Clear Air Turbulence Detection Methods: A Review

Abstract

Of the various methods proposed for CAT detection, only ground-based ultrasensitive radars have demonstrated partial success. However, their size and cost makes an operational system of doubtful economic feasibility. Nevertheless a limited system is proposed in the Appendix. It can now be predicted with some assurance that a 20 db improvement in sensitivity of airborne radars will permit CAT detection at a range of about 10 n. mi., although not with 100% confidence. Serious efforts should thus be made to advance radar technology sufficiently to provide the required improvement. Meanwhile we recommend the use of chaff to mark turbulent regions and appropriately modified existing airborne radars to measure the turbulence intensity. The radar-CAT research has also provided new insight into the nature of CAT. In particular, vertical wind shear in excess of 10⁻² sec⁻¹ appears to be a suitable criterion for its occurrence; while this generally implies Richardson (Ri) numbers less than 1, such a yardstick does not appear to improve the specification of CAT over that attainable with the shear alone. The implication is that any method capable of monitoring wind shear will be a valuable tool in CAT warnings. Also, the greater the thermal stability, the greater the possible shear and the more violent the CAT when it is finally released. This suggests that the magnitude of the refractivity structure constant C ²_n , and thus the radar reflectivity, should be a useful CAT indicator in the upper troposphere, but only at the time and place of its origin. The radar results also imply that ground-based bistatic radio tropo-scatter techniques should be at least equally effective in detecting CAT. These techniques also offer the promise of measuring winds and wind shear.

Dual pulse lidar techniques (using aerosol scatter) for the measurement of differential air velocity in adjacent volumes ahead of the aircraft would become feasible for CAT detection with a 100-fold increase in both sensitivity and coherence of pulsed lasers. Such improvements in lidar technology should thus be encouraged. While IR and microwave radiometric methods do not appear very promising, their relative simplicity and low cost suggests further trials. However, the basic approach should be altered to attempt to detect kilometer-scale temperature perturbations instead of temperature gradients, and temperature inhomogeneities perpendicular to the aircraft path. While the latter are not associated with the turbulence to be expected along the flight path, they are easier to detect and may be almost as well correlated.