Mountain effects on the atmosphere investigated by Doppler lidar measurements

Because the atmosphere is stable on average at large scales, it resists vertical motions and transports, and flow tends to be horizontal. Mountains produce major disruptions to the horizontal flow by forcing atmospheric motions and transports to deflect vertically, generally on time scales that are shorter than those produced by synoptic-scale uplift in baroclinic systems. Processes that affect large-scale NWP model results and verification include vertical momentum flux, mountain torque on the atmosphere, wave drag, and others, and it is important to assure that many aspects of these effects are properly represented. These effects can be produced by large-amplitude mountain waves, which are often manifested at the earth's surface by downslope windstorms. In the Front Range of the Rocky Mountains and in the European Alps, we have observed mountain-waves and windstorms under a variety of conditions. In this study, examples of several mountain-wave/windstorm structures observed in the lower troposphere are presented and implications for deep propagation of waves are discussed.

Other features routinely observed include streamwise banded structures perpendicular to mountain barriers and valley outflow jets. The streamwise banded structures observed during periods of cross-mountain flow are believed to be observational representations of "potential-vorticity banners" predicted by numerical models. These banners have climatological significance as determining geographical regions of preferred lee cyclogenesis. The valley nocturnal outflow jets (exit jets) have more local effects on the near-surface climatology of a region, including temperature, wind, and air quality. Globally many major cities are located in locations where a mountain valley spills onto a plain, so interest in the behavior of outflow jets is high in these regions.