The mesoscale dynamics of freezing rain storms over eastern Canada
Szeto, K.K., A. Tremblay, H. Guan, D.R. Hudak, R.E. Stewart, and Z. Cao
J. Atmos. Sci.
56, 1261-1281.
ABSTRACT
A severe freezing rain-producing winter storm affected the East Coast of Canada was observed during the CASP II field project. A hierarchy of cloud-resolving model simulations of this storm was performed with the objective of enhancing our understanding of the physical nature of freezing rain storms. The observed features of the system were well replicated in the high resolution simulation. It is found that potential instability and conditional symmetric instability are the prime mechanisms that produced the various banded precipitation features in the model storm. The change of surface roughness from ocean to land in the vicinity of the surface warm front accelerates warm-frontogenesis and intensifies the cross-frontal circulation which in turn lead to the development of an extensive above-freezing inversion layer in the model storm. The phase change of precipitation within the inversion layer and the subsequent evolution of surface precipitation types in the model winter storm was reasonably well simulated. Model results show that perturbations induced by the cooling effects of melting snow on the cross-front flow near the nose of the above-freezing inversion layer might trigger the formation of symmetric or convective overturning above the region and consequently enhance the local precipitation production via a positive feedback mechanism. Model results also show that ageostrophic circulations induced by surface effects, CSI within the frontal zone and cooling-by-melting near the nose of the above-freezing inversion layer can affect significantly the local frontal topography and local baroclinicity of the synoptic frontal zone in the model storm as well as in the observed system. Based on the model results, the prospect for reliable dynamic forecasting of severe winter storm events will be assessed.