A comprehensive observational filter for satellite infrared limb sounding of gravity waves

Abstract

Abstract. This paper describes a comprehensive observational filter for satellite infrared limb sounding of gravity waves. The filter considers instrument visibility and observation geometry with a high level of accuracy. It contains four main processes: visibility filter, projection of the wavelength on the tangent-point track, aliasing effect, and calculation of the observed vertical wavelength. The observation geometries of the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) and HIRDLS (High Resolution Dynamics Limb Sounder) are mimicked. Gravity waves (GWs) simulated by coupling a convective GW source (CGWS) scheme and the gravity wave regional or global ray tracer (GROGRAT) are used as an example for applying the observational filter. Simulated spectra in terms of horizontal and vertical wave numbers (wavelengths) of gravity wave momentum flux (GWMF) are analyzed under the influence of the filter. We find that the most important processes, which have significant influence on the spectrum are the visibility filter (for both SABER and HIRDLS observation geometries) and aliasing for SABER and projection on tangent-point track for HIRDLS. The vertical wavelength distribution is mainly affected by the retrieval as part of the "visibility filter" process. In addition, the short-horizontalscale spectrum may be projected for some cases into a longer horizontal wavelength interval which originally was not populated. The filter largely reduces GWMF values of very short horizontal wavelength waves. The implications for interpreting observed data are discussed.

Satellite observations of middle atmosphere gravity wave absolute momentum flux and of its vertical gradient during recent stratospheric warmings

“…Only gravity waves with horizontal wavelengths longer than about 100-200 km are visible for those instruments. For details about the observation geometry and the resulting sensitivity for gravity waves see, for example, Preusse et al (2009a) or Trinh et al (2015). Second, no directional information is available, and only absolute values of gravity wave momentum flux and potential drag can be derived.…”

“…In our study, we cover horizontal wavelengths longer than about 100-200 km (e.g., Ern and Preusse, 2012), and vertical wavelengths in the range 2-25 km for HIRDLS, and 4-25 km for SABER (see also Ern et al, 2011). For a detailed discussion of the observational filter of limb observations from satellite see Trinh et al (2015).…”

Global distributions of overlapping gravity waves in HIRDLS data

“…Our measured results will not fully capture the true spectrum of wavelength data, due to the observational filter (Alexander, 1998;Preusse et al, 2000Preusse et al, , 2008Trinh et al, 2015) of the instrument and to other effects arising as a result of the analysis process. We discuss here the key limitations inherent in our measurements and analysis technique; Table 1 summarises these effects.…”

Intercomparison of AIRS and HIRDLS stratospheric gravity wave observations

“…Each type of current satellite instruments can detect only a certain part of the full vertical and horizontal wave number spectrum of gravity waves, which is determined by its observational filter (Alexander, 1998;Preusse et al, 2008;Alexander et al, 2010;Trinh et al, 2015). For AIRS the sensitivity to vertical and horizontal wavelengths was determined using an approach similar to Hoffmann et al (2014).…”

“…The calculation of the HIRDLS sensitivity function follows the approach of Preusse et al (2002) and Trinh et al (2015), with additional vertical filtering being applied. This additional filtering was added because in the analysis by Ern et al (2011) gravity wave amplitudes are determined in sliding windows of 10 km vertical extent.…”