High Resolution Dynamics Limb Sounder Research Articles

The non-LTE correction to the vibrational component of the internal partition sum for atmospheric calculations

In this paper, we develop the theoretical basis for the non-local thermodynamic equilibrium (non-LTE) correction to the vibrational component of the internal partition sum Q v , and we show how this correction enters into the radiative transfer calculation for a limb-viewing radiometer. The importance of the non-LTE correction to Q v depends, for the most part, on the degree to which the lowest vibrational state of the molecule is in non-LTE for a given altitude, since it is this vibration that contributes most to the partition sum. The affect on high-altitude emission depends on the principle excitation mechanism of the radiating molecule, and we compare CO 2 and O 3. In the case of CO 2, to first order, the high-altitude radiance measured by a limb-viewing radiometer is dependent on the value of the inverse of the partition sum in the region just above the tangent point. We have shown that in the case of the high-altitude temperature sounding channel of the High Resolution Dynamics Limb Sounder (HIRDLS) the non-LTE correction can result in a difference in calculated radiance of some 30% at 130 km. This suggests that the non-LTE correction to Q v is an important consideration for high-altitude non-LTE radiative transfer models.

The Radiometric Calibration Budget for the High-Resolution Dynamics Limb Sounder

The High-Resolution Dynamics Limb Sounder is a 21-channel infrared filter radiometer due to be launched on the Earth Observing System Chemistry platform in 2002. The channels range in wavelength from 6.1 to 17.8 μm, with fractional widths of between 1% and 8%. The scientific objectives include the retrieval of atmospheric temperature and distributions of a range of minor constituents, the retrieval of geopotential height gradients, and the detection of atmospheric wave activity, by sounding the atmospheric radiance viewed at the earth's limb. These objectives require extremely accurate radiometry, with stringent limits on both errors that scale with the radiance and systematic offsets. The analysis presented shows explicitly how systematic errors in the in-flight calibration propagate into radiometric errors. Initial budget allocations to these error sources are made. An indirect calibration view of a small blackbody, rather than a full-aperture calibrator, is shown to have advantages in the case that the blackbody and the telescope optics are at the same temperature. The origins of scan-dependent offsets are discussed and shown to be significant error sources unless in-flight correction is possible. Two approaches for this are discussed.

Selection of sounding channels for the High Resolution Dynamics Limb Sounder

We describe the scientific design work behind the selection of the IR spectral passbands for the 21 sounding channels of the High Resolution Dynamics Limb Sounder (HIRDLS), which is scheduled to fly aboard the Earth Observing System chemistry platform at the beginning of the next century. At least one radiometer channel must be used for each gas that is being measured. Preferably the interfering contributions to the radiance by other gases in a channel should be small, but the principal requirements are that the desired emission be measured with high signal-to-noise ratio and that there be separate channels for the measurement of interfering species. However, more than one channel is required for providing full altitude coverage of those target gases such as CO(2), H(2)O, and O(3), which have emission bands whose centers become optically thick in the middle atmosphere. Further channels, in which gaseous absorption is low, are required for the characterization of aerosol effects. We describe the HIRDLS channels selected for each gas, with emphasis on signal-to-noise considerations and altitude coverage, the elimination of contaminating signal between channels, and nonlocal thermodynamic equilibrium processes for high-altitude sounding and space view definition.

Non-local thermodynamic equilibrium limb radiance from O 3 and CO 2 in the 9–11 μm spectral region

Satellite remote sensing of mesospheric and thermospheric O 3 abundance in the terrestrial atmosphere often uses 9–11 μm thermal emission. In this paper, we apply a line-by-line non-local thermodynamic equilibrium (non-LTE) radiance model to this spectral region and investigate the conditions of LTE breakdown and the effect that this has on the limb radiance measured by an i.r. sounder. Monochromatic and band-integrated radiance calculations have been performed for limb view tangent heights between 55 and 105 km under daytime and nightime conditions. Non-LTE emission from both O 3 and CO 2 are shown to be important with the divergence of radiance from LTE values and the diurnal variation being band dependent. We have shown that the contribution of the CO 2 bands to the Limb Infrared Monitor of the Stratosphere O 3 channel is significant for daytime conditions at tangent heights above about 60 km. A study has been made to choose O 3 sounding channel spectral passbands for the High Resolution Dynamics Limb Sounder. High resolution calculations are required to determine those spectral intervals that will filter radiance from selected bands and characterize their non-LTE behavior. This will allow for improved O 3 retrievals above 70 km and non-LTE studies.