P 2 . 1 SIMULATIONS OF ENVIRONMENTAL CONDITIONS CONDUCIVE TO THE FORMATION OF LAKE-TO-LAKE BANDS

Lake-effect snow has a large influence on the Great Lakes region, often enhancing snowfall downwind of the lakes. Several different types of lake-effect bands exist, but lake-to-lake (L2L) bands remain the least studied. L2L bands are cloud bands that extend from one lake across intervening land and a downwind lake, and occur due to preconditioning in the heat and moisture fields of the atmosphere by the upwind lakes. The purpose of this study is to examine conditions conducive to the development of L2L bands. The Weather and Research Forecasting-Advanced Research WRF (WRF-ARW) Version 2.2 model is used to simulate and compare two cases of cold air outbreaks over the western Great Lakes. The case of 02 December 2003 produced L2L bands while the case of 15 February 2007 started with broken lake-effect band production and later converted into a L2L band case. Sensitivity studies involving both an increase and a decrease of 5°K in the lake surface temperatures, as well as a change in the surface roughness length of the land surrounding the western Great Lakes, were conducted to investigate impacts on the L2L bands. Examination of cross-sections and plan-view images of model output reveal that the two cases were accurately simulated by the model. Of significant importance for the formation of lake-effect bands of any type include instability, wind speed and direction, and lake surface-850 hPa temperature difference. For the December case, the northwesterly flow of air over Lake Superior created superadiabatic lapse rates over the water surfaces with mixing continuing over land surfaces. Only a slight decrease in boundary layer depth occurred as air passed over the Upper Peninsula of Michigan. With the lake surface temperatures increased by 5°K, the well-mixed boundary layer and cloud bands expanded in depth and more lake-effect band formation occurred to the south as more heat and moisture became available from the lake surface. The opposite effect happened when the lake surface temperatures were decreased by 5°K; a reduction in bands over land masses was apparent. The February case showed similar boundary layer conditions to the December case initially. However, when the lake surface temperatures were increased by 5°K, the boundary layer remained similar to the base simulation, but L2L bands were delayed, suggesting that very large lakeland temperature differences may hinder L2L bands. The decrease in 5°K again produced thinner cloud and boundary layers. For both events, the change in land-surface roughness length produced no variations in the simulations. Overall, inversion heights were comparable with time between the two cases. Lake surface-850 hPa temperature differences were typically higher for the February simulations, but L2L bands were generally observed to form between 15°C and 24°C in the simulations. Wind speed and direction appears to affect L2L band formation the most in this study. Surface wind speeds at the start of L2L convection were approximately 15 m s -1 in both events. With diminished wind speeds, a change of wind direction from northwest to westerly resulted in the end of L2L bands.