Estimates of Cn2 from numerical weather prediction model output and comparison with thermosonde data

Area-averaged estimates of Cn² from high-resolution numerical weather prediction (NWP) model output are produced from local estimates of the spatial structure functions of refractive index with corrections for the inherent smoothing and filtering effects of the underlying NWP model. The key assumptions are the existence of a universal statistical description of small-scale turbulence and a locally universal spatial filter for the NWP model variables. Under these assumptions, spatial structure functions of the NWP model variables can be related to the structure functions of the atmospheric variables and extended to the smaller underresolved scales. The shape of the universal spatial filter is determined by comparisons of model structure functions with the climatological spatial structure function determined from an archive of aircraft data collected in the upper troposphere and lower stratosphere. This method of computing Cn² has an important advantage over more traditional methods that are based on vertical differences because the structure function-based estimates avoid reference to the turbulence outer length scale. To evaluate the technique, NWP model-derived structure-function estimates of Cn² are compared with nighttime profiles of Cn² derived from temperature structure-function sensors attached to a rawinsonde (thermosonde) near Holloman Air Force Base in the United States.