Considerations For Computing Realistic Atmospheric Distortion Parameter Profiles

Nonlinear propagation phenomena such as thermal blooming, thermally amplified scintillations, and breakdown often have thresholds below which their respective effects are negligible. Depending upon the gain associated with the phenomena, the threshold may be either "hard" (i.e., the onset is sudden and dramatic as the threshold is passed) or "soft" (i.e., the effect grows steadily more important). Critical propagation parameters, such as the distortion number (ND) and coherence length (r0) are often computed from atmospheric data to represent these phenomena. Thresholds for the onset of the nonlinear effects and adaptive optics correctability are commonly expressed in terms of these parameters. This paper examines practical considerations in computing and applying critical propagation parameters for realistic slant-path propagation scenarios in an inhomogeneous atmosphere. Issues such as correlation of wind speed and direction at different altitudes, location of blooming and turbulence layers, and the effect of averaging techniques on the computed parameter values are discussed. Calculations are presented contrasting predictions for model atmospheric profiles with actual meteorological data from specific rawinsonde launches.