Lake Michigan lake breezes: Climatology, local forcing, and synoptic environment

A method was developed to identify the occurrence of lake-breeze events alo ng the eastern, western, and both shores of Lake Michigan during a 15-yr pe riod (1982-96). Comparison with detailed observations from May through Sept ember of 1996-97 showed that the method reasonably identified Lake Michigan lake-breeze events. The method also demonstrated the important ability to distinguish non-lake-breeze events; a problem experienced by previously dev eloped lake-breeze criteria. Analyses of the 15-yr climatological data indi cated that lake breezes tended to occur more frequently along the eastern s hore of Lake Michigan than along the western shore. On average, a maximum n umber of lake-breeze events occurred during August at each location. This m aximum is most closely associated with weaker monthly average wind speeds. Even though the air-lake temperature difference DT provides the local forci ng for the development of the lake-breeze circulation, large temperature di fferences are not required. Nearly 70% of all events occurred with a daytim e maximum DeltaT less than or equal to 12 degreesC. The evaluation of a lak e-breeze index epsilon used in past studies and many forecasting applicatio ns showed indices computed using offshore or shore-perpendicular wind speed s (U or |U|, respectively) at inland sites resolved greater than or equal t o 95% of identified events based on critical epsilon values of 2-6. When wi nd speed, irrespective of wind direction, was used to calculate epsilon, th e success of the critical indices decreased by as much as 26%. Results also showed that the lake-breeze index has a considerable tendency to overestim ate the number of events. Although the possibility was suggested by previou s investigations, the critical value of epsilon may not be appreciably affe cted by changes in location along the shoreline. In addition, noteworthy di fferences in the position of synoptic-scale sea level pressure and wind fie lds with respect to Lake Michigan were found to occur during eastern, weste rn, and both-shore lake-breeze events.