Time-dependence of freezing arises because any freezing can only progress as fast as latent heat can be dissipated to the ambient air. This time-dependence may influence the fall-speeds of the precipitation particles and their trajectories through the cloud. Two types of time-dependent freezing are considered here.
First, 'wet growth' occurs when supercooled liquid is accreted onto ice precipitation, only a fraction of it freezes. So, the surface of the ice precipitation becomes permanently wet, if the supercooled cloud-liquid cannot freeze faster on the ice particle than it is accreted. Second, raindrop-freezing is not instantaneous. Laboratory experiments show that it takes up to a minute or so for large supercooled raindrops to freeze when forming hail or graupel. Raindrop-freezing is a major source of ice precipitation particles aloft in vigorous deep convection. Rain, in turn, is generated by coalescence of cloud-droplets in convection.
In this presentation, a new treatment of both types of time-dependent freezing is described. For wet growth, we have created a theoretical model that treats the inhomogeneity of surface temperature observed in laboratory experiments. The new model treats both wet and dry components of the surface of each hail/graupel particle. Both components of the surface typically co-exist at high enough liquid water contents and warm enough sub-zero temperatures in cloudy air, as seen in observational studies. The laboratory experiments with the wind tunnel at the University of Toronto are simulated off-line by our novel '2-component wet growth scheme'. Results from this off-line validation are shown.
Cloud simulations are presented, after including the new freezing schemes in the Hebrew University Cloud Model (HUCM). This model has spectral bin microphysics and 6 species of ice. Impacts on precipitation development and on the simulated radar reflectivity from such time-dependence of freezing are shown. Dependencies on aerosol conditions of the distributions of freezing and of its latent heating are shown, for wet growth and raindrop-freezing. Repercussions for the dynamics of deep convective cells are discussed.