Atmospheric temperature HIRS and AIRS

Atmospheric vertical temperature profiles obtained by infra-red remote sensing from satellites are now routinely used by the meteorological services to augment traditional sources such as balloon-borne radiosondes. The High-resolution Infrared Radiation Sounder or HIRS, which has been in use for many years for this purpose, featured a large number of channels (20) with the highest spectral resolution that can be obtained with interference filters (about 10 cm-1), in order to separate the emission intensities from different parts of the vibration-rotation bands of CO2 at 15 and 4.3 ¡m. These originate from different height ranges in the atmosphere, and permit the reconstruction of a vertical profile of temperature. Crude profiles of water vapour are achieved by the inclusion of three channels in the 6.7 ¡m band of H2O.

The HIRS optical system, like MODIS, uses dichroic beamsplitters to provide three channels in the visible, near-IR and mid-IR, further multiplexing being by means of a single filter wheel carrying two rings of filters. The latest, advanced temperature sounders use fixed-grating spectrometry or interferometry to have large numbers of narrow channels sampled simultaneously. The AIRS (Atmospheric Infra-Red Sounder) on the Aqua spacecraft alongside MODIS measures simultaneously in 2378 spectral channels from 0.4 to 1.7 ¡m and 3.4 to 15.4 ¡m, with a spectral resolution of A/AA «1200, which corresponds to about 0.01 ¡m in the temperature-sounding channels. A diffraction grating disperses the

Track flG. 10.9. The MODIS optical layout. The scan mirror reflects the Earth view into the telescope assembly and a system of dichroic beamsplitters separates the incoming radiation by wavelength into four channels; visible (VIS) (0.412 to 0.551 pm), near-infra-red (NIR) (0.650 to 0.940 pm), short/mid-infra-red (MIR) (1.240 to 4.565 pm), and longwave infra-red (LIR) (6.715 to 14.235 pm).

Track flG. 10.9. The MODIS optical layout. The scan mirror reflects the Earth view into the telescope assembly and a system of dichroic beamsplitters separates the incoming radiation by wavelength into four channels; visible (VIS) (0.412 to 0.551 pm), near-infra-red (NIR) (0.650 to 0.940 pm), short/mid-infra-red (MIR) (1.240 to 4.565 pm), and longwave infra-red (LIR) (6.715 to 14.235 pm).

radiation from the scene onto 17 linear arrays of HgCdTe detectors on the focal plane (Fig. 10.10), giving 90 simultaneous ground footprints across the flight track of the satellite. Each detector element has a field of view of 1.1°, which corresponds to a spatial resolution at the surface of about 10 km. The detectors are cooled to 60 K by a Stirling cycle pulse tube cryocooler, while the rest of the spectrometer is cooled to 155 K by a two-stage passive radiative cooler to reduce the background flux and achieve the required instrument sensitivity. A scan mirror that makes a full cycle every 2.67 s provides spatial coverage and views of cold space, hot calibration targets and a visible radiometric source. The scan mirror is radiatively coupled to the spectrometer so that it operates at a temperature of 250 K. Since the mirror by definition has a low emissivity, this is cool enough to reduce the flux emitted from the mirror into the instrument to an acceptable level.

The AIRS instrument also includes four visible/near-IR channels between 0.40 and 0.94 pm, primarily for the diagnostics of cloud in the IR field-of-view. Cloud contamination is a major problem for tropospheric sounders: even with the relatively high spatial resolution of AIRS, it has been estimated that only about

flG. 10.10. Diagram of the AIRS instrument. The principle is similar to that of ERBE and MODIS (Figs. 10.4 and 10.9), except that the spectral channels are defined not only by filters, as shown, but then by an eschelle grating (a diffraction grating on a curved substrate, so that it focuses as well as disperses the radiation). The result is high spectral resolution and the capability to have a large number of spectral channels (see Fig. 10.11)

Folding l mirror array flG. 10.10. Diagram of the AIRS instrument. The principle is similar to that of ERBE and MODIS (Figs. 10.4 and 10.9), except that the spectral channels are defined not only by filters, as shown, but then by an eschelle grating (a diffraction grating on a curved substrate, so that it focuses as well as disperses the radiation). The result is high spectral resolution and the capability to have a large number of spectral channels (see Fig. 10.11)

5% of measurements made over the ocean have less than 0.6% change in radiance due to the presence of cloud in the field-of-view. For this reason, AIRS flies in tandem with a pair of microwave sounders, AMSU (Advanced Microwave Sounding Unit) and HSB (Humidity Sounder for Brazil) that sense millimetre-wavelength radiation that is insensitive to the presence of most types of cloud. Together, the three sounders can achieve temperature-profile retrievals with an accuracy of better than 1 K in 1-km vertical layers in the troposphere under clear and partly cloudy conditions, the goal set in the 1990s by the World Meteorological Organisation for accurate numerical weather prediction. In addition to atmospheric temperature profiles, AIRS routinely produces data sets on each of the following:

sea-surface temperature;

land-surface temperature and emissivity;

relative humidity profiles and total precipitable water vapour; fractional cloud cover;

cloud spectral IR emissivity; cloud-top pressure and temperature; total ozone burden of the atmosphere;

column abundances of minor atmospheric gases such as CO2, CH4, CO, and N2O;

outgoing longwave radiation and longwave cloud-radiative forcing; precipitation rate.

In nadir temperature sounding, as practiced by HIRS and AIRS, the upwelling radiation is measured by an instrument that is looking vertically downwards, and so viewing the complete atmospheric column. Vertically resolved information about the temperature, including the surface temperature at the lower boundary, is obtained by selecting several spectral intervals of different opacity in which to measure the radiance. Inside the strong absorption bands of infra-red-active atmospheric gases such as carbon dioxide the atmosphere is highly opaque, and the radiation measured originates near the top. In a more transparent region, the radiation originates in the lower atmosphere and from the surface. At wavelengths where the absorption has an intermediate value, the radiance originates somewhere in the middle of the atmosphere. The weighting of the emission that contributes to the measured radiance thus varies with the wavelength at which the measurement is made, and the contribution as a function of height is described by a wavelength-dependent weighting function.

The radiance at the top of the atmosphere in a narrow interval AA centred on wavelength A is

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