Imaging gravity waves in lower stratospheric AMSU-A radiances, Part 1: Simple forward model

Abstract

Abstract. Using a simplified model of in-orbit radiance acquisition by the Advanced Microwave Sounding Unit (AMSU-A), we derive three-dimensional temperature weighting functions for Channel 9 measurements (peaking at ∼60-90 hPa) at all 30 cross-track beam positions and use them to investigate the sensitivity of these radiances to gravity waves. The vertical widths of the weighting functions limit detection to waves with vertical wavelengths of 10 km, with slightly better vertical wavelength sensitivity at the outermost scan angles due to the limb effect. Fourier Transforms of two-dimensional cross-track weighting functions reveal optimal sensitivity to cross-track wavelengths at the near-nadir scan angles, where horizontal measurement footprints are smallest. This sensitivity is greater for the AMSU-A on the Aqua satellite than for the identical instruments on the NOAA meteorological satellites, due to a lower orbit altitude and thus smaller horizontal footprints from antenna spreading. Small cross-track asymmetries in the radiance response to gravity waves are found that peak at the mid-range scan angles, with more symmetric responses at near-nadir and far off-nadir scan angles. Three-dimensional simulations show gravity wave oscillations imaged in horizontal AMSU-A radiance maps swept out by the scan pattern and satellite motion. A distorting curvature is added to imaged wave phase lines due to vertical variations in weighting function peaks with cross-track scan angle. This wave distortion is analogous to the well-known "limb darkening" and "limb brightening" of microwave radiances acquired from purely vertical background temperature profiles by crosstrack scanners. Waves propagating along track are more visible in these images at the outermost scan angles than those propagating cross track, due to oversampling and narrower widths of the horizontal measurement footprints in the along track direction. Based on nominal noise floors and repreCorrespondence to: S. D. Eckermann (stephen.eckermann@nrl.navy.mil) sentative lower stratospheric wave temperature amplitudes, our modeling indicates that Channel 9 AMSU-A radiances can resolve and horizontally image gravity waves with horizontal wavelengths of 150 km and vertical wavelengths of 10 km.

Imaging gravity waves in lower stratospheric AMSU-A radiances, Part 2: Validation case study

“…Here we attempt an absolute observational validation of the forward modeling predictions of Eckermann and Wu (2006) of gravity wave signals in AMSU-A Channel 9 radiances. We focus on wavelike structures imaged in swathscanned Channel 9 radiances over southern Scandinavia on 14 January 2003.…”

“…Horizontal gravity wave imaging is an important new satellite measurement capability, heretofore only hinted at by a few limited observational case studies (Dewan et al, 1998;Wu and Zhang, 2004) and never previously modeled. Second, if the three-dimensional wavelength and amplitude structure of the gravity wave is known, the forward model of Eckermann and Wu (2006) makes specific predictions about the absolute brightness temperature amplitudes that should be seen by Channel 9. Indeed, given a complete gridded three-dimensional temperature field, the forward model can convert these temperatures into brightness temperature maps that can be compared directly with the AMSU-A data.…”

“…To provide estimates of three-dimensional gravity wave temperature perturbations in this region of the lower stratosphere on this day, we analyze high-resolution forecast and hindcast fields from NWP models, validating them against available suborbital observations in this region, such as DC-8 lidar data acquired during the SAGE III Ozone Loss and Validation Experiment (SOLVE II). We then apply the forward model of Eckermann and Wu (2006) to convert these validated NWP temperature fields into swath-scanned Channel 9 brightness temperature maps, using the actual AMSU-A scanning patterns during each of the 8 satellite overpasses that occurred at different times on this day. These S. D. Eckermann et al: Imaging gravity waves in lower stratospheric AMSU-A radiances 3345 NWP-based radiance perturbations are compared to those measured by AMSU-A, to assess how closely the observed horizontal structure, wavelengths, amplitudes and time evolution of the measured fluctuations are reproduced.…”

“…To provide some theoretical insight, Eckermann and Wu (2006) developed a simplified model of the in-orbit acquisition of radiances by AMSU-A Channel 9 on both the NOAA and EOS Aqua satellites. The three-dimensional temperature weighting functions that this modeling generated were in turn used to specify how gravity waves with different temperature amplitudes, horizontal propagation directions, and vertical and horizontal wavelengths manifested as oscillatory signals in swath-scanned Channel 9 radiance imagery.…”

“…These NWPCorrespondence to: S. D. Eckermann (stephen.eckermann@nrl.navy.mil) based brightness temperatures contain two-dimensional oscillations due to this resolved stratospheric mountain wave that have an amplitude, wavelength, horizontal structure and time evolution that closely match those observed in the AMSU-A data. These comparisons not only verify gravity wave detection and horizontal imaging capabilities for AMSU-A Channel 9, but provide an absolute validation of the anticipated radiance signals for a given three-dimensional gravity wave, based on the modeling of Eckermann and Wu (2006). …”