Lake-Effect Rain Events

Abstract

Seven years of autumnal (September?November) precipitation data are examined to determine the characteristics of lake-effect precipitation downwind of Lake Erie. Atmospheric conditions for each lake-effect event are compiled and the mean atmospheric environment for rain events is constructed and compared to conditions for lake-effect snow events. It is found that lake-effect precipitation occurs approximately one out of every five days with a diurnal peak in precipitation intensity during the afternoon and evening. The greatest number of lake-effect days occurs in October followed by November and then September. Comparison of these results to regional precipitation climatologies strongly suggests that the season of lake enhanced precipitation begins in late summer. Precipitation is predominantly rain throughout September and October and snow after the first week of November. A transition period of both rain and snow occurs in early November. Analysis of thunder events for the 7-yr period show a late September to mid-October peak with a decline in frequency by November. The decline in thunder events is due to a seasonal decrease in the depth of the conditionally unstable layer. As might be expected, the mean atmospheric conditions during rain events are similar to those found during lake-effect snow events. This is particularly true with regard to the overall positions of transient synoptic features. Differences are most apparent in the thermodynamic profile of the lower troposphere. Extreme low-level instabilities typically observed in lake-effect snow events are absent from lake-effect rain events. However, in contrast to most snow events, a much deeper layer of conditionally unstable air is usually present during rain events.