California Institute for Energy and Environment (CIEE)

Recent publications suggest that anthropogenic aerosols suppress orographic precipitation in California and elsewhere. A field campaign (SUPRECIP: Suppression of Precipitation) was conducted to investigate this hypothesized aerosol effect. The campaign consisted of aircraft measurements of the polluting aerosols, the composition of the clouds ingesting them, and the way the precipitation‐forming processes are affected. SUPRECIP was conducted during February and March of 2005 and February and March of 2006. The flights documented aerosols and orographic clouds flowing into the central Sierra Nevada from upwind densely populated industrialized/urbanized areas and contrasted them with the aerosols and clouds downwind of the sparsely populated areas in the northern Sierra Nevada. SUPRECIP found that the aerosols transported from the coastal regions are augmented by local sources in the Central Valley, resulting in high concentrations of aerosols in the eastern parts of the Central Valley and the Sierra foothills. This pattern is consistent with the detected patterns of suppressed orographic precipitation that occur primarily in the southern and central Sierra Nevada but not in the north. The precipitation suppression occurs mainly in the orographic clouds that are triggered from the boundary layer over the foothills and propagate over the mountains, although the elevated orographic clouds that form at the crest are minimally affected. The clouds are affected mainly during the second half of the day and the subsequent evening, when solar heating mixes the boundary layer up to cloud bases. Local, yet unidentified non‐urban sources are suspected to play a major role.