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Regional aerosol effects on precipitation: an observational study


There have been a multitude of studies on the effects increased amounts of aerosols may have on clouds. The connection between increased cloud condensation nuclei (CCN) and cloud microphysics has been established by in situ observations as well as modeling studies. However, the impact on precipitation is less well established. Of the studies that have assessed aerosol effects on precipitation most have been limited to modeling studies or global studies using satellite data. The few observational studies that have examined these relationships have been mainly limited to data from short-lived field campaign, such as oceanic stratocumulus decks or biomass burning areas. This study attempts to examine regional aerosol effects on precipitation in areas not previously examined in field campaigns, using data from two different sites, one from an Atmospheric Radiation Measurement (ARM) Program permanent facility in Oklahoma and the other from a mobile facility located in the Azores. These two sites were chosen in order to illustrate the differences between a marine and a continental location. Meteorological conditions were taken into account in both locations through surface and sounding data and trends in precipitation were found with increasing aerosol concentrations. The marine site witnessed a suppression of precipitation, consistent with past studies and proposed theories of aerosol effects. This was not true for clouds with liquid water paths exceeding 200g/m2. These clouds appear to contain sufficient amounts of water to overcome the aerosol effect. The continental site, however, experienced an opposite trend, with enhancement of precipitation witnessed in all clouds examined in this study. This is thought to be due to a buffering mechanism in these types of clouds, as introduced by Stevens and Feingold (2009). Results were separated by season and cloud type using the horizontal variability of radar reflectivity at cloud top height. The seasonal results generally either were in line with the year round results or were too noisy to interpret. The results separated by cloud type give a concrete result, illustrating the fact that differing cloud dynamics may lead to opposing trends in precipitation with increasing aerosols. Competing effects of aerosols within clouds appear to dampen any effect on precipitation to the point that it is not detectable from the in-situ observations considered here.


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