Coastal and Orographic Influences on Lake-Effect Systems:Results from Recent Observational and Modeling Studies

Landfalling lake- and sea-effect (hereafter lake-effect) systems often interact with complex coastlines and downstream orography, altering the distribution and intensity of precipitation. In this presentation we synthesize findings from observational and modeling studies of lake-effect systems over the Sea of Japan and the formidable downstream orography of the Japanese Archipelago and Lake Ontario and the 500-m high Tug Hill Plateau.

Japanese meteorologists distinguish between Satoyuki storms with heavy lowland (coastal) snowfall and Yamayuki storms with heavy mountain snowfall. In central Honshu, snowfall is frequently produced by broad-coverage lake-effect systems with longitudinal-mode precipitation bands associated with horizontal roll convection. An analysis of nine winters of radar data from the Japanese Meteorological Agency radar on the central coast of Honshu and real-data simulations of a major multi-day event illustrate that the inland penetration and orographic enhancement of lake-effect precipitation in this region are strongly dependent on the boundary layer wind speed and sea-induced convective available potential energy (CAPE). Higher values favor inland lake-effect penetration and a large ratio of upland to lowland precipitation. These storm characteristics are well described by the non-dimensional mountain height, , with < 1 associated with heavy mountain snowfall and > 1 associated with flow blocking, coastal-front development, and lowland snowfall.

Oval-shaped Lake Ontario frequently produces intense long-lake-axis-parallel (LLAP) precipitation bands during westerly flow. Real-data simulations show that bulges in the south shoreline can produce two land-breeze fronts, one that serves as the locus for LLAP-band formation and another that forms downstream, cuts obliquely across the LLAP band, and enhances precipitation over the Tug Hill Plateau. Sensitivity studies show that an inland precipitation maximum would exist without orography, but the Tug Hill Plateau enhances precipitation through orographic ascent and reduced sub-cloud sublimation.

Idealized numerical simulations indicate that broad-coverage lake-effect systems experience considerable orographic enhancement during flow over large topographic features due to increased convective vigor. In contrast, an inland precipitation maximum appears ubiquitous to LLAP bands even in the absence of orography as unmodified continental airstreams circumscribe the oval lake and form a cold pool downstream of the lake. Ascent at the coastal front at the leading edge of this cold pool enhances hydrometeor growth within the LLAP band, with hydrometeor transport and fallout creating the inland maximum. The addition of a plateau only weakly enhances this inland precipitation, whereas larger topographic obstacles shift the coastal front and associated precipitation offshore with little mountain precipitation