Interesting FM-CW Radar Data

Since the late 1960's, only a few frequency-modulated continuous-wave (FM-CW) radar systems have been built in order to sense clear-air backscatter within the planetary boundary layer. These systems typically have high average power, very good sensitivity, and the ability to measure backscatter with range gate sizes as small as 0.5 meters with averaging times less than 5 seconds. FM-CW radars are normally not used for wind measurements however, but instead are pointed vertically to obtain continuous reflected power measurements as the clear-air turbulence, hydrometeors, birds, and insects drift over the system. Historically the resulting data was important in showing that very thin turbulence layers (non-beam filling for horizontal pointing radar systems) do exist and were not necessarily insects or other aerobiota. Later the FM-CW technique helped in identifying migrating and foraging birds and bats as a problem for newly developed radar wind profilers and the new more sensitive NEXRAD systems.

The U. S. Army Atmospheric Sciences Laboratory took delivery of an FM-CW radar system in late 1989. Once operational, the system operated continuously for many years in the Tularosa Basin of southern New Mexico. While located in the Chihuahuan Desert, far away from farmland, the data images showed many more (assumed) biological targets than were expected, including many episodes of migrating insects and birds and extraordinary bursts of insects, bats and night-feeding birds shortly after sunset. Other very interesting atmospheric data was seen over seasons and the years, from clear-air turbulence to rain events to strange winter point targets. The data is still important today to help in understanding modern sky-pointed radar systems. This paper will briefly review FM-CW technology and then present many interesting and significant data images obtained from the radar during this time period.