Because gravity wave effects control middle atmosphere circulation
patterns, and because those effects depend sensitively on the
properties of the waves, reseachers have been trying for decades to
ascertain the global properties of gravity waves to better constrain global circulation models. Space-based observations hold promise for providing the needed information, but the small scales of gravity waves have posed observational challenges. Traditional analyses of averaged temperature variance do not provide the needed information. We will present statistics from alternative analyses of Atmospheric Infrared Sounder (AIRS) images of gravity waves in the stratosphere.
The high spatial resolution of the AIRS observations permit resolution of gravity waves with horizontal wavelengths as small as 50 km. We present the results of wavelet analyses of AIRS images at 667cm-1 (in the CO2 15 μm band) that spatially resolve gravity wave amplitudes, horizontal wavelengths, and propagation directions. These analyses reveal both local maximum amplitudes as well as frequencies of wave occurrence, while in contrast these quantities are inseparably blended within traditional wave temperature variance or Fourier analysis methods. The AIRS observations are known to detect only long vertical wavelength waves, whose occurrences are in turn known to be highly dependent on the strength of background wind speeds. The AIRS data permit for the first time detailed studies of the relationships between the occurrence of these waves and their propagation directions relative to the background winds.
We consider the example of the CRyogenic Infrared Spectrometers and
Telescopes for the Atmosphere (CRISTA) experiment to deduce the sensitivity of an infrared emission limb sounder to gravity waves of different horizontal and vertical wavelength. The sensitivity studies show that gravity waves with horizontal wavelengths of the order of 100-200 km or longer can be detected. The deduced sensitivity factors are validated by comparing CRISTA and data sonde temperature profiles. Analysis of CRISTA temperature data
reveals large gravity wave amplitudes in the stratosphere over southernmost South America. The horizontal structure is compared to
model calculations. Global distributions are discussed with respect to convective sources, wind modulation, and Coriolis force modulation. It is shown that even the very dense spatial sampling of the CRISTA instrument is insufficient to fully resolve the horizontal structure of the waves which are seen in the vertical.
Hence, increased spatial resolution of about 50 X 50 km or better
is required to obtain all information the limb sounding technique is capable to provide.
The Waves middle class Explorer mission (WE) is proposed to observe and quantify the effects of small-scale internal Gravity Waves (GW) in the Earth's atmosphere from source regions in the troposphere and lower stratosphere to the mesosphere, lower thermosphere, and ionosphere (MLTI) where the GW have their most dramatic effects. These are now understood to be a key element in defining large-scale circulation, thermal and constituent structures, and variability of the stratosphere and MLTI. The WE instrumentation consists of 5 nadir and limb viewing sensors of the wave perturbed emission structure due to GW throughout the source and affected regions. The WE PI is Prof. G.R. Swenson. This paper addresses the measurement strategy and implementation for two of these instruments, the Source Wave And Propagation Imager (SWAPI), and the Hydroxyl Airglow Wave Imager (HAWI). The SWAPI uses multi-spectral sublimb imaging measurements in the CO2 (nu) 3 band near 4210 nm to identify GW sources, and their propagation through the stratosphere. Its measurement strategy is driven by data, particularly sublimb images in the CO2 (nu) 3 band that were obtained by instrumentation deployed on the Midcourse Space Experiment (MSX) satellite, and by the WE team member's data analysis and models. Similarly team member's ground based observational experience and data analysis drives the HAWI measurement strategy.
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