Aircraft measurements of the impacts of pollution aerosols on clouds and precipitation over the Sierra Nevada

Abstract

Satellite measurements in onshore-flowing clouds showed that they become more microphysically continental downwind of areas of major emissions of anthropogenic aerosols. Rain gauge analyses of orographic precipitation showed that the upslope precipitation in mountain ranges downwind was decreased with respect to the coastal precipitation during the 20th century along with the assumed increase in pollution aerosols. Following the publication of these findings a research effort called SUPRECIP (Suppression of Precipitation) was conducted to make in situ aircraft measurements of the polluting aerosols, the composition of the clouds ingesting them, and the way the precipitation forming processes are affected. Preliminary results of SUPRECIP Phase 2 2006 are reported here. The program is funded by the PIER (Public Interest Energy Research) Program of the California Energy Commission. The flights documented the aerosols and orographic clouds downwind of the densely populated areas in the central Sierra Nevada and contrasted them with the aerosols and clouds downwind of the sparsely-populated areas in the northern Sierra Nevada. The main results from the February 2005 campaign of SUPRECIP-1 are: 1. The in situ aircraft measurements of the cloud microstructure validated the satellite retrievals of cloud particle effective radius and microphysical phase. 2. Ample supercooled drizzle were found in the pristine orographic clouds with only few tens of drops cm-3, and no drizzle with small concentrations of graupel were found in clouds with drop number concentrations of ∼ 150 cm-3. 3. The pristine clouds occurred in air masses that were apparently decoupled from the boundary layer in the early morning, whereas the more microphysically continental clouds occurred during the afternoon, when the surface inversion over the Central Valley disappeared. Based on what was learned during the first season, a second field campaign was conducted in February and March 2006, called SUPRECIP-2. The cloud physics instruments were enhanced with another cloud drop spectrometer, and a second low level aerosol airplane was added. Two cloud physics aircraft were involved, making measurements of CCN, CN, cloud drop size distribution, hydrometeor images and size distribution, thermodynamic properties of the air and air 3-D winds. SUPRECIP-2 was augmented also by surface measurements of aerosols and chemical composition of the hydrometeors, made by collaborating research groups from the Desert Research Institute of the University of Nevada, The University of California Davis, and the SCRIPPS Oceanographic Institute of the University of California at San Diego. This provided coincident measurements of the low level aerosols and the properties of the clouds that ingest them. Preliminary results, reported here, confirm the link between anthropogenic aerosols and suppressing precipitation forming processes in the clouds in the context of California.