The role of upstream lakes in determining downstream severe lake-effect snowstorms

A severe lake-effect snowstorm on 25–26 December 1993 was successfully modeled using the Penn State/NCAR MM5 model at 7 km grid resolution. Another simulation performed with Lake Michigan removed from the domain resulted in a two-thirds reduction in snowfall maxima, reduced vertical ascent (50% smaller maxima), and cloud depth (50–100 mb in vertical depth) upwind and over Lake Erie. The downstream snowband shifted northward and eastward as a consequence of stronger background flow and weaker shoreline convergence in the removed case. An idealized model of two-lake interaction was devised using an alternative set of MM5 preprocesses, allowing the definition of simple boundary conditions subject to a single thermal profile applied across the domain. Fixed background winds, temperature, and humidity were then allowed to interact with a surface boundary composed of flat land and elliptical lakes. In the absence of orography and synoptic-scale transients, model responses could be attributed directly to mesoscale forcing via the thermal and frictional disparity between land and water. Results showed that multi-lake interaction did occur once heat and moisture advected from the upstream to downstream lakes (12–18 hours under 10 m s–1 background flow). When background conditions known to produce strong lake-effect snowband development are imposed, the heat and moisture plume from the upstream lake warmed the CIBL between the two lakes by 4– 6°C, and elevated cloud liquid water by amounts exceeding 0.20 g kg–1. The dynamical adjustment resulting from the upstream surface-forcing lowered pressure by 1.5–2.0 mb downwind of the upstream lake; produced a mesoscale low pressure with flow reversal along the northern one-third of the upstream lake; and accelerated flow downwind of the lower one-third of this lake. Removal of turbulent heat fluxes from the upstream lake demonstrated that sensible heating is directly responsible for the establishment and maintenance of the upstream local pressure perturbation through dynamic adjustment or thermal troughing. Suppression of moisture availability or latent heat flux from the upstream lake greatly reduces precipitation amounts and areal extents over the downstream lake, much more so than removal of both latent and sensible heat fluxes, or removal of the lake itself.