Examination Of The Potential Impacts Of Dust and Pollution Aerosol On Orographic Precipitation In The Colorado River Basin

The Southwest United States has huge demands on water resources. The Colorado River Basin (CRB) covers the states of Colorado, Wyoming, Utah, Nevada, Arizona, New Mexico, and California as well as Mexico and serves 27 million people. The clouds in the CRB are potentially affected by anthropogenic aerosol pollution and dust acting as cloud-nucleating aerosol as well as impacting snowpack albedo.

The specific objectives of this research is to quantify the impacts of both dust and pollution aerosols on wintertime precipitation in the Colorado Mountains. The combined effects of dust serving as ice nuclei (IN), giant cloud condensation nuclei (GCCN), and cloud condensation nuclei (CCN) on precipitation, in combination with anthropogenic pollution aerosol acting as CCN and, in particular, on water resources in the CRB has been examined. This has been done for the entire winter season of 2005. We hypothesized that dust will enhance precipitation, while aerosol pollution will reduce water resources in the CRB via the “spill-over” effect. Since dust is more episodic and aerosol pollution is more pervasive throughout the winter season, we anticipate that the combined response to dust and aerosol pollution is a net reduction of water resources in the CRB. The question is by how much are those water resources affected? The Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS) version 6.0 is used for this study. RAMS was modified to ingest GEOS-CHEM output data and periodically update aerosol fields. GEOS-CHEM is a chemical transport model which uses assimilated meteorological data from the NASA Goddard Earth Observation System (GEOS). The aerosol data comprise a sum of hydrophobic and hydrophilic black carbon and organic aerosol, hydrophilic SOAs, hydrocarbon oxidation and inorganic aerosols (nitrate, sulfate and ammonium). In addition, a RAMS-based dust source and transport model is used for prediction of regional dust sources.

It was found that the combined effects of dust and aerosol pollution lead to a 1.9 % loss of precipitation in the CRB or 4,020,000 acre-feet loss compared to a run with no anthropogenic sources and no dust.

In addition to the seasonal runs, sensitivity studies were also performed. It was found that the precipitation increases when dust is increased 3 times. However, the response is non-monotonic when dust is increased 10 times and the response depends on the environmental conditions. The sensitivity studies show that adding dust in a wet system increases precipitation when IN affects are dominant. For a relatively dry system high concentrations of dust can result in over-seeding the clouds and reductions in precipitation. However, when adding dust to a system with warmer cloud bases, the response is non-monotonic, and when CCN affects are dominant, reductions in precipitation are found. Dust acting as CCN acts in opposition to dust acting as IN. In general, dust acting as IN dominates over dust acting as CCN and thus dust tends to enhance precipitation in wintertime orographic clouds but not as much as it is decreased by anthropogenic pollution.