Gravity wave spectra from GPS/MET occultation observations

Abstract

The potential utility of radio occultation data in general, and of data from the Global Positioning System/Meteorology (GPS/MET) experiment in particular, for studying atmospheric gravity waves is discussed. Based on a validated set of ~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) within the lower stratosphere (~15-30 km), where data accuracy was proven highest. The Fresnel diffraction limited vertical resolution and the limited 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 spectrum predicted by gravity wave saturation theory and generally found consistent with the theoretical saturation limit. The low-latitude power spectra exhibited almost saturation, with spectral power about twice as high than at mid- to high latitudes. Dominant fluctuations were observed to occur at wavelengths of about 3-5 km, indicating the wave structures to be interpreted either as inertio-gravity waves or Rossby-gravity waves. A tendency was found, though, of increasingly underestimating spectral power toward shorter wavelengths. This very likely roots in weaknesses of the 'standard' GPS/MET temperature retrieval applied here, which involves a geometrical optics, a local spherical symmetry, and a hydrostatic equilibrium assumption, all of which are increasingly violated toward smaller-scale wave structures and lead to an (artificial) wave smoothing in the retrieved temperature profiles. More elaborated future wave analyses should therefore employ improved retrieval methodology alleviating these assumptions and the authors indicate clear directions to this end. Generally, the results demonstrate that radio occultation data indeed bear high potential not only for fields like weather 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.