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N. A. Scott and A. Chedin

Abstract

A computationally fast line-by-line method for the determination of atmospheric absorption is described. This method is based on the creation of an Automatized Atmospheric Absorption Atlas (4A) covering all possible plausible atmospheric conditions (temperature, mixing ratios of absorbing gases, zenith angle). It is applied to synthetic computations of atmospheric transmittances and radiant energies associated with three types of satellite observations: radiometric measurements made by HIRS/2 (High-Resolution Infrared Sounder) on TIROS-N; infrared images taken from the geostationary satellite METEOSAT; interferometric experiment IRIS (Infrared Interferometer Spectrometer) on VOYAGER (NASA's mission to Jupiter, Saturn and possibly Uranus). For all three experiments, comparisons were made with real observations and are presented associated with radiosonde data for the first. Concerning computation times, a gain of a factor varying between 15 and 40 is obtained when using the 4A line-by-line method rather than a standard line-by-line method.

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A. Chedin, N. A. Scott, and A. Berroir

Abstract

An experimental simulation of a single-channel, double-angle viewing technique for the determination of sea surface temperature from satellite is presented. This method relies upon the fact that the same area can be viewed simultaneously at two different angles (different air masses) by the geostationary satellite METEOSAT and by the polar orbiting satellite TIROS-N. Extrapolating the two air mass observations to zero air mass is shown to give a value of the temperature in good agreement with the true sea surface temperature. A discussion concerning the viewing angles is presented.

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D. Imbault, N. A. Scott, and A. Chedin

Abstract

Synthetic computations of atmospheric transmissions and radiant energies are presented in order to simulate remote soundings of the sea surface temperature in two pairs of channels: ∼8.7 and 11.6 μm, on the one hand, and 11 and 12 μm, on the other. This study takes into account absorptions by the line spectra of the various absorbers and by the water vapor continuum, first separately and then simultaneously. In the latter case, the accuracy of the linear parametric retrieval scheme previously presented by Imbault et al. (1978) is shown to depend mostly on the choice of wavelengths for the two channels and two criteria for optimization are introduced. Applying this parametric scheme to the analysis of the experimental results obtained, with the help of an airborne scanning radiometer, over a relatively humid atmosphere within the midlatitudes has led us to conclude that the usual expression for the water vapor absorption coefficient seems to overestimate this phenomenon.

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E. Péquignot, A. Chédin, and N. A. Scott

Abstract

Atmospheric Infrared Sounder (AIRS; NASA Aqua platform) observations over land are interpreted in terms of monthly mean surface emissivity spectra at a resolution of 0.05 μm and skin temperature. For each AIRS observation, an estimation of the atmospheric temperature and water vapor profiles is first obtained through a proximity recognition within the thermodynamic initial guess retrieval (TIGR) climatological library of about 2300 representative clear-sky atmospheric situations. With this a priori information, all terms of the radiative transfer equation are calculated by using the Automatized Atmospheric Absorption Atlas (4A) line-by-line radiative transfer model. Then, surface temperature is evaluated by using a single AIRS channel (centered at 12.183 μm) chosen for its almost constant emissivity with respect to soil type. Emissivity is then calculated for a set of 40 atmospheric windows (transmittance greater than 0.5) distributed over the AIRS spectrum. The overall infrared emissivity spectrum at 0.05-μm resolution is finally derived from a combination of high-spectral-resolution laboratory measurements of various materials carefully selected within the Moderate-Resolution Imaging Spectroradiometer/University of California, Santa Barbara (MODIS/UCSB) and Advanced Spaceborne Thermal Emission and Reflection Radiometer/Jet Propulsion Laboratory (ASTER/JPL) emissivity libraries. It is shown from simulations that the accuracy of the method developed in this paper, the multispectral method (MSM), varies from about 3% around 4 μm to considerably less than 1% in the 10–12-μm spectral window. Three years of AIRS observations (from April 2003 to March 2006) between 30°S and 30°N have been processed and interpreted in terms of monthly mean surface skin temperature and emissivity spectra from 3.7 to 14.0 μm at a spatial resolution of 1° × 1°. AIRS retrievals are compared with the MODIS (also flying aboard the NASA/Aqua platform) monthly mean L3 products and with the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies baseline-fit method (UW/CIMSS BF) global infrared land surface emissivity database.

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C. Claud, N. A. Scott, and A. Chedin

Abstract

A 4½-yr monthly stratospheric temperature record derived from TIROS-N Operational Vertical Sounder satellite observations has been used to study the global variability of the stratosphere in the Tropics. A comparison with an independent set of temperatures (Free University of Berlin) is first discussed. Among the different parameters that influence the tropical stratosphere, 1) the regular seasonal cycle, 2) the quasi-biennal oscillation (QBO), and 3) the El Niño–Southern Oscillation (ENSO) effects are studied in detail. A transition level has been found at about 30 hPa. Below this level, the standard stratospheric seasonal cycle in the temperatures is modulated by ENSO and the QBO, while above, ENSO has no discernible influence. In addition, longitudinal variations of monthly mean temperatures show minima during northern winter months from the tropopause up to 50 hPa over some areas, in relation to convection. Results presented here are also discussed in the view of recently published studies based on either radiosonde reports or microwave satellite measurements. While there is a fair agreement with radiosonde-based studies, more finescale details on the horizontal are obtained due to a much better sampling. Differences with other satellite-based studies are due to a better description of the temperature behavior along the vertical.

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C. J. Stubenrauch, N. A. Scott, and A. Chedin

Abstract

Onboard the NOAA satellites, the High-Resolution Infrared Sounder (HIRS) with its 20 channels, combined with the Microwave Sounding Unit (MSU), provides a powerful tool for cloud field classification at a spatial resolution of about 100 km. The 3I (improved initialization inversion) algorithm-developed to obtain atmospheric temperature and water vapor profiles as well as cloud and surface properties-has been modified in order to extract more reliable information on cloud-top pressure and effective cloud amount. These cloud parameters have been compared to cloud types identified by an operationally working threshold algorithm based on Advanced Very High Resolution Radiometer measurements over the North Atlantic. The improved 3I cloud algorithm provides cloud parameters not only for high clouds but also greatly improves the determination of low clouds. The algorithm has also been extended to give cloud information over partly cloudy situations. The 3I cloud field classification yields 11 different cloud field types for spatial elements of 100 km according to cloud height, cloud thickness, and cloud cover. The radiative effects of these different cloud field types are studied by combining the 3I results with Earth Radiation Budget Experiment (ERBE) fluxes. A simple radiative transfer theory can relate the ERBE outgoing longwave flux to all 3I cloud field types to within 5 W m−2. This encourages a detailed analysis of cloud radiative effects on a global scale. Especially during night, as shown in this study, International Satellite Cloud Climatology Project (ISCCP) cloud information can be extended by the HIRS-MSU analysis, because the ISCCP provides information on cloud thickness only during day.

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C. J. Stubenrauch, A. Chédin, R. Armante, and N. A. Scott

Abstract

First comparisons of improved initialization inversion (3I) cloud parameters determined from TIROS-N Operational Vertical Sounder observations with time–space-collocated clouds from the recently reprocessed International Satellite Cloud Climatology Project (ISCCP) dataset have shown a reasonable agreement between all cloud types, with exception of the stratocumulus regions off the western coasts. Here, 3I clouds were found systematically thinner and higher than ISCCP clouds. These results have initiated a careful investigation of the methods used to convert measurements from IR sounders into cloud parameters. All existing methods get very sensitive to the chosen temperature profile toward lower cloud heights, due to a denominator approaching zero. This leads to a bias like the one seen in the comparison with ISCCP. Therefore, a new 3I cloud scheme has been developed, based on a weighted-χ 2 method, which calculates the effective cloud amount from the CO2-band radiances, but weighted differently according to the effect of the brightness temperature uncertainty within an air mass on these radiances at the various cloud levels. This physically much more correct method led to unbiased 3I cloud parameters for homogeneous cloud types. The ISCCP comparison agrees much better now, especially in the stratocumulus regions where the cloud type matching improved from about 50% to 75%. In 1° grid boxes covered uniformly with the same ISCCP cloud type, the matching reaches even 87%. Remaining discrepancies in cloud classification can be explained by partly cloudy fields and differences in temperature profiles and cloud detection.

The weighted-χ 2 method can be used in other IR sounder inversion algorithms, if the empirical weights, taking care of the effect of temperature profile uncertainties on the difference between clear sky and cloudy radiances for different cloud levels and spectral channels, have been reevaluated so that they can be calculated automatically by the corresponding inversion algorithm.

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M. M. Poc, M. Roulleau, N. A. Scott, and A. Chedin

Abstract

In this paper, quantitative studies of water vapor Meteosat imagery, based on an accurate transmittance and radiance model are described. Using conventional radiosonde data, a linear relation between cloudless radiances and digital counts is obtained. It is shown that the distribution with altitude of the peak contribution to the radiances varies from about the 550 mb level to about the 450 mb level for pictures acquired in July over the western part of Europe and Mediterranean Sea. The influence of the temperature profile on the altitude of the maximum contribution layer is pointed out. A relation between the radiative field and the water vapor mass above the 600 mb level is found.

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N. Beriot, N. A. Scott, A. Chedin, and P. Sitbon

Abstract

A method is presented for the calibration of infrared radiometers on geostationary satellites using calibrated infrared radiometers on an orbiting satellite. This method relies on similarities between the weighting functions corresponding to the radiometers on geostationary satellites like METEOSAT or the GOES series and the weighting functions of some of the channels on the TIROS-N Operational Vertical Sounder (TOVS). It makes use of iso-secant observations of the same scene from both satellites. Many such observations are available every day resulting in daily calibration curves defined by several hundred points. This calibration method is shown to be very sensitive, accurate and tractable. This method does not require the collection of radiosonde data or any kind of in situ experiments and may be completely automated.

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F. Chevallier, F. Chéruy, N. A. Scott, and A. Chédin

Abstract

The authors have investigated the possibility of elaborating a new generation of radiative transfer models for climate studies based on the neural network technique. The authors show that their neural network–based model, NeuroFlux, can be used successfully for accurately deriving the longwave radiative budget from the top of the atmosphere to the surface. The reliable sampling of the earth’s atmospheric situations in the new version of the TIGR (Thermodynamic Initial Guess Retrieval) dataset, developed at the Laboratoire de Météorologie Dynamique, allows for an efficient learning of the neural networks. Two radiative transfer models are applied to the computation of the radiative part of the dataset: a line-by-line model and a band model. These results have been used to infer the parameters of two neural network–based radiative transfer codes. Both of them achieve an accuracy comparable to, if not better than, the current general circulation model radiative transfer codes, and they are much faster. The dramatic saving of computing time based on the neural network technique (22 times faster compared with the band model), 106 times faster compared with the line-by-line model, allows for an improved estimation of the longwave radiative properties of the atmosphere in general circulation model simulations.

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