Gravity wave spectra from GPS/MET occultation observations

The potential utility of radio occultation data in general, and of data fro m the Global Positioning System/Meteorology (GPS/MET) experiment in particu lar, for studying atmospheric gravity waves is discussed. Based on a valida ted set of similar to 270 GPS/MET-derived temperature profiles, the authors produced and analyzed mean vertical wavenumber power spectra of normalized temperature fluctuations in three latitude bands (low, middle, high) withi n the lower stratosphere (similar to 15-30 km), where data accuracy was pro ven highest. The Fresnel diffraction limited vertical resolution and the li mited height range of the dataset restricted this initial investigation to medium-to large-scale waves with vertical wavelengths of about 2-5 km. The deduced vertical wavenumber power spectra were compared with a saturation s pectrum predicted by gravity wave saturation theory and generally found con sistent with the theoretical saturation limit. The low-latitude power spect ra exhibited almost saturation, with spectral power about twice as high tha n at mid- to high latitudes. Dominant fluctuations were observed to occur a t wavelengths of about 3-5 km, indicating the wave structures to be interpr eted either as inertio-gravity waves or Rossby-gravity waves. A tendency wa s found, though, of increasingly underestimating spectral power toward shor ter wavelengths. This very likely roots in weaknesses of the "standard" GPS /MET temperature retrieval applied here, which involves a geometrical optic s, a local spherical symmetry, and a hydrostatic equilibrium assumption, al l of which are increasingly violated toward smaller-scale wave structures a nd lead to an (artificial) wave smoothing in the retrieved temperature prof iles. More elaborated future wave analyses should therefore employ improved retrieval methodology alleviating these assumptions and the authors indica te clear directions to this end. Generally, the results demonstrate that ra dio occultation data indeed bear high potential not only for fields like we ather and climate prediction and climate monitoring but also for the study of atmospheric wave activity and its important role in atmospheric momentum and energy budgets.