The Atmospheric Infrared Sounder (AIRS) on board NASA's Aqua satellite platform is a hypersectral IR temperature and humidity sounder for numerical weather prediction and climate change studies. We use the rich spectral information available in the AIRS thermal infrared radiances to study the spectral signatures of dust over ocean for four case studies, and to retrieve dust optical depths using a fast two‐stream radiative transfer model. Retrieved optical depths for one case, a dust storm spreading over the Eastern Mediterranean in October 2002, are compared with visible imagery and MODIS optical depth retrievals. This work represents a preliminary step to removing the effects of dust on the retrieval of temperature and water vapor from the AIRS measurements.

[1] AIRS thermal infrared radiance data are used with a fast infrared scattering radiative transfer model to physically retrieve the dust column amount and dust height over both ocean and land. The retrieved optical depths are compared against those retrieved using visible and ultraviolet instruments on the A-Train, while dust layer heights are evaluated against lidar data. The synergistic use of AIRS data is explored by using dust layer heights constrained by CALIPSO retrievals and coarse mode particle sizes over ocean from PARASOL. Optical depths from AIRS correlate well with those from other instruments over ocean (R ≥ 0.9) and are lower over land when compared to MODIS Deep Blue and OMI retrievals (R ≤ 0.8). AIRS-derived dust top heights compare favorably with CALIPSO data and can be used to improve OMI optical depth retrievals over a much larger area than CALIPSO can provide. AIRS data can also provide estimates of dust longwave radiative forcing. For the examples examined here, the forcings are estimated to be about +1.5 and +4.5 W/m 2 per unit visible optical depth over ocean and land, respectively, compared to a shortwave forcing estimate of −50 W/m 2 over ocean. AIRS dust retrievals are possible day or night, can provide dust column amount information over land or ocean, and are unaffected by areas of the oceans covered by sun glint.

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Retrieval algorithms for downlooking infrared sounders typically avoid using channels from the 4.3 μm CO2 region that probe the mid‐ and upper‐atmosphere due to very high altitude Non Local Thermodynamic Equilibrium (NLTE) emission, which can add as much as 10 K to the measured daytime brightness temperatures (BT). In this paper we report a fast radiative transfer model for a nadir sounding instrument (AIRS) that includes the effects of NLTE, allowing the retrieval algorithm to use many short wave CO2 channels for upper‐air soundings. Model biases and standard deviations are very similar for both day and night. This work allows an infrared sounder to probe the upper atmosphere much more completely using only short wave 4.3–4.5 μm channels.

Data from hyperspectral infrared sounders are routinely ingested worldwide by the National Weather Centers. The cloud‐free fraction of this data is used for initializing forecasts which include temperature, water vapor, water cloud, and ice cloud profiles on a global grid. Although the data from these sounders are sensitive to the vertical distribution of ice and liquid water in clouds, this information is not fully utilized. In the future, this information could be used for validating clouds in National Weather Center models and for initializing forecasts. We evaluate how well the calculated radiances from hyperspectral Radiative Transfer Models (RTMs) compare to cloudy radiances observed by AIRS and to one another. Vertical profiles of the clouds, temperature, and water vapor from the European Center for Medium‐Range Weather Forecasting were used as input for the RTMs. For nonfrozen ocean day and night data, the histograms derived from the calculations by several RTMs at 900 cm−1 have a better than 0.95 correlation with the histogram derived from the AIRS observations, with a bias relative to AIRS of typically less than 2 K. Differences in the cloud physics and cloud overlap assumptions result in little bias between the RTMs, but the standard deviation of the differences ranges from 6 to 12 K. Results at 2,616 cm−1 at night are reasonably consistent with results at 900 cm−1. Except for RTMs which use full scattering calculations, the bias and histogram correlations at 2,616 cm−1 are inferior to those at 900 cm−1 for daytime calculations.