Real-Time Prediction of the Lake Breeze on the Western Shore of Lake Michigan

Abstract

A forecast verification study of the occurrence and inland penetration of the lake breeze on the western shore of Lake Michigan was conducted. A real-time version of The Pennsylvania State University?National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) with 5-km grid spacing was evaluated for a set of 71 (68) dates at 12?24-h (36?48-h) forecast range from summer 1998 and spring 1999. Contingency measures showed skill in predicting lake-breeze development at both day 1 and day 2 [Kuipers skill score (KSS) of 0.80 and 0.74, respectively]. These skill levels exceeded capabilities demonstrated by a simple lake-breeze index computed for the identical set of cases and also surpassed the published performance of the operational 29-km Eta Model for another site. Skill in the prediction of the inland penetration of the lake breeze (relative to a baseline defined by the sample average distance of the lake-breeze front from the shoreline) peaked at 2100 UTC at both day 1 and day 2 (KSS of 0.28 and 0.22, respectively) but fell thereafter due to a westward forecast bias. The origins of this bias are tied in part to errors in the static model specification of the lake surface temperature, resulting from both errors in initialization and the observed diurnal cycle. Skill is also constrained by the sensitivity of the lake breeze to the prevailing synoptic flow; modest errors in forecasts of coast-normal winds can lead to substantial errors in the forecast position of the lake-breeze front by 0000 UTC. It is suggested that future research should focus on coupling a meteorological model to a dynamic lake model, improved initialization of lake water temperatures, and further refinements of the planetary boundary layer physics to improve near-surface winds. Such a model may be needed to allow reliable extension of forecasts out to timescales beyond 48 h.