microphysical parameters play important roles in the glaciation process, a model of ice‐particle growth and splinter production in a simple multi‐thermal framework is developed. the characteristics of the model are prescribed with values that are based on the above‐mentioned field studies. the model cloud possesses four distinct regions: the main updraught, a quiescent (debris) region, the cloud top, and a downdraught region.The trajectories of all primary ice particles introduced into the cloud at t = O, 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 multiplication 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 rapid growth of graupel pellets, large number of thermals, and efficient transport between cloud top and the Hallett‐Mossop temperature band yield the most rapid ice‐particle multiplication.