Climatology of Diffusion Potential Classes for Minneapolis–St. Paul

Abstract

This climatological study reports on the potential for atmospheric diffusion at Minneapolis?St. Paul, Minnesota, cities located in the heart of the North American continent. As such, the results can be considered typical of an urban setting within a continental climate. Data were obtained from a nearly continuous 8-yr record of vertical temperature and wind measurements made on a 152.4-m tower. Temperature lapse rates between 21.3 and 152.4 m were grouped into three stability categories: 1) isothermal?inversion, 2) subadiabatic, and 3) superadiabatic. A subdivision of each was based upon wind speeds of less than 4 m s?1 and greater than or equal to 4 m s?1, resulting in six classes that were examined according to wind direction, time of day, time of year and, most importantly, the associated synoptic conditions. The isothermal?inversion condition was limited to nighttime periods, especially when high pressure centers were dominant and winds were less than 4 m s?1. The highest frequency of occurrence was during midsummer, while the lowest was during late fall and early winter. The subadiabatic condition was primarily a nighttime phenomenon, except for the winter season when it was also common during the day. An interesting feature of the diurnal frequencies was that a morning and evening subadiabatic peak occurred due to the transition between nighttime stable and the daytime unstable conditions. The superadiabatic condition was mainly a daytime phenomenon and dominated the early afternoon period throughout the year. The lowest diffusion potential, a result of very stable air and light winds, occurred during the nighttime period, particularly when under the influence of a high pressure center. Weak to moderate diffusion potential, found to occur with weakly stable air and light to moderate winds, was associated with the perimeter of the high pressure center and also with overcast skies near a low pressure center. This condition normally occurred during the night as well as during windy days. Moderate to high diffusion potential, resulting from superadiabatic conditions and light to moderate winds, dominated the early afternoon period. Strong nighttime ventilation was restricted to the winter season when northwesterly winds dominated the region immediately behind a cold front.