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Mountain waves and orographic precipitation in a northern Colorado winter storm

2015; Wiley; Volume: 142; Issue: 695 Linguagem: Inglês

10.1002/qj.2685

1477-870X

David E. Kingsmill, Ola Persson, Samuel Haimov, Matthew D. Shupe,

Tropical and Extratropical Cyclones Research

This study characterizes mountain waves and orographic precipitation associated with a winter storm passing over the ∼3.5 km above mean sea level ( MSL ) P ark R ange of northern C olorado on 15 D ecember 2010. Observations from an airborne vertically pointing D oppler radar are used to document reflectivity and horizontal and vertical velocity in 13 two‐dimensional vertical planes extending across the P ark R ange from upstream of the windward slope, over the crest and downstream of the lee slope. The winter storm investigated in this study is associated with a general zonal flow over the western continental USA and significant vertical wind shear between 700 and 500 hPa . A vertically propagating wave forced by the P ark R ange is most evident above 4 km MSL and associated with relatively wide, upstream‐tilted updraughts and downdraughts located above the P ark R ange windward and lee slopes, respectively. The P ark R ange also forces a trapped lee wave that manifests itself as a relatively erect updraught ∼15–20 km east of the crest. Smaller‐scale trapped lee waves forced by terrain upstream of the P ark R ange are evident below 4 km MSL and associated with rotor circulations composed of relatively narrow updraughts and downdraughts located above the Y ampa V alley and the P ark R ange windward slope. A ∼1 km thick layer of strong vertical shear exists between the mountain waves forced by the P ark R ange and those forced by upstream terrain. This shear layer exhibits a large vertical displacement over the P ark R ange, with relatively strong westerly winds plunging to low levels over the lee slope. While precipitation on the P ark R ange windward slope is generally enhanced for the event, data analysed for this case surprisingly does not show a spatially and temporally consistent correlation between mountain‐wave kinematic structures and orographic precipitation. Transient processes such as wave‐regime interactions may have masked this correlation.