Evidence of Orographic Precipitation Suppression by Air Pollution–Induced Aerosols in the Western United States

Abstract

Analyses of trends of the orographic winter precipitation enhancement factor Ro along the coastal mountain ranges of the west coast of the United States show a pattern of decreasing Ro during the last century by as much as −24% from the southern border to central California, to no decrease in northern California and Oregon, and to a renewed decrease of Ro (−14%) in Washington to the Seattle area east of the Puget Sound. Similar decreases also occurred well inland, over Arizona, New Mexico, Utah (this study), and the east slopes of the Colorado Rockies (already documented prior to this study). Both absolute precipitation amounts and Ro are affected by fluctuations in the atmospheric circulation patterns, such as those associated with the Pacific decadal oscillation and the Southern Oscillation index. However, these climatic fluctuations cannot explain the observed trends in Ro. Although the trends of aerosols are available only since 1988, aerosol measurements from the Interagency Monitoring of Protected Visual Environments (IMPROVE) aerosol monitoring network show that negative trends in Ro are associated with elevated concentrations of fine aerosols [particulate matter less than 2.5 μm in diameter (PM 2.5)]. The PM 2.5 showed stability or some increase in the areas where their levels were elevated and decreasing trends of Ro were noted. Strong decreasing trends of the coarse aerosols (PM 10–PM 2.5) were noted, especially in the areas with elevated levels of PM 2.5. The decreasing trend in coarse aerosols (the coarse aerosols may act to initiate and enhance precipitation), in conjunction with the constancy and/or increases of the small aerosols (small aerosols suppress precipitation), can explain the continuing decreases in orographic precipitation during the last two decades despite the indicated improvement in the conventional air quality standards.

Localized Climate and Surface Energy Flux Alterations across an Urban Gradient in the Central U.S.

“…A third possible mechanism for an urban influence on precipitation includes destabilization due to the UHI [7]. While it is difficult to generalize the aerosol effect on precipitation across seasons and atmospheric conditions, increased aerosol concentrations lead to decreased precipitation in stratocumulus clouds, small cumulus clouds, orographic clouds, cold-based convective clouds, and dry environment deep convective clouds [33,34]. Increased aerosol concentrations lead to increased precipitation in warm-based deep convective clouds, convective cloud ensembles and squall lines in moist environments [33,34].…”

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

“…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). Slowing the precipitation forming processes in shallow and short lived orographic clouds is expected to cause a net decrease in precipitation amount in the upwind slope of the mountains (Griffith et al, 2005), often with some compensation at the downwind slope Rosenfeld, 2004, 2005;Jirak and Cotton, 2005;Rosenfeld and Givati, 2006;Givati and Rosenfeld, 2007;Rosenfeld et al, 2007;Cotton et al, 2010;Yang et al, 2013).…”

“…Two studies of Givati and Rosenfeld (2004) and Rosenfeld and Givati (2006) showed the decreasing pattern of the ratio between the precipitation amounts over the hills to the precipitation amounts in the upwind lowland at the west coast of the United States during the 20th century. This pattern was associated with a decreasing trend of coarse aerosols, which act as giant CCN, while maintaining or increasing the concentrations of the PM2.5 aerosols.…”

“…This might explain why the orographic enhancement factor continued to decrease despite the efforts to clean the air. An alternative explanation based on changing weather patterns could not be identified, at least not for the western USA (Rosenfeld and Givati, 2006). This question motivated two field campaigns called Suppression of Precipitation (SUPRECIP), which took place in the late winters of 2005 and 2006.…”

Cold and transition season cloud condensation nuclei measurements in western Colorado

“…4304 D. S. Ward and W. R. Cotton: Cold and transition season supercooled cloud droplets reduces the riming efficiency which may lead to a decrease in the liquid water content of precipitation falling as snow or a change in the spatial distribution of the precipitation (Borys et al, 2003;Saleeby et al, 2009). Investigations of precipitation climatology in mountainous regions in the western US have demonstrated a longterm decrease in accumulated precipitation downwind and upslope of urban areas (Givati and Rosenfeld, 2004;Rosenfeld and Givati, 2006). Jirak and Cotton (2006) found similar results along the Colorado Front Range, attributing the lower precipitation to increases in pollution from the urban corridor.…”