The 13th Symposium on Boundary Layers and Turbulence

### 3B.5

MIDDLE ATMOSPHERE SLANT-PATH OPTICAL TURBULENCE CONDITIONS DERIVED FROM VHF RADAR OBSERVATIONS

Frank D. Eaton, Air Force Research Laboratory, Kirkland AFB, NM; and G. D. Nastrom and A. R. Hansen
Middle Atmosphere Slant-path Optical Turbulence Conditions Derived from VHF Radar Observations
by
Frank D. Eaton1, Gregory D. Nastrom2, Anthony R. Hansen2
1US Air Force Research Laboratory, Kirtland AFB, NM 87117
2St. Cloud State University, St. Cloud MN 56301
Abstract
Slant path calculations are shown of the transverse coherence length, the isoplanatic angle, and the Rytov variance, using a six-year data set of refractive index structure parameter (C-sub-N-squared) from 49.25 MHz radar observations at White Sands Missile Range, NM. The calculations were for a spherical wave condition, a wavelength of electromagnetic radiation of 1 micron, four different elevation angles (3, 10, 30, and 60 degrees), two path lengths (50 km and 150 km), and a platform, such as an aircraft, at 12.5 km MSL. Over 281,000 radar-derived C-sub-N-squared profiles sampled at three minute intervals with 150-m height resolution were used for the calculations. The approach, an "onion skin" model, assumes horizontal stationarity over each entire propagation path and is consistent with Taylor's Hypothesis. The results show that refractivity turbulence effects are greatly reduced for the three propagation parameters (isoplanatic angle, coherence length, and Rytov variance) as the elevation angle increases from 3 to 60 degrees. A pronounced seasonal effect is seen on the same parameters which is consistent with climatological variables and with seasonal changes in gravity wave activity. Interactions with the enhanced turbulence in the vicinity of the tropopause with the range weighting functions of each propagation parameter is evaluated. Results of a two region model relating the three propagation parameters to the wind speed at 5.6 km MSL are shown. This statistical model can be understood in terms of upward propagating gravity waves that are launched by strong winds over complex terrain.

The 13th Symposium on Boundary Layers and Turbulence