Orographic Effects and Lake Geometry in Idealized Simulations of Banded and Cellular Lake- and Sea-Effect Precipitation Systems

The distribution and intensity of lake- and sea-effect (hereafter lake-effect) precipitation interacting with downstream orography varies with lake-effect mode for reasons that are poorly understood. Here, we use idealized large-eddy simulations to examine the interaction of landfalling lake-effect systems generated by an oval or open lake with a sea-level plain (i.e. no downstream terrain), 500-m bell-shaped peak, or 2000-m smooth ridge. This yields six simulations, with the oval lake/500-m peak and open lake/2000-m ridge simulations representing idealizations of the shape and downstream topography of Lake Ontario and the Sea of Japan, respectively.

Under identical environmental conditions, the oval lake generates thermally forced land-breeze convergence, a coherent mesoscale precipitation band along the major lake axis, and a downstream cold pool. In contrast, the open lake generates open cellular convection with discrete precipitation cells. In the absence of terrain, the band generated by the oval lake produces a narrow strip of heavy precipitation. In contrast, the cells generated by the open lake produce widespread, light precipitation amounting to only one-third the total precipitation produced by the band. The addition of the 500-m peak reduces sub-cloud sublimation for both modes and enhances the snow growth in individual cells, resulting in weak (moderate) orographic enhancement in banded (cellular) lake-effect. Replacing the peak with a 2000-m ridge leads to downstream flow reversal and an over-water precipitation maximum for banded lake-effect. Conversely, the ridge results in a rapid growth in buoyant updrafts and an increase in precipitation by roughly a factor of 9 for cellular lake-effect. These results highlight the distinct influences of lake geometry and orography on individual lake-effect precipitation modes.