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

Abstract

Slant path calculations are shown of the transverse coher- ence length (r 0), the isoplanatic angle (u 0), and the Rytov variance (s R ), using a 6-yr data set of refractive index structure parameter (C n ) from 49.25-MHz radar observations at White Sands Missile Range, New Mexico. The calculations are for a spherical wave condition; a wave- length (l) of electromagnetic radiation of 1 mm; 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 Cn profiles sampled at 3 min intervals with 150-m height resolution are used for the calculations. The approach, an ''onion skin'' model, assumes horizontal stationarity over each entire propaga- tion path and is consistent with Taylor's hypothesis. The results show that refractivity turbulence effects are greatly reduced for the three propagation parameters (r 0 , u 0 , and s R ) as the elevation angle in- creases from 3 to 60 deg. A pronounced 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 weighting functions of each propagation parameter is evaluated. Results of a two region model re- lating r 0 , u 0 , and s R to wind speed at 5.6 km MSL are shown. This statistical model can be understood in terms of upward propagating grav- ity waves that are launched by strong winds over complex terrain.