The Mesoscale Dynamics of Freezing Rain Storms over Eastern Canada

Abstract

A severe ice storm affected the east coast of Canada during the Canadian Atlantic Storms Project II. A hierarchy of cloud-resolving model simulations of this storm was performed with the objective of enhancing understanding of the cloud and mesoscale processes that affected the development of freezing rain events. The observed features of the system were reasonably well replicated in the high-resolution simulation. Diagnosis of the model results suggests that the change of surface characteristics from ocean to land when the surface warm front approaches Newfoundland disturbs the (quasi-) thermal wind balance near the frontal region. The cross-frontal circulation intensifies in response to the thermal wind imbalance, which in turn leads to the development of an extensive above-freezing inversion layer in the model storm. Depending on the depth of the subfreezing layer below the inversion, the melted snow may refreeze within the subfreezing layer to form ice pellets or they may refreeze at the surface to form freezing rain. Such evolution of surface precipitation types in the model storm was reasonably well simulated in the model. Model results also show that the horizontally differential cooling by melting near the nose of the above-freezing inversion layer enhances the local baroclinicity, which in turn induces perturbations on the cross-front flow. Depending on stability of the ambient flow, such local flow perturbations may trigger symmetric or convective overturning above the region and consequently enhance the local precipitation production via a positive feedback mechanism.