Computing two-dimensional unambiguous horizontal wavenumber spectra from OH airglow images

There is an inherent 180° ambiguity in the derived wave-propagation direction when using conventional spectral-analysis techniques on OH-imager observations. A processing technique for computing the unambiguous two-dimensional (2D) horizontal wavenumber spectrum from images of gravity wave perturbations in OH emission intensities is presented. The technique involves computing the (w, k,l)=(w, h, φ) spectrum of OH images collected over a several-hour period, where k is the zonal wavenumber, l is the meridional wavenumber, h=(k²+ l²)^1/2, φ=tan^-1 (k/l), and w is the temporal frequency. Before computing the spectra, the all-sky images are processed by first flat fielding each image, then removing stars by using a median filter, subtracting an estimate of the background continuum, and finally computing the relative OH intensity perturbations. Each pixel is then mapped onto a 256×256 rectilinear grid of geographic coordinates by using a 9×9 Hamming-weighted sinc-interpolation function. The interpolated image is restricted to the 600×600-km² region centered at zenith so that the resulting horizontal resolution is 2.34 km. Spatial and temporal prewhitening is employed prior to computing spectra to minimize artifacts in the derived unambiguous spectrum. The authors illustrate the method with a real sequence of images acquired on February 3, 1995, at the Starfire Optical Range (SOR), near Albuquerque, NM. The predominant direction of wave propagation is determined and the k, l, h, and φ spectra presented