Marco Matricardi

Hyperspectral Earth Observation from IASI: Five Years of Accomplishments

The Infrared Atmospheric Sounding Interferometer (IASI) forms the main infrared sounding component of the European Organisation for the Exploitation of Meteorological Satellitess (EUMETSATs) Meteorological Operation (MetOp)-A satellite (Klaes et al. 2007), which was launched in October 2006. This article presents the results of the first 4 yr of the operational IASI mission. The performance of the instrument is shown to be exceptional in terms of calibration and stability. The quality of the data has allowed the rapid use of the observations in operational numerical weather prediction (NWP) and the development of new products for atmospheric chemistry and climate studies, some of which were unexpected before launch. The assimilation of IASI observations in NWP models provides a significant forecast impact; in most cases the impact has been shown to be at least as large as for any previous instrument. In atmospheric chemistry, global distributions of gases, such as ozone and carbon monoxide, can be produ...

Fast radiative transfer model for simulation of infrared atmospheric sounding interferometer radiances.

A fast radiative transfer model has been developed for prelaunch simulation studies of Infrared Atmospheric Sounding Interferometer (IASI) data and for the exploitation of IASI radiances within the framework of a numerical weather prediction variational analysis scheme. The model uses profile-dependent predictors to parameterize the atmospheric optical depths and is fast enough to cope with the processing of observations in near real time and with the several thousands of transmittance calculations required to simulate radiances from a full range of atmospheric conditions. The development of the model has involved the selection of a training set of atmospheric profiles, the production of a line-by-line transmittance database, the selection of optimal predictors for the gases considered in the study, and the production of regression coefficients for the fast transmittance scheme. The model fit to the line-by-line radiances shows that it can reproduce the line-by-line radiances to a degree of accuracy that is at or below the instrumental noise.

The ISSWG line-by-line inter-comparison experiment

Abstract To document the performance of current line-by-line radiative transfer models, a study was performed to compare the model simulations with real observations and also inter-compare the simulations themselves. Two broadband mid-IR observed spectra with high spectral resolution were analyzed. The observations were done in nadir mode, and at the same time the atmospheric state was carefully monitored. The first dataset consisted of radiance observations using the HIS interferometer during the CAMEX-1 campaign off the east coast of the USA. The second dataset consisted of observations from the ARIES interferometer collected during the Ascension Island campaign over the tropical Atlantic. These two cases are very different with the Ascension Island case being much warmer and more humid than the CAMEX case. In total 13 different research groups participated with seven different line-by-line models. The results of this study indicated that in many spectral regions the models are capable of reproducing the observations to within the observed noise. In some spectral regions relatively large differences between the simulations and observations exist.

Estimating atmospheric CO2 from advanced infrared satellite radiances within an operational 4D‐Var data assimilation system: Methodology and first results

[1] Atmospheric CO2 concentrations have been obtained from the Atmospheric Infrared Sounder (AIRS) radiance data within the European Centre for Medium-Range Weather Forecasts data assimilation system. A subset of channels from the AIRS instrument on board the NASA Aqua platform has been assimilated providing estimates of tropospheric and stratospheric column-average CO2 mixing ratios. Although global estimates are obtained, the information content of the tropospheric estimates at middle and high latitudes is limited, and results are therefore only presented for the tropical region. First results for February and August 2003 show considerable geographical variability compared to the background with values ranging between 371 and 380 ppmv. These CO2 values are representative for a layer between the tropopause and about 600 hPa. The monthly mean random error is about 1%. Careful error analysis has been carried out to minimize any systematic errors. This study has demonstrated the feasibility of global CO2 estimation using AIRS data in a numerical weather prediction data assimilation system. In the future the system will be improved to treat CO2 as a full three-dimensional atmospheric variable, including transport. INDEX TERMS: 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 1610 Global Change: Atmosphere (0315, 0325); KEYWORDS: AIRS, carbon dioxide, data assimilation

Homomorphism between cloudy and clear spectral radiance in the 800–900-cm -1 atmospheric window region

The sensitivity of a new algorithm for cloud detection over a sea surface has been assessed on the basis of extensive simulations of clear and cloudy radiance spectra, including water and ice and low- and high-altitude clouds. The new algorithm makes use of autocorrelation and cross correlation between an observed spectrum and either a synthetic or a laboratory spectrum and can be used to determine quantitatively the degree of homogeneity of two spectra in the 800-900-cm(-1) region (11.11-12.5 microm). The scheme is intended for high-spectral-resolution observations and could form the basis for an operational stand-alone cloud-detection algorithm for next-generation sounding spectrometers. Application of the scheme to real observations is presented and discussed.

Simulation of uplooking and downlooking high-resolution radiance spectra with two different radiative transfer models

Measurements of up-looking spectral radiances measured during the Convection and Moisture Experiment and down-looking spectral radiances measured at one of the Atmospheric Radiation Measurement sites are compared with simulations with use of two different line-by-line models. Simulations are performed in tightly controlled conditions to verify the behavior of the models. Spectra computed at higher samplings are used to study the spectral structure of the differences between simulations and measurements. A revised list of water vapor spectroscopic parameters is used to test the impact of improved spectroscopic data on the accuracy of the line-by-line calculations. The sensitivity of the results to errors that result from uncertainties in the input atmospheric temperature and humidity profiles is also investigated.

Partially scanned interferogram methodology applied to IASI for the retrieval of CO, CO 2 , CH 4 and N 2 O

The technique of partially scanned interferograms is applied to the retrieval of trace gases from Infrared Atmospheric Sounding Interferometer (IASI) observations. For the specific case of CO, CO₂, CH₄ and N₂O, we show that this methodology allows us to retrieve trace gases column abundances at an unprecedented accuracy at the level of the single IASI footprint. The technique consists in transforming the IASI spectra back to the interferogram domain where we identify small regions that are mostly sensitive to single gas species. The retrieval is then performed by directly applying Least Squares estimation to these small segments of interferometric radiances. One of the main advantages of the technique is that it allows the efficient use of the information contained in all the IASI channels that are available in the absorption bands of a specific gas species. The retrieval technique has been applied to IASI radiances measured over the Mediterranean sea during the month of July 2010, one of the hottest months on record. Results have been validated against ground-based measurements. We have also carried out a comparison with Atmospheric Infrared Radiometer Sounder data and IASI retrievals obtained with usual variational approaches in the spectral domain.

Retrieving N2O from nadir-viewing infrared spectrometers

The paper describes and demonstrates a methodology for the physical retrieval of nitrous oxide that uses the spectral radiance measured by the next generation of high-resolution satellite-borne infrared sensors. The performance of the retrieval scheme has been assessed on the basis of numerical exercises. Examples of retrievals based on Interferometric Monitoring of Greenhouse Gases (IMG) spectra measured over the sea surface are given to demonstrate the ability of the scheme to obtain accurate N2O concentration values.

IMG evidence of chlorofluorocarbon absorption in the atmospheric window region 800–900 cm−1

Abstract Infrared spectra recorded by the interferometric monitoring of greenhouse gases sensor have been analysed to assess the effect of the molecule CCl3F in the atmospheric window region 800–900 cm −1 . The analysis has been carried out at a sampling rate of 0.25 cm −1 which is typical of next generation space borne infrared sensors such as the European infrared atmospheric sounding interferometer. It has been found that at this spectral resolution absorption by CCl3F is clearly evident and mostly effective in the spectral range 840–860 cm −1 where it may account for up to 1K depletion in the brightness temperature spectrum. It will be shown that a CCl3F concentration of 270pptv (mixing ratio) fits very well to the spectral observations. Our findings show that chlorofluorocarbons may play an interfering role in determining surface or close to surface geophysical parameters. On the other hand, the fact that these compounds are clearly identifiable in infrared spectra opens the way to an effective monitoring of their presence and abundance from space.

Evaluation of Radiative Transfer Models With Clouds

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 . 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. Plain Language Summary Getting the right clouds of the right type, at the right time and location in Global Circulation Models, is key to getting the local energy balance right. This is key to an accurate forecast. If the clouds are of the wrong type or at the wrong location or time, the accuracy of the forecast is degraded. We evaluate the accuracy of the best currently available cloud description (produced by the European Center for Medium-Range Weather Forecasting) by comparing the radiances calculated using Radiative Transfer Models (RTMs) from six major development teams to cloudy radiances observed by the Atmospheric Infrared Sounder at the same location and time. The better RTMs fit statistically reasonably well in the 11-μm atmospheric window area, with little latitude (zonal) and day/night cloud-type related bias. None of the RTMs fit well in the 4-μm atmospheric window area during daytime, unless the calculations use full scattering. With the current state of art, all major RTMs would be suitable to start the validation of cloud effects in the National Weather Center models using just one 11-μm atmospheric window channel.