Cloud Detection and Clearing for the Earth Observing System Terra Satellite Measurements of Pollution in the Troposphere (MOPITT) Experiment

The Measurements of Pollution in the Troposphere (MOPITT) instrument, which was launched aboard the Earth Observing System (EOS) Terra spacecraft on 18 December 1999, is designed to measure tropospheric CO and CH(4) by use of a nadir-viewing geometry. The measurements are taken at 4.7 mum in the thermal emission and absorption for the CO mixing ratio profile retrieval and at 2.3 and 2.2 mum in the reflected solar region for the total CO column amount and CH(4) column amount retrieval, respectively. To achieve the required measurement accuracy, it is critical to identify and remove cloud contamination in the radiometric signals. We describe an algorithm to detect cloudy pixels, to reconstruct clear column radiance for pixels with partial cloud covers, and to estimate equivalent cloud top height for overcast conditions to allow CO profile retrievals above clouds. The MOPITT channel radiances, as well as the first-guess calculations, are simulated with a fast forward model with input atmospheric profiles from ancillary data sets. The precision of the retrieved CO profiles and total column amounts in cloudy atmospheres is within the expected ?10% range. Validations of the cloud-detecting thresholds with the moderate-resolution imaging spectroradiometer airborne simulator data and MOPITT airborne test radiometer measurements were performed. The validation results showed that the MOPITT cloud detection thresholds work well for scenes covered with more than 5-10% cloud cover if the uncertainties in the model input profiles are less than 2 K for temperature, 10% for water vapor, and 5% for CO and CH(4).     

Tropospheric CO observed with the NAST-I retrieval methodology, analyses, and first results

High-resolution infrared spectra from aircraft and space-based observations contain information about tropospheric carbon monoxide (CO) as well as other trace species. A methodology for retrieving tropospheric CO from such remotely sensed spectral data has been developed for the National Polar-Orbiting Operational Environmental Satellite System's Airborne Sounder Testbed-Interferometer (NAST-I). CO profiles of the troposphere, together with its thermodynamic properties, are determined by use of a three-stage retrieval approach that combines the algorithms of physically based statistical eigenvector regression, simultaneous and iterative matrix inversion, and single-variable error-minimization CO profile matrix inverse retrieval. The NAST-I is collecting data while it is aboard high-altitude aircraft throughout many field campaigns. Detailed retrieval analyses based on the NAST-I instrument system along with retrieval results from several recent field campaigns are presented to demonstrate NAST-I CO retrieval capability.     

Radiometric calibration for the airborne interferometric monitor for greenhouse gases simulator

The Advanced Earth Observation Satellite (ADEOS), launched in the summer of 1996, has a high-resolution infrared Fourier transform spectrometer, with the interferometric monitor for greenhouse gases (IMG) onboard. The IMG has a high spectral resolution of 0.1 cm(-1) for the purpose of retrieving greenhouse gas profile maps of the Earth. To meet the requirements of the retrieval algorithms for greenhouse gas profiles, atmospheric emission spectra must be calibrated to better than 1 K accuracy. Prior to the launch of the ADEOS with the IMG, we developed an airborne simulator called the tropospheric infrared interferometric sounder (TIIS). We explain the calibration procedure for the TIIS, which determines the points with the same optical path difference on interferograms for complex Fourier transformation, using the retained phase term on the calibrated spectrum. The downward atmospheric radiation, measured with the TIIS, was well calibrated using this algorithm. Furthermore, calibration of the spectra obtained from the IMG initial checkout mission observation was carried out.     

Sensitivity studies for space-based measurement of atmospheric total column carbon dioxide by reflected sunlight

The feasibility of making space-based carbon dioxide (CO2) measurements for global and regional carbon-cycle studies is explored. With the proposed detection method, we use absorption of reflected sunlight near 1.58 microm. The results indicate that the small (degrees 1%) changes in CO2 near the Earth's surface are detectable provided that an adequate sensor signal-to-noise ratio and spectral resolution are achievable. Modification of the sunlight path by scattering of aerosols and cirrus clouds could, however, lead to systematic errors in the CO2 column retrieval; therefore ancillary aerosol and cloud data are important to reduce errors. Precise measurement of surface pressure and good knowledge of the atmospheric temperature profile are also required.     

Simultaneous retrieval of atmospheric profiles, land-surface temperature, and surface emissivity from Moderate-Resolution Imaging Spectroradiometer thermal infrared data: extension of a two-step physical algorithm

An extension to the two-step physical retrieval algorithm was developed. Combined clear-sky multitemporal and multispectral observations were used to retrieve the atmospheric temperature-humidity profile, land-surface temperature, and surface emissivities in the midwave (3-5 microns) and long-wave (8-14.5 microns) regions. The extended algorithm was tested with both simulated and real data from the Moderate-Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator. A sensitivity study and error analysis demonstrate that retrieval performance is improved by the extended algorithm. The extended algorithm is relatively insensitive to the uncertainties simulated for the real observations. The extended algorithm was also applied to real MODIS daytime and nighttime observations and showed that it is capable of retrieving medium-scale atmospheric temperature water vapor and retrieving surface temperature emissivity with retrieval accuracy similar to that achieved by the Geostationary Operational Environmental Satellite (GOES) but at a spatial resolution higher than that of GOES.     

Pressure sounding of the middle atmosphere from ATMOS solar occultation measurements of atmospheric CO(2) absorption lines

A method for retrieving the atmospheric pressure corresponding to the tangent point of an infrared spectrum recorded in the solar occultation mode is described and applied to measurements made by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier-transform spectrometer. Tangent pressure values are inferred from measurements of isolated CO(2) lines with temperature-insensitive strengths by measuring the slant-column CO(2) amount and by adjusting the viewing geometry until the calculated column matches the observed column. Tangent pressures are determined with a spectroscopic precision of l%-3%, corresponding to a tangent-point height precision of 70-210 m. The total uncertainty is limited primarily by the quality of the spectra and ranges between 4% and 6% (280-420 m) for spectra with signal-to-noise ratios of 300:1 and between 4% and 10% for spectra with signal-to-noise ratios of 100:1. The retrieval of atmospheric pressure increases the accuracy of the retrieved-gas concentrations by minimizing the effect of systematic errors introduced by climatological pressure data, ephemeris parameters, and the uncertainties in instrumental pointing.     

Methods for determining regularization for atmospheric retrieval problems

The atmosphere of Earth has already been investigated by several spaceborne instruments, and several further instruments will be launched, e.g., NASA's Earth Observing System Aura platform and the European Space Agency's Environmental Satellite. To stabilize the results in atmospheric retrievals, constraints are used in the iteration process. Therefore hard constraints (discretization of the retrieval grid) and soft constraints (regularization operators) are included in the retrieval. Tikhonov regularization is often used as a soft constraint. In this study, different types of Tikhonov operator were compared, and several new methods were developed to determine the optimal strength of the constraint operationally. The resulting regularization parameters were applied successfully to an ozone retrieval from simulated nadir sounding spectra like those expected to be measured by the Tropospheric Emission Spectrometer, which is part of the Aura platform. Retrievals were characterized by means of estimated error, averaging kernel, vertical resolution, and degrees of freedom.     

Retrieval of geophysical parameters from moderate resolution imaging spectroradiometer thermal infrared data: evaluation of a two-step physical algorithm

A two-step physical algorithm that simultaneously retrieves geophysical parameters from Moderate Resolution Imaging Spectroradiometer (MODIS) measurements was developed. The retrieved geophysical parameters include atmospheric temperature-humidity profile, surface skin temperature, and two surface emissivities within the shortwave (3-5-mum) and the longwave (8-14.5-mum) regions. The physical retrieval is accomplished in two steps: (i) The Tikhonov regularization method is employed to generate a regularization solution along with an optimum regularization parameter; (ii) the nonlinear Newtonian iteration algorithm is carried out with the regularization solution as a first-guess profile to obtain a final maximum probability solution for geophysical parameters. The algorithm was tested with both simulated and real MODIS Airborne Simulator (MAS) data. Sensitivity studies on simulated MAS data demonstrate that simultaneous retrievals of land and atmospheric parameters improve the accuracy of the retrieved geophysical parameters. Finally, analysis and accuracy of retrievals from real MAS data are discussed.     

Bandpass-resampling effects for the retrieval of surface emissivity

The retrieval of surface emissivity in the 8-14-microm region from remotely sensed thermal imagery requires channel-averaged values of atmospheric transmittance, path radiance, and downwelling sky flux. Band-pass resampling introduces inherent retrieval errors that depend on atmospheric conditions, spectral region, bandwidth, flight altitude, and surface temperature. This simulation study is performed for clear sky conditions and moderate atmospheric water vapor contents. It shows that relative emissivity retrieval errors can reach as much as 3% for broadband sensors (1-2-microm bandwidth) and 0.8% for narrowband instruments (0.15 microm), even for constant surface emissivity. For spectrally varying surface emissivities the relative retrieval error increases for the broadband instrument by approximately 2% in channels with strong emissivity changes of 0.05-0.1. The corresponding retrieval errors for narrowband sensors increase by approximately 3-4%. The channels in the atmospheric window regions with lower transmittance, i.e., 8-8.5 and 12.5-14 microm, are most sensitive to retrieval errors.     

Thermodynamic product retrieval methodology and validation for NAST-I

The National Polar-Orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounder Testbed (NAST) consists of two passive collocated cross-track scanning instruments, an infrared interferometer (NAST-I) and a microwave radiometer (NAST-M), that fly onboard high-altitude aircraft such as the NASA ER-2 at an altitude near 20 km. NAST-I provides relatively high spectral resolution (0.25-cm(-1)) measurements in the 645-2700-cm(-1) spectral region with moderate spatial resolution (a linear resolution equal to 13% of the aircraft altitude at nadir) cross-track scanning. We report the methodology for retrieval of atmospheric temperature and composition profiles from NAST-I radiance spectra. The profiles were determined by use of a statistical eigenvector regression algorithm and improved, as needed, by use of a nonlinear physical retrieval algorithm. Several field campaigns conducted under varied meteorological conditions have provided the data needed to verify the accuracy of the spectral radiance, the retrieval algorithm, and the scanning capabilities of this instrumentation. Retrieval examples are presented to demonstrate the ability to reveal fine-scale horizontal features with relatively high vertical resolution.     

Mid-infrared measurements of the atmospheric emission over the South Pole using a radiometrically calibrated Fourier transform spectrometer

We conducted year-round measurements of the downwelling atmospheric infrared emission over the South Pole in 1992. The instrument covered the 550-1600-wave-number region with 1-wave-number resolution. We calculated the water vapor content for clear-sky cases and found a good correlation with the surface temperature, with values ranging from 0.2 to 0.8 mm. Ozone-sonde profiles were compared with total column abundances of O(3) retrieved from the spectra. The experiment is explained in detail, including the instrumentation, calibration, and retrieval methods used. The calibrated spectra contain information about several trace gases, water, clouds, temperature profiles, and aerosols.     

GMTR: two-dimensional geo-fit multitarget retrieval model for michelson interferometer for passive atmospheric sounding/environmental satellite observations

We present a new retrieval model designed to analyze the observations of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), which is on board the ENVironmental SATellite (ENVISAT). The new geo-fit multitarget retrieval model (GMTR) implements the geo-fit two-dimensional inversion for the simultaneous retrieval of several targets including a set of atmospheric constituents that are not considered by the ground processor of the MIPAS experiment. We describe the innovative solutions adopted in the inversion algorithm and the main functionalities of the corresponding computer code. The performance of GMTR is compared with that of the MIPAS ground processor in terms of accuracy of the retrieval products. Furthermore, we show the capability of GMTR to resolve the horizontal structures of the atmosphere. The new retrieval model is implemented in an optimized computer code that is distributed by the European Space Agency as "open source" in a package that includes a full set of auxiliary data for the retrieval of 28 atmospheric targets.     

Frequency-resolved absorption tomography with tunable diode lasers

A tunable diode laser was used for absorption tomography in an axisymmetric atmospheric pressure flat-flame burner. A rapid tomographic inversion algorithm was used to facilitate the many reconstructions at a relatively sparse set of projections typical of laser absorption tomography. Profiles of temperature and CO2 mole fraction were measured simultaneously in methane-air flames. Absorption measurements were made near the R-branch bandhead at 4.17 microm to minimize interferences with other species, while providing good temperature and concentration sensitivity at flame conditions. The procedure showed the advantage of reconstructing detailed spectra at each radial node.     

Optical remote sensing of marine constituents in coastal waters: a feasibility study

Optical remote sensing of ocean color is a well-established technique for inferring ocean properties. However, most retrieval algorithms are based on the assumption that the radiance received by satellite instruments is affected only by the phytoplankton pigment concentration and correlated substances. This assumption works well for open ocean water but becomes questionable for coastal waters. To reduce uncertainties associated with this assumption, we developed a new algorithm for the retrieval of marine constituents in a coastal environment. We assumed that ocean color can be adequately described by a three-component model made up of chlorophyll a, suspended matter, and yellow substance. The simultaneous retrieval of these three marine constituents and of the atmospheric aerosol content was accomplished through an inverse-modeling scheme in which the difference between simulated radiances exiting the atmosphere and radiances measured with a satellite sensor was minimized. Simulated radiances were generated with a comprehensive radiative transfer model that is applicable to the coupled atmosphere-ocean system. The method of simulated annealing was used to minimize the difference between measured and simulated radiances. To evaluate the retrieval algorithm, we used simulated (instead of measured) satellite-received radiances that were generated for specified concentrations of aerosols and marine constituents, and we tested the ability of the algorithm to retrieve assumed concentrations. Our results require experimental validation but show that the retrieval of marine constituents in coastal waters is possible.     

Use of high-resolution measurements for the retrieval of temperature and gas-concentration profiles from outgoing infrared spectra in the presence of cirrus clouds

We explore ways in which high-spectral-resolution measurements can aid in the retrieval of atmospheric temperature and gas-concentration profiles from outgoing infrared spectra when optically thin cirrus clouds are present. Simulated outgoing spectra that contain cirrus are fitted with spectra that do not contain cirrus, and the residuals are examined. For those lines with weighting functions that peak near the same altitude as the thin cirrus, unique features are observed in the residuals. These unique features are highly sensitive to the resolution of the instrumental line shape. For thin cirrus these residual features are narrow (< or = 0.1 cm(-1)), so high spectral resolution is required for unambiguous observation. The magnitudes of these unique features are larger than the noise of modern instruments. The sensitivities of these features to cloud height and cloud optical depth are also discussed. Our sensitivity studies show that, when the errors in the estimation of temperature profiles are not large, the dominant contribution to the residuals is the misinterpretation of cirrus. An analysis that focuses on information content is also presented. An understanding of the magnitude of the effect and of its dependence on spectral resolution as well as on spectral region is important for retrieving spacecraft data and for the design of future infrared instruments for forecasting weather and monitoring greenhouse gases.     

Testing carbon sequestration site monitor instruments using a controlled carbon dioxide release facility

Two laser-based instruments for carbon sequestration site monitoring have been developed and tested at a controlled carbon dioxide (CO(2)) release facility. The first instrument uses a temperature tunable distributed feedback (DFB) diode laser capable of accessing the 2.0027-2.0042 microm spectral region that contains three CO(2) absorption lines and is used for aboveground atmospheric CO(2) concentration measurements. The second instrument also uses a temperature tunable DFB diode laser capable of accessing the 2.0032-2.0055 mum spectral region that contains five CO(2) absorption lines for underground CO(2) soil gas concentration measurements. The performance of these instruments for carbon sequestration site monitoring was studied using a newly developed controlled CO(2) release facility. A 0.3 ton CO(2)/day injection experiment was performed from 3-10 August 2007. The aboveground differential absorption instrument measured an average atmospheric CO(2) concentration of 618 parts per million (ppm) over the CO(2) injection site compared with an average background atmospheric CO(2) concentration of 448 ppm demonstrating this instrument's capability for carbon sequestration site monitoring. The underground differential absorption instrument measured a CO(2) soil gas concentration of 100,000 ppm during the CO(2) injection, a factor of 25 greater than the measured background CO(2) soil gas concentration of 4000 ppm demonstrating this instrument's capability for carbon sequestration site monitoring.     

Adaptive semianalytical inversion of ocean color radiometry in optically complex waters

To address the challenges of the parameterization of ocean color inversion algorithms in optically complex waters, we present an adaptive implementation of the linear matrix inversion method (LMI) [J. Geophys. Res.101, 16631 (1996)], which iterates over a limited number of model parameter sets to account for naturally occurring spatial or temporal variability in inherent optical properties (IOPs) and concentration specific IOPs (SIOPs). LMI was applied to a simulated reflectance dataset for spectral bands representing measured water properties of a macrotidal embayment characterized by a large variability in the shape and amplitude factors controlling the IOP spectra. We compare the inversion results for the single-model parameter implementation to the adaptive parameterization of LMI for the retrieval of bulk IOPs, the IOPs apportioned to the optically active constituents, and the concentrations of the optically active constituents. We found that ocean color inversion with LMI is significantly sensitive to the a priori selection of the empirical parameters g0 and g1 of the equations relating the above-surface remote-sensing reflectance to the IOPs in the water column [J. Geophys. Res.93, 10909 (1988)]. When assuming the values proposed for open-ocean applications for g0 and g1 [J. Geophys. Res.93, 10909 (1988)], the accuracy of the retrieved IOPs, and concentrations was substantially lower than that retrieved with the parameterization developed for coastal waters [Appl. Opt.38, 3831 (1999)] because the optically complex waters analyzed in this study were dominated by particulate and dissolved matter. The adaptive parameterization of LMI yielded consistently more accurate inversion results than the single fixed SIOP model parameterizations of LMI. The adaptive implementation of LMI led to an improvement in the accuracy of apportioned IOPs and concentrations, particularly for the phytoplankton-related quantities. The adaptive parameterization encompassing wider IOP ranges were more accurate for the retrieval of bulk IOPs, apportioned IOPs, and concentration of optically active constituents.     

Multiparameter retrieval of water optical properties from above-water remote-sensing reflectance using the simulated annealing algorithm

Water-leaving radiance, measured just above the ocean surface, contains important information about near-surface or subsurface processes that occur on or below the deep ocean and coastal water. As such, retrieving seawater inherent optical properties (IOPs) is an important step to determining water type, subsurface light field, turbidity, pigment concentration, and sediment loading. However, the retrieval (or inversion) of seawater IOPs from just above water radiance measurements is a multiparameter nonlinear problem that is difficult to solve by conventional optimization methods. The applicability of the simulated annealing algorithm (SA) is explored as a nonlinear global optimizer to solve this multiparameter retrieval problem. The SA algorithm is combined with widely known semianalytical relations for seawater's IOPs to parameter invert these properties from simulated and measured water-leaving reflectance spectra. Furthermore, given the versatility of the SA algorithm, the scheme is extended to retrieve water depth from input reflectance data. Extensive tests and comparisons with in situ and simulated data sets compiled by the International Ocean-Color Coordinating Group are presented. Field data include reflectance spectra acquired with a handheld GER 1500 spectroradiometer and absorption measurements, performed with the AC-9 instrument on waters around Singapore's nearby islands.     

Validation of a rotational coherent anti-Stokes Raman spectroscopy model for carbon dioxide using high-resolution detection in the temperature range 294-1143 K

Experiments were performed in the temperature range of 294-1143 K in pure CO(2) using high-resolution rotational coherent anti-Stokes Raman spectroscopy (CARS), in the dual-broadband approach. Experimental single-shot spectra were recorded with high spectral resolution using a single-mode Nd:YAG laser and a relay imaging lens system on the exit of a 1 m spectrometer. A theoretical rotational CARS model for CO(2) was developed for evaluation of the experimental spectra. The evaluated mean temperatures of the recorded single-shot dual-broadband rotational coherent anti-Stokes Raman spectroscopy (DB-RCARS) spectra using this model showed good agreement with thermocouple temperatures, and the relative standard deviation of evaluated single-shot temperatures was generally 2-3%. Simultaneous thermometry and relative CO(2)/N(2)-concentration measurements were demonstrated in the product gas of premixed laminar CO/air flames at atmospheric pressure. Although the model proved to be accurate for thermometry up to 1143 K, limitations were observed at flame temperatures where temperatures were overestimated and relative CO(2)/N(2) concentrations were underestimated. Potential sources for these discrepancies are discussed.     

Simultaneous retrievals of temperature and volume mixing ratio constituents from nonoxygen odin submillimeter radiometer bands

We present the retrieval of temperature and O(3) volume mixing ratio profiles in the middle atmosphere from a single strong O(3) line. We performed the study using simulated limb-sounding measurements in the frame of the submillimeter radiometer (SMR) instrument that will be carried by the Odin satellite that is due to be launched in early 2001. This study is interesting for the Odin SMR data analysis because we first provide additional temperature measurements, and second reduce significantly the O(3) retrieval error that is due to the temperature and pressure uncertainties. Nonlinear retrievals are performed to retrieve the O(3), CO, H(2)O, and temperature profiles simultaneously from the spectral band 576.27-576.67 GHz. The pressure profile is deduced from the hydrostatic equilibrium equation after each iteration. Temperature and O(3) can be retrieved throughout the stratosphere from 15-50 and 20-50 km, respectively, with a vertical resolution of 3 km. The altitude domain corresponds to the parts of the atmosphere where the signal intensity saturates in some spectrometer channels. A total error of 4-6 K has been found in the temperature profile, mainly because of the instrumental thermal noise and to a lesser extent the calibration. The total error in the O(3) profile is 5-10% and is dominated by the O(3) line-broadening parameter. The total error on the retrieved pressure profile is 2-10% because of the errors in calibration and reference pressure.