Meteorological Factors in Spring Dust Trends in the Southwestern U.S.

Kristi Gebhart1, Jenny L. Hand2, Warren H. White3, Nicole P. Hyslop4, Bret A. Schichtel5 and Thomas E. Gill6

It has recently been observed that fine soil (<2.5 µm diameter) concentrations in the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network in the southwestern United States have increased during March, while remaining relatively stable during other months.  Fine soil concentrations at these mostly rural and remote sites typically peak in spring and early summer, so the March increase is interpreted as an earlier onset of the dust season.  The rate of increase has been approximately 5% per year over 1995-2014 and is consistent with an approximately two weeks earlier beginning of the season.  The March increase in fine soil is ubiquitous over a large four-state region and so is unlikely to be due to local sources or small scale meteorological fluctuations.  There are implications for ecosystems, human health, the hydrologic cycle and visibility.

The temporal trends in March fine soil concentrations are correlated with the Pacific Decadal Oscillation (PDO) which, like the more familiar El Niño and La Niña, is related to sea surface temperatures in the Pacific Ocean, though with a longer periodicity. The PDO is believed to influence the position of storm tracks around the world. To further investigate how the PDO relates to meteorology and ultimately fine soil concentrations, hourly Automated Surface Observing System (ASOS) and Remote Automated Weather Station (RAWS) meteorological data were examined for the southwestern U.S. and back trajectories were generated for IMPROVE sites in Colorado, Utah, Arizona, and New Mexico for 1995-2014. The surface based meteorological data were examined for trends in wind speed, wind direction, temperature, precipitation, and humidity. The back trajectory endpoints, which track transport patterns, were grouped by the dust source regions that they traversed and then meteorology associated with endpoints in each source region were examined for number of endpoints (indicating transport direction), wind speed, temperature, precipitation, humidity, solar radiation, mixing depth, and transport height. The most significant findings are that during the second half of March, in the later years the transport patterns shifted so that IMPROVE sites in the Four Corners states received more air masses from the Sonoran and Mohave Deserts, the Colorado River Basin, and the San Joaquin Valley. Later years were drier than the earlier years as indicated by both lower average relative humidity and lower amounts of and less frequent precipitation. Wind speeds were somewhat higher.

 

1National Park Service, kristi_gebhart@nps.gov
2Colorado State University, jlhand@colostate.edu
3University of California, Davis, whwhite@ucdavis.edu
4University of California, Davis, nmhyslop@ucdavis.edu
5National Park Service, bret_schichtel@nps.gov
6University of Texas, El Paso, tegill@utep.edu