The topography of Middle Park, a mountain basin surrounded on three sides by higher mountains and on the west side by a topographic constriction through which the Colorado River flows, induces the frequent development of cold-air pools during all times of the year. Using wind profiler and surface meteorological station data obtained from a two-year measurement campaign, this paper provides a climatology of cold-air pooling within Middle Park, examining the frequency and structure of these pools during the 1994-1995 and 1995-1996 winters and the 1995 and 1996 summers.

Westerly ambient winds within and above Middle Park dominate during both the winter and summer months. The presence of these winds simplifies the identification of cold-air pooling events, which appear as periods of surface-based stagnant winds embedded within the westerly flows. For this study, subjective techniques were employed to identify these events using data obtained from a wind profiler located within Middle Park. The cold-air pool was defined as a surface-based region of stagnant air (wind speeds < 1.25 m s-1) that becomes decoupled from the ambient winds. During the summer months these cold pools formed only during the nighttime and early-morning hours while during the winter, cold pools could form at any hour of the day, and at times, last for several days. The winter season showed a large month-to-month variability in the frequency distribution of cold-air pooling with monthly frequency distributions ranging from 27 to 53%. Percentages represent the fraction of total hours within a given month. Winter pooling events were highly correlated with 500-mb ridging over the region and negatively correlated with approaching 500-mb troughs and the associated cold-air advection. The summer season showed very little month-to-month variability in the frequency of cold-air pooling events with monthly frequency distributions ranging from 25 to 33%. While cold pool evolution during the winter months appears to be driven by the synoptic forcing, the daily heating and cooling cycles govern cold pool evolution during the summer months.

A feature somewhat unique to the winter months is the occurrence of cold-air pooling events lasting for at least 24 h and on one occasion for nearly five days. During the two winters analyzed for this study, multi-day cold-air pooling events occurred on ten occasions during the 1994-1995 winter and on three occasions during the 1995-1996 winter. A knowledge of the evolution of these events has important implications to understanding the meteorological conditions associated with poor air quality events in mountain basins during the winter months.