Robert R. O'Neil

MSX satellite observations of thunderstorm-generated gravity waves in mid-wave infrared images of the upper stratosphere

Data from the Midcourse Space Experiment (MSX) has provided the first observations of thunderstorm-generated gravity waves imaged from space. Gravity wave theory predicts that isolated, sufficiently convective thunderstorms can launch waves and create a unique intensity pattern of concentric circles on a radiating surface of constant altitude above such a storm. Among the MSX constant-nadir-angle mid-wave infrared (MWIR) observations, two instances of such patterns have been identified. It was confirmed from meteorological satellite images that highly convective isolated thunderstorms occurred at the locations and times expected.

Remote sensing of discrete stratospheric gravity-wave structure at 4.3-µm from the MSX satellite

Distinctive structure in the 4.3-µm spectral region has been imaged by the SPIRIT 3 radiometer on the MSX satellite observing the cloud-free atmosphere. We show nadir, high-nadir-angle (NA) sublimb, and limb images which, coupled with radiative transfer analysis, indicate that this structure originates from internal gravity waves (GWs). Such structure occurs in a significant fraction of both below-the-horizon (BTH), or sublimb, and above-the-horizon (ATH), or limb, observations in both MSX 4.3-µm channels. The structure has different morphology from clouds and has spatial scales appropriate for atmospheric GWs. Calculation of contribution functions (CFs), or weighting functions, for MSX filters and viewing conditions confirms that the BTH GW structure originates from altitudes near 40 km. We believe this is the first high-resolution imaging of atmospheric GWs from space in the mid-wave infrared (MWIR) spectral region. In addition, the technique provides structure imaging capabilities at upper stratospheric altitudes inaccessible to airglow imagery.

Midcourse Space Experiment: Auroral enhancement of nitric oxide medium-wave infrared emission observed by the Spatial Infrared Imaging Telescope III radiometer

Enhanced medium-wave infrared nitric oxide fundamental (Δv = -1) vibration-rotation (VR) band emission observed by the Midcourse Space Experiment radiometer band A (6.8-10.8 μm) at nighttime in the 110- to 130-km tangent altitude range on November 10, 1996, is shown to be the result of auroral dosing. The energy flux Q (ergs cm -2 s -1 ), the average energy (E) (keV) of the incident electrons, as well as the location of the dosed region along the line of sight (LOS) were previously derived [Strickland et al., 1997]. Combining these results with the N + 2 first negative band emission at 391.4 nm gives the auroral energy deposition rate in all three spatial dimensions. The portion of the LOS NO VR band radiance transmitted by the band A filter calculated by a more elaborate version of the auroral model in the Strategic High-Altitude Radiance Code agrees remarkably well with the enhancements in the signal observed in that band of the Spatial Infrared Imaging Telescope III radiometer.

Structure in Middle and Upper Atmospheric Infrared Radiance

An extensive database on spatial structure in the infrared radiance of the middle and upper atmosphere has been collected by the Mid-Course Space Experiment (MSX). The observed radiance contains spatial structure down to the scale of hundreds of meters. This spatial structure results from local fluctuations in the temperature and densities of the radiating states of the emitting molecular species as well as fluctuations in radiation transport from the emitting regions to the observer. A portion of this database has been analyzed to obtain statistical parameters characterizing stochastic spatial structure in the observed radiance. Using simple models, the observed statistics have been shown to agree with prior observations and theoretical models of stochastic spatial structure generated by gravity waves for special viewing geometries. The SHARC model has been extended to predict the statistics of stochastic fluctuations in infrared radiance from the statistics characterizing temperature fluctuations in the middle and upper atmosphere for arbitrary viewing geometries. SHARC model predictions have been compared with MSX data and shown to be in generally good agreement. Additional work is in progress to account for the statistics characterizing small spatial scale fluctuations.

Structure of radiance from an atmosphere perturbed by superposed gravity waves

Analysis of 4.3-μm CO2 radiance data from the MSX (Midcourse Space Experiment) satellite has shown that gravity waves dominate the fluctuations of radiance at 4.3 μm for both earthlimb (above-the-horizon) and downlooking (below-the-horizon) lines-of-sight under a broad class of conditions. We review previous work on the spectra of known sources of gravity waves and on wave filtering mechanisms by M. J. Alexander and others, as well as the characteristics of gravity-wave power spectra. We then consider the power spectra of line-of-sight radiance fluctuations emitted and self-absorbed by an atmosphere perturbed by gravity waves, discussing the shape of the spectrum and the spectral slopes. We show examples of radiance spectra from gravity-wave-perturbed atmospheres that have two different slopes, with a steeper slope at large wavenumber, and discuss mechanisms that can account for this effect. The effect of latitude and season on the 4.3-μm fluctuations will also be considered.

MSX: remotely sensed observations of atmospheric infrared radiance and spatial structure

The MSX SPIRIT III infrared radiometer and interferometer/spectrometer completed 233 episodic data collection events of remotely sensed atmospheric limb and terrestrial scenes from April 1996 to February 1997. The below the horizon (BTH) or terrestrial scenes were recorded in two mid-wavelength infrared (MWIR) radiometer bands centered near 4.3 microns. The above the horizon (ATH) measurements were measured simultaneously in the MWIR bands and four long wavelength infrared (LWIR) bands extending over the wavelength range from 6.8 to 25.1 microns. MSX results included the initial space based MWIR observations of gravity waves in the stratosphere. This source of atmospheric structure was detected in approximately 30 percent of the BTH and low limb MWIR measurements. As anticipated the MSX MWIR BTH results also included structure produced by clouds with large contrast ratios produced by high altitude clouds in the equatorial region. The MSX MWIR and LWIR measurements include observations of the radiance and structure associated with a range of atmospheric phenomena including clouds, gravity waves, airglow, day- night terminator transitions and aurora. Representative samples of these and other significant finding in the MSX measurements of infrared terrestrial and earth limb backgrounds are presented.