Middle atmosphere slant-path optical turbulence conditions derived from very high frequency radar observations

Slant path calculations are shown of the transverse coherence length (r(0)) , the isoplanatic angle (theta(0)), and the Rytov variance (sigma(R)(2)), u sing a 6-yr data set of refractive index structure parameter (C-n(2)) from 49.25-MHz radar observations at White Sands Missile Range, New Mexico. The calculations are for a spherical wave condition; a wavelength (lambda) of e lectromagnetic radiation of 1 mu m; four different elevation angles (3, 10, 30, and 60 deg), two path lengths (50 and 150 km); and a platform, such as an aircraft, at 12.5 km MSL (mean sea level). Over 281,000 radar-derived C -n(2) profiles sampled at 3 min intervals with 150-m height resolution are used for the calculations. The approach, an "onion skin" model, assumes hor izontal stationarity over each entire propagation path and is consistent wi th Taylor's hypothesis. The results show that refractivity turbulence effec ts are greatly reduced for the three propagation parameters (r(0), theta(0) , and sigma(R)(2)) as the elevation angle increases from 3 to 60 deg. A pro nounced seasonal effect is seen on the same parameters, which is consistent with climatological Variables and gravity wave activity. Interactions with the enhanced turbulence in the vicinity of the tropopause with the range w eighting functions of each propagation parameter is evaluated. Results of a two region model relating r(0), theta(0), and sigma(R)(2) to 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 co mplex terrain. (C) 1999 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(99)01202-7].