HR: 11:00h
AN: A52A-04 INVITED     [Abstracts]
TI: Air Pollution Induced Suppression of Orographic Precipitation and Loss of Water Resources in the Western USA
AU: * Rosenfeld, D
EM: daniel.rosenfeld@huji.ac.il
AF: The Hebrew University, Earth Sciences, Jerusalem, 91904 Israel
AU: Givati, A
EM: givati@vms.huji.ac.il
AF: The Hebrew University, Earth Sciences, Jerusalem, 91904 Israel
AU: Leung, R
EM: ruby.leung@pnl.gov
AF: Pacific Northwest Laboratories, P.O. Box 999, MSIN K9-30, Richland, WA 99352 United States
AB: Suppression of orographic precipitation was observed over the California Sierra and over the hills in Israel, as published by Rosenfeld and Givati (J. Appl. Met., 2004). The suppression was detected as a reduction of the orographic enhancement factor downwind of major urban areas during the last century, which occurred along with the urban development and associated emissions. In contrast, stability in the orographic enhancement factor was observed in pristine coastal ranges. Additional analyses of the whole western USA during wintertime show that the reduction in the orographic precipitation extends eastward all the way to the continental divide. The suppressed precipitation over the hills is compensated by enhanced precipitation over the downwind low lands. We propose the following mechanism: Orographic precipitation occurs from relatively short living and shallow clouds, where slowing down the conversion of cloud water to precipitation can actually reduce the total precipitation falling from the clouds. With less precipitation more water vapor remains in the air downstream. Winter clouds over lowland can be induced only by synoptic uplifting, which creates long-living and relatively deep clouds in which aerosols do not play a major role in the precipitation efficiency. The added water vapor therefore would cause similar addition of precipitation. In the bottom line, precipitation is redistributed from the hills, where it contributes to snow pack and runoff, to the semi-desert low lands where it mostly evaporates. This kind of redistribution leads to a net loss of available hydrological water, in spite of area rainfall depth average not decreasing and even somewhat increasing. Model simulations of this effect are being performed, and initial results will be shown, if already available by the time of the presentation.
DE: 1803 Anthropogenic effects
DE: 1854 Precipitation (3354)
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting