10th conference on atmospheric radiation, Madison, WI · 1999

Validation of Radiative Transfer for Atmospheric Temperature Sensing

Haijun Hu, L. Larrabee Strow, David W. Keith, and James G. Anderson

Remote sounding of atmospheric temperature profiles, which is of paramount importance to numerical weather forecasting, has been conducted routinely with the 15-µm CO₂ band using low spectral resolution satellite remote sensors. Next generation atmospheric sounders, such as the Atmospheric Infrared Sounder, AIRS (Aumann and Pagano 1994), is also designed to use the 15-µm CO₂ band, but in addition will have spectral channels in the 4.3-µm CO₂ R branch. This region is useful because the weighting function are quite sharp (partially due to the lack of interfering hot bands and isotopes) and because the Planck function is more sensitive to the temperature in the short wave. AIRS has a nominal spectral resolution of 2 cm⁻¹ in this region, thus producing a good number of channels with sharp weighting functions. Like remote sounding using the 15-µm CO₂ band, implementation of the 4.3-µm band sounding requires an accurate forward transmittance model for temperature retrieval. The well-known extreme sub-Lorentz behavior of the CO₂ lineshape in this region is difficult to model, especially inside the bandhead between lines, which is where the best sounding channels are located. Moreover, there are very few studies of the lineshape at low temperatures relevant for atmospheric sounding. Thus, validation of forward model assumptions with inflight measurements is an important step toward improvements of model and temperature retrieval. Here we present our approach and preliminary findings from a recent field campaign using a newly developed high-resolution Interferometer for Emission and Solar Absorption (INTESA) flown on the NASA ER-2. The observed radiances and transmittances are compared to computed quantities using a new pseudo lineby-line algorithm, kCARTA (Strow et al. 1998). kCARTA uses compressed look-up tables for computation of transmittances and radiances which were derived from the GENLN2 line-by-line code (Edwards 1992), which uses a parameterization of the 4.3-µm CO₂ lineshape developed by Cousin et al. (1985).

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