Mountain waves and orographic precipitation in a northern Colorado winter storm

Abstract

This study characterizes mountain waves and orographic precipitation associated with a winter storm passing over the ∼3.5 km above mean sea level (MSL) Park Range of northern Colorado on 15 December 2010. Observations from an airborne vertically pointing Doppler radar are used to document reflectivity and horizontal and vertical velocity in 13 two‐dimensional vertical planes extending across the Park Range 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 Park Range is most evident above 4 km MSL and associated with relatively wide, upstream‐tilted updraughts and downdraughts located above the Park Range windward and lee slopes, respectively. The Park Range 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 Park Range are evident below 4 km MSL and associated with rotor circulations composed of relatively narrow updraughts and downdraughts located above the Yampa Valley and the Park Range windward slope. A ∼1 km thick layer of strong vertical shear exists between the mountain waves forced by the Park Range and those forced by upstream terrain. This shear layer exhibits a large vertical displacement over the Park Range, with relatively strong westerly winds plunging to low levels over the lee slope. While precipitation on the Park Range 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.