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The Sensitivity of Orographic Precipitation to Flow Direction: An Idealized Modeling Approach

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
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Abstract

A major question regarding orographic precipitation is its sensitivity to flow direction, with some research suggesting substantial sensitivity. To examine this issue, this paper describes a full physics model with realistic three-dimensional terrain that is forced by a single input sounding. This system is used to investigate the sensitivity of orographic precipitation to wind direction over the Pacific Northwest for conditions approximating an atmospheric river. The model results show considerable modulation of regional precipitation as flow direction changes, with results for four Washington State river drainages agreeing well with previous observational studies. It is shown that precipitation amounts over such drainages can vary substantially with very small changes in the direction of the incoming flow. To explore the origin of directional sensitivity of precipitation over the Olympic Mountains of western Washington State, additional experiments were carried out using modified terrain fields with smoothed or idealized Olympic Mountains, or with nearby orography removed. These simulations suggest that the sensitivity of Olympic Mountain precipitation to wind direction is more strongly modulated by the presence of surrounding orography than by the specific geometry of the Olympic Mountains.

Current affiliation: Fleet Numerical Meteorology Oceanography Center, Monterey, California.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Clifford Mass, cmass@uw.edu

Abstract

A major question regarding orographic precipitation is its sensitivity to flow direction, with some research suggesting substantial sensitivity. To examine this issue, this paper describes a full physics model with realistic three-dimensional terrain that is forced by a single input sounding. This system is used to investigate the sensitivity of orographic precipitation to wind direction over the Pacific Northwest for conditions approximating an atmospheric river. The model results show considerable modulation of regional precipitation as flow direction changes, with results for four Washington State river drainages agreeing well with previous observational studies. It is shown that precipitation amounts over such drainages can vary substantially with very small changes in the direction of the incoming flow. To explore the origin of directional sensitivity of precipitation over the Olympic Mountains of western Washington State, additional experiments were carried out using modified terrain fields with smoothed or idealized Olympic Mountains, or with nearby orography removed. These simulations suggest that the sensitivity of Olympic Mountain precipitation to wind direction is more strongly modulated by the presence of surrounding orography than by the specific geometry of the Olympic Mountains.

Current affiliation: Fleet Numerical Meteorology Oceanography Center, Monterey, California.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Clifford Mass, cmass@uw.edu
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