High wind events in the lee of the Sierra Nevada – High wind events in the lee of the Sierra Nevada – are they downslope windstorms?are they downslope windstorms?

C. David WhitemanC. David WhitemanDepartment of Meteorology, University of UtahDepartment of Meteorology, University of Utah

Sharon ZhongSharon ZhongDepartment of Geography and Center for Global Climate Change and Earth Department of Geography and Center for Global Climate Change and Earth

ObservingObservingMichigan State UniversityMichigan State University

Ju LiJu LiDepartment of Geosciences, University of HoustonDepartment of Geosciences, University of Houston

Institute of Urban Meteorology, BeijingInstitute of Urban Meteorology, Beijing

Xindi BianXindi BianUSDA Forest Service Northern Research StationUSDA Forest Service Northern Research Station

Motivation

© 2004 Matthew Trump© 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. Marith C. ReheisReheisU.S. Geological U.S. Geological SurveySurvey

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

BishoBishopp

Big PineBig Pine

IndependencIndependencee

Lone PineLone Pine

KeelerKeeler

OlanchaOlancha

Surface ObservationsSurface Observations

6 GBUAPCD surface stations

Ideally located along the valley axis

Records are near 20 years

Data quality are generally very good

Upper air Upper air data data

DRADRA

REVREV

OAKOAK

VBGVBG

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 ValleyNARR Grid and the Owens Valley

650-mb wind roses 650-mb wind roses

Big PineBig Pine Lone PineLone Pine

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

FEBFEB MARMAR

APRAPR MAYMAY

Wind roses for each Wind roses for each stationstation

Wind Wind roses for roses for winds winds greater greater than 7 than 7 m/sm/s

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

Pressure-Driven Channeling

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

© 2000 Whiteman© 2000 Whiteman

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

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

Consistent with the theory of forced Consistent with the theory of forced channelingchanneling

Seasonal variation of high wind events

Speed > 7 m/sSpeed > 7 m/s

Speed > 13 Speed > 13 m/sm/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.