Clear Air Turbulence and Refractive Turbulence in Upper Troposphere and Lower Stratosphere

Refractive Turbulence is the main contributor to scintillation of laser communication and directed energy systems. The US Air Force is concerned with global operation of such systems and wishes to know in what regions and seasons their operations could be degraded by refractive turbulence. To answer this a program to directly measure refractive turbulence between 8 and 15 km was developed. The aircraft chosen was the GROB 520 T EGRETT. It is owned and operated by Airborne Research Australia. The NOAA/FRD "BAT" turbulence probe, which is a nine hole pressure sphere with a thermistor for temperature measurement mounted in the central dynamic pressure port, was chosen and three were installed on EGRETT. To determine where and when to measure, the NOAA web site was invaluable in determining the choice the Perth-Adelaide corridor in the southern hemisphere winter (August) and southern coastal Japan in the northern hemisphere winter (February).

Two measurement campaigns were performed in Australia in Aug/Sept 98 and Jul/Aug 99.The single Japan campaign was in Feb 99. In Japan the strongest jet stream wind shears did not always produce the strongest refractive turbulence. and the reason was the lack of local isotropy in the turbulent velocity components. To have local isotropy in the inertial sub-range the ratio of the vertical velocity structure parameter to the longitudinal velocity structure parameter should be 4/3. The EGRETT measurements of this ratio ranged from 0.1 to 0.7 in Japan. The physical cause for this result should be sought in the correlation of fluctuating velocity components with fluctuating pressure gradients. In Australia, the one case where a clear air turbulence event was clearly encountered, local isotropy was observed to be present. Implications of these and other turbulence statistical results from these measurements for turbulence dynamics and for a future measurement strategy to measure clear air and refractive turbulence will be discussed.