Imaging gravity waves in lower stratospheric AMSU-A radiances, Part 1: Simple forward model S. D. Eckermann1 and D. L. Wu2 1E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, D.C., USA 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
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.
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 cross-track 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 representative
lower stratospheric wave temperature amplitudes, our
modeling indicates that Channel 9 AMSU-A radiances can resolve and
gravity waves with horizontal wavelengths of ≳150 km and
vertical wavelengths of ≳10 km.
Citation: Eckermann, S. D. and Wu, D. L.: Imaging gravity waves in lower stratospheric AMSU-A radiances, Part 1: Simple forward model, Atmos. Chem. Phys., 6, 3325-3341, doi:10.5194/acp-6-3325-2006, 2006.