Further Numerical Studies of the Effects of the Great Lakes on Winter Cyclones

Abstract

The effects of the Great Lakes on two vigorous winter cyclones are studied with the aid of an 8-level primitive equations model. The model includes fluxes of heat and water vapor from water surfaces, orography, surface and internal friction, large-scale release of latent heat, and long wave radiation from clouds and water vapor. The grid size is 190 km at 60N. In the first synoptic case, a standing wave is evident in the 1000-mb height field when a grid square is assumed either all water or all land. This is reduced by using realistic fractions of open water for each grid square obtained from ice observations. In the second synoptic case, 36-hr forecasts are made both with a grid square all water or all land (identified as LNS prognoses) and with a grid square containing the actual fraction of open water as computed from ice observations (LS predictions). The standing wave was not apparent in the LNS prognosis, suggesting that its occurrence in the first synoptic case simulation was due to the configuration of heat sources and not to the discontinuity from water to land. After 36 hr the average influences over the Great Lakes as deduced by the LS prognosis are as follows. Isobaric heights are decreased by 18 m at 1000 mb but are increased 7 m at 500 mb. Temperatures and dew points at 850 mb are raised 4C and 5C respectively. Large-scale precipitation is augmented 1.5 mm. In the LNS prediction, the effects are 25?65% greater than in the LS forecast. In both prognoses, the surface low is deepened an additional millibar or so as a result of having crossed the Great Lakes. In the LS prediction, approximately 52% of the water evaporated from the Great Lakes falls out as precipitation and the remaining 48% increases the amount of water vapor in the air.