Time-dependent freezing in deep convection and dependencies on aerosol conditions

Ice microphysics may be important for precipitation development in severe storms. Ice production affects their latent heating, vigor and production of intense precipitation, such as damaging hail. Physically, ice is remarkable in the complexity of its diverse morphologies. Owing to their lower bulk density, large ice particles have a wider cross-sectional area for accretion of cloud and can convert cloud to precipitation more efficiently than raindrops of the same mass.

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.