The Met Office Cloud Resolving Model (CRM) and the UMIST Explicit Microphys
ics Model (EMM) have been employed in the analysis of data from airborne st
udies of a multi-thermal cumulus cloud which developed over New Mexico in t
he summer of 1987. The principal goal was to establish a quantitative under
standing of the observed development of glaciation of this cloud.
The EMM was utilized in a series of tests designed to assess the sensitivit
y of cloud glaciation via the Hallett-Mossop (H-M) process to cloud paramet
ers such as the concentration of cloud condensation nuclei, the cloud-base
temperature, entrainment, and the freezing and splintering of supercooled r
aindrops. These tests with the EMM demonstrate that reductions in the mean
droplet diameter can inhibit the rates of H-M splinter production and auto-
conversion, reducing the rate of accumulation of precipitation at the groun
d and reducing the concentration of ice particles. The warm-rain process in
the EMM is fundamental to the production of graupel, H-M splinters and pre
cipitation.
Good agreement was found between the predictions of the CRM and the availab
le dynamical and microphysical field observations. Analysis of results from
both models indicated that the cloud glaciation is explicable in terms of
the H-M process, with ice production being dominated by the freezing of sup
ercooled raindrops in the H-M band, and the immediate and continuous produc
tion of ice splinters as supercooled droplets freeze onto them.