Bias in mean vertical wind measured by VHF radars: Significance of radar location relative to mountains

Measurements by VHF wind-profiling radars worldwide have, for many years, shown long-term downward atmospheric motion in most of the troposphere. Various explanations have been proposed, some atmospheric and some caused by the radar measurement method. At many sites, except those in the Tropics, the authors suggest a new model, in which the long-term downward vertical wind ((W) over bar) is real and atmospheric, a consequence of radars being constructed on low ground near or within mountain ranges, so that they tend to view on average the same phase of mountain waves. A coincidence consisting of five factors-low-lying radar locations near mountains, the upwind slope of untrapped mountain wave phase lines with height, the inability of wind-profiling radars to measure both the lowest few kilometers of the troposphere and far into the stratosphere, decreasing tropospheric static stability with height, and increasing tropospheric wind speed with height-has led to downward W measurements in most of the troposphere. The new mountain wave model applies to all types of wind-profiling radars and lidars, and may account for unexplained (W) over bar observations at many sites.