Radar Investigations of Atmospheric Structure and CAT in the 3 to 20-km Region

Abstract

Sensitive radars regularly detect thin horizontal layers in the clear atmosphere. The radar backscattering from these clear-air regions is related to the degree of turbulence and to the mean gradient of potential refractive index. A qualitative description of the atmospheric structure which may exist in regions of CAT is given and arguments are presented to indicate that the radar backscattered signal intensity will be at least a crude estimate of the intensity of high altitude CAT.

Some of the key results of sensitive radar observations above 3 km in the clear atmosphere are reviewed. Most of the clear-air echoes in the upper troposphere and lower stratosphere appear in stratified layers. On occasion, wave patterns in the layers are also apparent, and there is evidence that some of these patterns arise by the breaking of gravity waves. Using the JAFNA radars at Wallops Island, Virginia, it was found that all clear-air radar layers between 3 and 15 km, when probed with an uninstrumented jet aircraft, were turbulent. The ability of these radars to detect CAT decreased with increasing altitude, however, and only the more intense turbulence was detected above 12 km. Preliminary observations of stratospheric echoes, observed with an ultrasensitive L-band radar in Massachusetts, also indicated that the clear-air layers were associated with turbulence sufficiently intense to affect aircraft. This radar has a sensitivity approximately 10 times greater than that of the JAFNA facility and has been able to detect turbulent regions up to altitudes of about 20 km and out to ranges of 200 km. It is concluded from the large number of ultrasensitive radar observations now available that not only are these sensitive systems an important research tool for detecting and investigating atmospheric structures associated with the production and perhaps life cycle of turbulence in the free atmosphere, but that these radars also offer considerable promise of becoming an operational tool for clear-air turbulence detection and warning.