High wind events in the lee of the Sierra Nevada – are they downslope windstorms? C. David Whiteman Department of Meteorology, University of Utah Sharon Zhong Department of Geography and Center for Global Climate Change and Earth Observing Michigan State University Ju Li Department of Geosciences, University of Houston Institute of Urban Meteorology, Beijing Institute of Urban Meteorology, Beijing Xindi Bian USDA Forest Service Northern Research Station

Motivation © 2004 Matthew Trump  The Owens Valley is one of the largest sources of fugitive dust in the US  The dust storms are a result of high wind and dry, alkali soils in the Owens Lake at the southern end of the valley  Forecasting high wind events and the dust storms has proven difficult

© Marith C. Reheis U.S. Geological Survey Motivation for the study

Objective: To help improve the forecasting ability of high wind events in the Owens Valley and therefore improve dust storm prediction by understanding the driving mechanisms for these events in the Owens Valley. Approach:   Comprehensive analysis of long-term climate data from surface stations within the Owens Valley   Relate the surface wind behavior to synoptic scale winds above the Sierra Nevada

Bishop Big Pine Independence Lone Pine Keeler Olancha Surface Observations  6 GBUAPCD surface stations  Ideally located along the valley axis  Records are near 20 years  Data quality are generally very good

Upper air data DRA REV OAK VBG   No standard upper air sounding stations are within 150 km distance   The data may not be representative of winds above the Owens Valley   Given the complex topography, a simple interpolation of rawinsonde data in the region may not be sufficient

NARR Grid and the Owens Valley

650-mb wind roses Big Pine Lone Pine

Frequency of westerly winds at each station and for each hour of the day FEBMAR APRMAY

Wind roses for each station

Wind roses for winds greater than 7 m/s

Forced Channeling Forced channeling occurs when upper winds are brought down into valleys from aloft and turned to flow along the valley axis © 2000 Whiteman

Pressure-Driven Channeling Pressure-driven channeling occurs when winds blow along valley axis from the high pressure end of the valley to the lower pressure end, while winds aloft, in conformance with the Buys-Ballot rule, blow parallel to the isobars with low pressure to the left. © 2000 Whiteman

Joint frequency distribution of surface and upper-level wind direction for all winds

Joint frequency distribution of surface and upper level wind direction for wind speed greater than 7 m/s

Consistent with the theory of forced channeling

Seasonal variation of high wind events Speed > 7 m/s Speed > 13 m/s

Conclusions:  High wind events occur more frequently in spring and winter season.  Upper level winds are predominantly from westerly quadrants, While surface winds in the Owens Valley appear to be in alignment with the local valley axis, blowing either up or down valley.  In general, wind speeds are higher during the day and lower at night, as expected from the changes in the valley stability between day and night.  High winds are the result of forced channeling of winds aloft. Ambient winds, which are in geostrophic balance above the valley, are channeled by the valley sidewalls, so that within the valley, winds are aligned with the valley axis. Valley wind will shift from up to down valley when geostrophic wind shift from southwest to northwest across a line normal to the valley axis.