Simulations of the glaciation of a frontal mixed-phase cloud with the Explicit Microphysics Model

Simulations with the Explicit Microphysics Model (EMM) of a case of lightly precipitating, glaciated stratiform cloud are presented. This frontal cloud was observed by the UK Met Office C-130 aircraft and the dual-polarization radar at Chilbolton in southern England. The Hallett-Mossop (H-M) process was found to cause extremely high number concentrations of crystal columns of up to almost 1000 1(-1) in the H-M region (-3 to -8 degreesC) of the updraught in the EMM control simulation. Similarly high number concentrations of ice particles were seen in, and in the vicinity of, ascending thermals in the aircraft observations. Such concentrations are orders of magnitude higher than the ice nucleus (IN) concentration. Moreover, the dendritic growth of highly planar particles of primary ice produces a peak in differential reflectivity of almost 4 dB just below the cloud top (-15 degreesC). Primary ice particles are generally larger than H-M splinters and mostly determine the simulated radar properties of the model cloud. Tests with the EMM revealed significant sensitivities of the average ice number concentration, the cloud- and ice-water paths, and the surface precipitation rate, to atmospheric concentrations of IN and cloud condensation nuclei for this frontal-cloud case. The layer of supercooled cloud-water located just below cloud top in the control is depleted by evaporation when the IN concentration is augmented.