The WSR-88D weather radar at Denver, Colorado was installed in 1993, at a location approximately 80 km east of the Rocky Mountains. At this location, the radar is able to easily detect the mountains west of the Front Range urban corridor. Consequently, a rather sophisticated ground clutter suppression scheme is necessary to remove most of this unwanted ground clutter.

Despite the fact that this clutter suppression scheme is in effect, there are times when strong returns are seen in the radar data from the mountainous areas, even when little if any precipitation is occurring. These high reflectivity echoes appear to be ground returns, based on their near-zero Doppler velocities and low spectrum widths.

After seeing these odd "mountain" echoes on a number of occasions, it was discovered that their appearance seemed to be well correlated with the occurrence of strong winds in the foothills and the presence of mountain waves. In some instances, the location and appearance of these echoes changed with time, with surface winds exhibiting similar time dependencies. Later investigation revealed that this phenomenon was not unique to the Denver radar, but was seen to the lee of mountains on radars located in Pueblo, Colorado, Cheyenne and Riverton, Wyoming, Great Falls, Montana and Anchorage, Alaska during conditions when mountain lee waves were likely to be present.

The current working hypothesis explaining these anomalous returns is as follows: The radar beam leaves the antenna and propagates through a relatively homogeneous medium for several tens of kilometers. Upon encountering the mountain wave closer to the foothills, within which substantially different refractive properties likely exist, the beam is super-refracted. This allows more energy than is normally suppressed in the clutter suppression algorithm to return to the radar. Hence the radar detects more ground clutter during these mountain wave events than at other more quiescent times. Forecasters have been able to take advantage of this radar signature to aid in forecasting winds and windstorms.

As of late 2001, no proof of the hypothesis is yet available. However, an ongoing cooperative research project between the National Weather Service and the Environmental Technology Laboratory combining ray tracing, numerical modeling and more observational work is underway which will hopefully provide a deeper understanding of the aforementioned phenomenon and either prove or refute the working hypothesis. This presentation will report on the work of this project.