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Presentation
A Synoptic Climatology of Storms Producing Heavy Liquid Precipitation in Lee-side Basins of the Sierra Nevada
American Geophysical Union (2008)
  • S. Jeffrey Underwood, Georgia Southern University
Abstract
Fourteen lee-side heavy liquid precipitation events were identified based on their impact on the flood hydrology of the Truckee River Basin in Nevada. Moderate and extreme precipitation/flooding events were grouped based on the overbank discharge at the downtown Reno, NV USGS gauging station. A composite analysis of synoptic scale circulations preceding heavy rainfall in this lee-side basin was carried out for both the moderate and extreme groups of storms. The analysis confirms that there are two distinct circulation modes that define moderate rainfall events and extreme rainfall events in the lee of the Sierra Nevada. The extreme rainfall events were characterized by an omega blocking circulation pattern poleward of 45°N. This patter was evident from 850hPa through 500hPa. The eastward flanking low pressure area of the omega circulation amplified and reoriented its axis during a 48-hour period prior to heavy lee-side rainfall. This reorientation along with a building ridge centered on the Gulf of California compresss the pressure gradient and shifts the onshore flow from westerly to southwesterly as a surface cyclone matures in the Gulf of Alaska. The moderate rainfall storms did not have the omega blocking circulation pattern and exhibited a more westerly component in the onshore flow. The depth of moisture in the extreme rainfall cases was also greater than the moderate cases. The Oakland, CA sounding for the extreme group was saturated to 750hPa while the moderate group on average was saturated to 850hPa. West-southwesterly winds were more vigorous in the extreme group where 40knot and 60knot winds were common at 700hPa and 500hPa respectively. Both groups exhibited characteristics that distinguish them from storms that exclusively impact the California Coastal Mountains and the windward slopes of the Sierra Nevada. Most notably the lee-side storms exhibited tropospheric flow with a more prominent zonal wind component compared to the strong meridional component in storms producing heavy liquid precipitation on the windward Sierra. Also the lee-side storms possessed more moisture through a greater depth of the troposphere compared to windward storms. Finally it is suggested that the lee-side storms showed evidence of a mid-tropospheric atmospheric river that acted to transport moisture over the crest of the Sierra.
Keywords
  • 1821 Floods,
  • 3354 Precipitation (1854),
  • 3364 Synoptic-scale meteorology
Disciplines
Publication Date
2008
Citation Information
S. Jeffrey Underwood. "A Synoptic Climatology of Storms Producing Heavy Liquid Precipitation in Lee-side Basins of the Sierra Nevada" American Geophysical Union (2008)
Available at: http://works.bepress.com/stephen_underwood/29/