| Snowstorms associated with mid-latitude wintertime cyclones have large impacts on the central and eastern regions of the US. Heavy snowfall and blizzard conditions are commonly observed in the north and northwest quadrants of cyclones. The snowfall is usually organized on two different scales, a wide elongated area of precipitation (∼200--300 km wide, i.e., lower meso-alpha scale) extending along the warm front and wrapping within the trowal around the northwest part of the cyclone, and embedded narrow bands (∼20--80 km wide, i.e., meso-beta scale) propagating through the wide precipitation. In this study, fine scale MM5 model simulations are used to investigate the forcing for precipitation of these two scales in two cyclones observed during SNOWBAND project.; The key findings related to the wide precipitation areas are: (1) Shearing deformation dominated confluence deformation leading to frontolysis and an indirect circulation accounting for the wide swath of snowfall within the trowal region; (2) confluence deformation was dominant from the vicinity of the bifurcation axis eastward along the warm frontal zone leading to frontogenesis and a direct circulation accounting for the precipitation over the warm frontal zone; (3) Diabatic processes associated with latent heat release significantly modified the frontal circulations amplifying by a factor of 2 or greater the ascending motion; (4) Vertical tilting reduced the magnitude of the vertical circulation; (5) The atmosphere in the trowal region and warm frontal zone was subgeostrophic and supergeostrophic, respectively.; The key findings related to the narrow bands are: (1) The simulated embedded narrow bands were oriented vertically upright, their widths and evolution characteristics similar to radar observations; (2) Conditional symmetric instability did not contribute to the formation of the narrow bands, rather, the release of potential instability at the interface of the dry slot and the trowal led to the development of upright convection and initiated the narrow bands. As the initial precipitation moved northward and progressed into the mid-level deformation zone associated with the trowal and warm frontal zone, they were stretched into elongated narrow bands particularly along the strongest and narrow deformation regions; (3) As bands propagated across the trowal, new bands developed at the interface of the dry slot and trowal forming the multi-banded structure. |
