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Using a simplified model of in-orbit radiance acquisitionby the Advanced Microwave Sounding Unit (AMSU-A), we derivethree-dimensional temperature weighting functions for Channel 9measurements (peaking at ~60–90 hPa) at all 30 cross-trackbeam positions and use them to investigate thesensitivity of these radiances to gravity waves. The vertical widthsof the weighting functions limit detection to waves withvertical wavelengths of ≳10 km, with slightlybetter vertical wavelength sensitivity at the outermost scanangles due to the limb effect. FourierTransforms of two-dimensional cross-track weighting functions revealoptimal sensitivity to cross-track wavelengths at the near-nadirscan angles, where horizontal measurement footprints are smallest.This sensitivityis greater for the AMSU-A on the Aqua satellite than for theidentical instruments on the NOAAmeteorological satellites, due to a lower orbit altitude and thus smallerhorizontal footprints from antenna spreading.Small cross-track asymmetries in the radiance response togravity waves are found that peak at the mid-range scan angles,with more symmetric responses at near-nadir and far off-nadirscan angles. Three-dimensional simulationsshow gravity wave oscillationsimaged in horizontal AMSU-A radiance maps swept out by the scan pattern andsatellite motion. A distorting curvature is added to imagedwave phase lines due to vertical variations inweighting function peaks with cross-track scan angle. This wave distortionis analogous to the well-known "limb darkening" and"limb brightening" of microwave radiances acquired from purelyvertical background temperature profiles by cross-track scanners.Waves propagating along track are more visible in theseimages at the outermost scan anglesthan those propagating cross track, due to oversampling andnarrower widths of the horizontal measurement footprints in the alongtrack direction. Based on nominal noise floors and representativelower stratospheric wave temperature amplitudes, ourmodeling indicates that Channel 9 AMSU-A radiances can resolve andhorizontally imagegravity waves with horizontal wavelengths of ≳150 km andvertical wavelengths of ≳10 km

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