HR: 08:30h
AN: U51A-02 INVITED     [PDF]
TI: Urban Aerosol-Induced Changes of Precipitation
AU: * Rosenfeld, D
EM: daniel@vms.huji.ac.il
AF: Institute of Earth Sceinces, The Hebrew University of jerusalem, Jerusalem, 91904 Israel
AU: Givati, A
EM: givati@vms.huji.ac.il
AF: Institute of Earth Sceinces, The Hebrew University of jerusalem, Jerusalem, 91904 Israel
AU: Khain, A
EM: khain@vms.huji.ac.il
AF: Institute of Earth Sceinces, The Hebrew University of jerusalem, Jerusalem, 91904 Israel
AU: Kelman, G
EM: superk@pob.huji.ac.il
AF: Institute of Earth Sceinces, The Hebrew University of jerusalem, Jerusalem, 91904 Israel
AB: Precipitation has been shown often to be enhanced over and downwind of major urban areas. These effects were most noticeable in convective clouds of the warm season, and were ascribed to dynamic causes, such as the urban heat island and the urban topography. In contrast, recent observations have shown that urban and industrial aerosols suppress rain and snow, by providing large concentrations of small cloud condensation nuclei (CCN), which when ingested into the clouds create large number of small cloud droplets. The clouds need to grow to greater depth and colder cloud top temperatures for the onset of precipitation. Therefore it was expected that precipitation would be most suppressed when the pollution is ingested into relatively shallow and short-living clouds. Such clouds occur in winter over orographic barriers downwind of coastal cities. This was examined by measuring the orographic enhancement factor (RO, ratio of rainfall over the hills versus the upwind lowland rainfall) of the precipitation downwind of the urban areas in California and Israel. It was found that in both locations RO has decreased by 15 to 25 percent during the last century whereas no change occured in side wind little urbanized areas. Some compensation occurred further downwind on the drier downwind side of the mountains. The aerosols suppress the onset of precipitation also in warm-base convective clouds that develop in moist tropical air masses, such as occur during the summer in Houston and St. Louis. Simulations of clouds with explicit microphysical processes show that the delay of the onset of the warm rain in such clouds delays also the onset of the downdraft, and by that allows the updraft to invigorate further and produce more violent and precipitation producing thunderstorms. This is proposed as an additional enhancement mechanism that works at the same direction of the urban heat island. The conversion from warm to ice precipitation processes advects more cloud water to the supercooled levels higher in the cloud. The added cloud water and updrafts are the two ingredients that electrify clouds, thereby providing also for more intense and frequent thunderstorms. Such enhancement in thunderstorm activity over and downwind of Houston has been observed, and awaits in situ observations for confirmation of the proposed mechanism, in the form of the proposed HEAT project. In summary, air pollution aerosols can have large impacts on the precipitation and dynamics of the clouds for both suppression and enhancement of precipitation. The pollution tends to suppress precipitation from relatively shallow and short living clouds. In contrast, the delay of precipitation in deep convective clouds that form in tropical air mass can invigorate them and the dynamical feedback can result in overall enhanced precipitation amount.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
DE: 0345 Pollution--urban and regional (0305)
DE: 1854 Precipitation (3354)
DE: 3314 Convective processes
SC: U
MN: 2003 Fall Meeting