Changing temperature inversion characteristics in the U.S. Southwest and relationships to large-scale atmospheric circulation

To understand how temperature inversions could change with climate, we investigate inversion characteristics over 15 years and analyze associations between large-scale circulation and local inversion activity in a region of complex terrain. Upper air soundings in six cities in the American Southwest show remarkable declines in inversion activity between 1994 and 2008, though these declines are not necessarily linear. Inversion intensity and height appear to have simultaneously increased, suggesting weaker, near ground-level inversions preferentially decreased in number. Associations between wintertime inversion frequency and large-scale circulation are then quantified from a climatology perspective. The self-organizing map technique is used to create 24 representative circulation patterns for the 15-year period using 500 hPa geopotential height fields from the North American Regional Reanalysis; and inversion frequencies are calculated for each of the 24 circulation patterns by site. Inversion activity in Salt Lake City, Utah and Albuquerque and Santa Teresa, New Mexico is found to correspond well with large-scale circulation, however correlations between inversion activity and large-scale circulation in Denver, Colorado and Flagstaff and Tucson, Arizona are weak. These findings indicate dynamical changes with climate will not uniformly influence inversions and hence urban air quality conditions in the American Southwest. Furthermore, though we interpret our results cautiously due to the short length of the time series, we note that strong decadal variability in inversion activity may complicate long-term forecasts of air quality conditions.