The glaciation of a cumulus cloud over New Mexico

The Met Office Cloud Resolving Model (CRM) and the UMIST Explicit Microphysics Model (EMM) have been employed in the analysis of data from airborne studies of a multi-thermal cumulus cloud which developed over New Mexico in the summer of 1987. The principal goal was to establish a quantitative understanding of the observed development of glaciation of this cloud. The EMM was utilized in a series of tests designed to assess the sensitivity of cloud glaciation via the Hallett-Mossop (H-M) process to cloud parameters such as the concentration of cloud condensation nuclei, the cloud-base temperature, entrainment, and the freezing and splintering of supercooled raindrops. 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 ground 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 precipitation. Good agreement was found between the predictions of the CRM and the available dynamical and micro-physical 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 supercooled raindrops in the H-M band, and the immediate and continuous production of ice splinters as supercooled droplets freeze onto them.