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

Measurements by VHF wind-profiling radars worldwide, for many years, have shown downward atmospheric motion in the troposphere. Several explanations, both real and caused by the radar measurement method, have been proposed. At many sites, although perhaps not those in the Tropics, we suggest this long-term downward vertical wind (W) may be a consequence of radars being constructed on low ground near or within mountain ranges, so that they tend to view the same phase of mountain waves. A coincidence of five factors - the radar location, the upwind slope of untrapped mountain-wave phase lines with height, the inability of radars to measure the lowest few kilometers of the troposphere or far into the stratosphere, decreasing static stability with height in the troposphere, and increasing wind speed with height in the troposphere - together cause downward W in the mid-troposphere, the height range most commonly measured by these radars. The new model may account for unexplained observations at several sites.