6.2 On the parameterization of immersion freezing of raindrops (Based on Bigg, 1953)

Villanueva-Birrel, Cecille M., and Sonia Lasher-Trapp, Purdue University

Cloud models use microphysical parameterizations to represent small-scale processes that otherwise would not be explicitly resolved in the model, or to represent processes that are not well constrained by observations. Most microphysical schemes, including those within the Weather Research and Forecasting (WRF) model, use the laboratory results of Bigg (1953) to parameterize immersion freezing of supercooled raindrops in the cloud without explicit dependence upon ice nuclei number concentrations. In convective storms, frozen raindrops will add to the production of graupel that upon melting forms the bulk of convective rainfall. Using the standard parameterization, an appreciable percentage of frozen raindrops can be found at a temperature around -15 oC. However, recent observations that will be presented show that frozen raindrops and even graupel particles can be found at much higher temperatures. Because a considerable fraction (sometimes as much as 40%) of the surface rainfall can be attributed to raindrop freezing in a convective storm, it is important to understand this sensitivity to precipitation production within the cloud, and also how it affects the overall precipitation efficiency of the storm. Such an analysis using simulations of deep convective storms using the WRF model will be presented.