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

Abstract

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 in situ 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 the aerosols and orographic clouds flowing into the central Sierra Nevada from the 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 greatly 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, occurring 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. 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 nonurban sources are suspected to play a major role.

Polluting of winter convective clouds upon transition from ocean inland over central California: Contrasting case studies

“…This situation occurred on the morning of 16 February 2011. It was previously documented in the study area during rainy conditions by Rosenfeld et al (2008). Very low PCASPmeasured aerosol concentrations were observed near cloud base (6-30 cm −3 , Fig.…”

“…These previous studies (Andreae et al, 2004;Rosenfeld et al, , 2008Freud and Rosenfeld, 2012) showed that warm rain is initiated when the cloud drop effective radius, r e , exceeds 12-14 μm, or when the volume-weighted modal size of the cloud drop size distribution, D L , exceeds 24 μm. The cloud drops grow with D until r e or D L exceeds the values for initiation of warm rain.…”

“…This slows the drop coalescence and in turn the conversion of cloud water into rain drops (Rosenfeld, 2000;Hudson and Yum, 2001;McFarquhar and Heymsfield, 2001;Yum and Hudson, 2002;Borys et al, 2003;Andreae et al, 2004;Hudson and Mishra, 2007;Rosenfeld et al, 2008;Flossmann and Wobrock, 2010). It also slows the mixed phase precipitation forming processes by decreasing the riming and growth rate of ice hydrometeors (Borys et al, 2003;Saleeby et al, 2008).…”

“…Aircraft measurements of the interactions of clouds and aerosols were made over the Central Valley and Sierra Nevada, CA. The objective was understanding the relationships between aerosol sources and orographic precipitation (Rosenfeld et al, 2008). One of the key findings was that CN and CCN concentrations in the Central Valley are much higher than the concentrations in the coastal urban areas, which implies that the Central Valley itself is a source of high concentrations of pollution aerosols.…”

“…As a result, losses of orographic precipitation in the Sierra Nevada could be ascribed to the pollution from the Central Valley. In addition, nonurban sources may play a major role in determining the properties of the clouds over the foothills of the Sierra Nevada (Rosenfeld et al, 2008). The SUPRECIP campaigns were followed in the late winter of 2011 by field campaign called California Water (CalWater), aimed at better understanding of the impacts of the various aerosols (local pollution coming from the boundary layer, long range transport pollution coming aloft, and pristine marine air) on the properties and precipitation of the water and mixed phase clouds over central California.…”