The aerosol radiative effect in the longwave spectral range is often neglected in atmospheric aerosol forcing studies,
hence very few researches are conducted in this field at local scale, and even less at regional scale. However, strong
absorbing aerosols, like mineral dust, can have a small, but non-negligible heating effect in the longwave spectral range
which can slightly counteract the aerosol cooling effect in the shortwave. The objective of this research is to perform a
sensitivity study of an aerosol radiative transfer model as a function of dust particle properties. GAME model1, which
can compute vertically resolved shortwave and longwave values of aerosol radiative forcing, is used. Before developing
the sensitivity analysis, the aerosol radiative transfer model is validated by comparing its outputs with results published
previously. Radiative forcing simulations in the longwave have shown an important sensitivity to the following
parameters: aerosol size and refractive index, aerosol vertical distribution, humidity, surface temperature and albedo. A
couple of strong mineral dust intrusion observed by means of lidar and sun-photometer are also presented in terms of
shortwave and longwave radiative forcing.
Several techniques exist to correct the estimation of the cloud top pressure for semi-transparency effect, and the advent of
Meteosat Second Generation (MSG) enables the simultaneous use of the IR/CO2 ratioing methodology in addition to the
IR/WV intercept method. This paper presents the performances of these two methods using simulated data. The FASDOM
radiative code has been used to simulate MSG radiances for various types of clouds at different levels in the troposphere,
using different atmospheric profiles. Performances of the methods are presented as function of several atmospheric and cloud
parameters.
The feasibility of retrieving aerosol vertical distribution from the ratio of atmospheric radiance in, and out of, the
oxygen A-band is investigated. Two typical cases of aerosol vertical profiles are considered, namely an exponential
profile (aerosols concentrated near the surface), and a Gaussian profile (aerosols concentrated in altitude). The
problem is expressed as a linear inverse problem with a compact operator, and a Tikhonov regularization scheme
is implemented for its inversion. It is found that the exponential profile can be reconstructed accurately and in a
stable manner, while this is not the case for the profile with aerosols concentrated in altitude. These results are
explained by the spectral properties of the operator. Information on profile shape and/or utilization of spectral
ratios more sensitive to upper layers would improve reconstruction when aerosols are located in altitude.
A new methodology is proposed to estimate from space the solar-induced chlorophyll fluorescence of natural waters.
The methodology exploits absorption in the oxygen B-band around 687 nm, located near the peak of fluorescence
emission at 685 nm. Inside the oxygen absorption lines, the fluorescence signal enhances the reflected solar radiance. By
using a pair of spectral bands inside and outside the absorption region, or more generally spectral bands for which
oxygen absorption is sufficiently different, the emitted contribution to the measured radiance can be extracted.
Feasibility is demonstrated and retrieval accuracy quantified through simulations of the top-of-atmosphere reflectance by
a radiation transfer code that fully accounts for multiple scattering and interactions between scattering and absorption.
The differential absorption method works well from just above the surface. Pairs of spectral bands centered on the same
wavelength provide the best results. Using spectral bands of 686.8-688.3 nm and 683.1-692.0 nm, the expected accuracy
on fluorescence retrievals is <10% for chlorophyll concentrations above 1 mgm-3. Performance is degraded from space,
due to the influence of aerosol vertical structure on the oxygen transmittance associated with path reflectance. In this
case, knowledge of aerosol reflectance and optical thickness is required, but assuming an average aerosol vertical
distribution yields reasonable results. In comparison with the standard baseline technique, significant improvements in
retrieval accuracy are expected in Case II waters, especially in the presence of sediments.
Atmospheric Motion Vectors (AMVs) are one of the most important products generally derived from geostationary satellites,
and especially from Meteosat at EUMETSAT, because they constitute a very important part of the observational data fed to
Numerical Weather Prediction. The height estimation or 'assignment' (HA) is still the most challenging task in the AMV
extraction scheme. The advent of Meteosat Second Generation provides many new opportunities for improving the HA of
AMVs. Indeed, the existence of a CO2 absorption channel at 13.4 μm on the SEVIRI instrument enables the simultaneous use
of the IR/CO2 ratioing methodology in addition to the 'WV-IRW intercept method' (also called STC), for semi-transparent
cases. Due to the existence of several Water Vapour and Infrared channels on SEVIRI, each method is implemented in
slightly different configuration, and several pressures are then calculated for each AMV. It was expected at first to use the
agreement of these pressures as a quality check for the final AMV height. Unfortunately, the various methods (STC and CO2
slicing) have clearly their own sensitivity and domain of application, which makes a quality check very challenging. It
appeared then necessary to define these domains of application more precisely, in order that better use may be made of these
methods operationally.
This paper presents such results using simulated SEVIRI radiances calculated by the FASDOM radiative transfer code.
FASDOM accounts for gaseous absorption as well as cloud scattering and absorption and can precisely consider various types
of clouds with various microphysical properties. We then have the possibility to compare the outputs of the HA methods
knowing precisely the input to the model, especially the pressure of the simulated cloud.
For inferring cirrus optical and microphysical properties from satellite imagery, a common assumption is that
the radiative properties of a cirrus cloud may be represented by those associated with a specific ice crystal habit, a single
particle size distribution and Ice Water Content (IWC). Various algorithms have been developed to retrieve cirrus optical
and microphysical properties in the past 20 years. They can be categorized into the techniques based on either thermal
infrared or solar reflection measurements. However, in-situ measurements have shown that shapes, sizes and IWC of ice
crystals may vary substantially with height within the clouds. Given the different sensitivity of thermal infrared and solar
wavelength to cloud microphysics, it is unlikely that a single cloud layer with homogeneous cloud properties can be used
to reproduce both type of measurements. Thus, it is necessary to assess the effect of vertical inhomogeneity within cirrus
on the radiative transfer calculations and on the retrieval techniques.
The purpose of this study is to investigate a microphysical cirrus model composed of different layers in terms of
ice crystal habit, size and IWC. The vertical structure will be given by simple analytic formula derived from various
prescribed physical constraints. The primary goal of this study is to determine a simple cloud model that can be used to
retrieve consistent information from both solar and thermal measurements. For this purpose, we examine the sensitivity
of cirrus reflectances and brightness temperature to its vertical description for a suite of MODIS (MODerate-resolution
Imaging Spectroradiometer) bands spanning visible, near infrared and thermal infrared wavelengths. Results of this study
are presented and potential application to remote sensing of cirrus clouds with MODIS are discussed.
A methodology is presented for obtaining information on aerosol vertical structure, a key variable in studies of
aerosol climate forcing and atmospheric correction of satellite ocean-color imagery. The methodology employs
ground-based angular measurements of atmospheric radiance, total or polarized, in the oxygen A-band centered
on 763 nm. The radiance measured at different zenith angles is sensitive to different atmospheric layers, and
the measurements can be inverted to retrieve the vertical profile of aerosol concentration. To solve the inverse
problem, in which small errors in the data may yield large errors in the reconstructed profile, an iterative
regularization scheme, robust to noise and perturbing effects (e.g., due to multiple scattering and non-null surface
reflectance), is developed. Maximum entropy regularized solutions are introduced. The methodology is tested on
atmospheric radiance data simulated for typical aerosol profiles and aerosol types. The retrieved aerosol profiles
agree with the prescribed ones, indicating that the inversion scheme is efficient in achieving a proper balance
between goodness-of-fit to the data and stability of the solution. The methodology has the potential to extend
and complement surface observations of aerosol vertical structure made by lidar networks. This perspective
is significant, since current information on aerosol vertical structure is insufficient to constrain and verify key
assumptions in global aerosol models. The complementary information would contribute, via assimilation, to
improving predictions of aerosol radiative forcing and to reducing uncertainties in model simulations of climate
change. In addition, the methodology would help to evaluate the retrievals of aerosol vertical structure from
space-borne lidars, and would be useful to check the atmospheric correction of satellite ocean-color imagery and
develop improved correction algorithms in the presence of absorbing aerosols.
A methodology is presented to estimate aerosol altitude from reflectance ratio measurements in the O2 absorption A-band. Previous studies have shown the impact of the vertical distribution of scatterers on the reflectance ratio. The reflectance ratio is defined as the ratio of the reflectance in a first spectral band, strongly attenuated by O2 absorption, to the reflectance in a second spectral band, minimally attenuated. First, a sensitivity study is performed to quantify the expected accuracy for various aerosol loadings and models. An accurate, high spectral resolution, radiative transfer model that fully accounts for interactions between scattering and absorption is used in the simulations. Due to their adequate spectral characteristics, POLDER and MERIS instruments are considered for simulations. For a moderately loaded atmosphere (i.e., aerosol optical thickness of 0.3 at 760 nm), the expected error on aerosol altitude is about 0.3 km for MERIS and 0.7 km for POLDER. More accurate estimates are obtained with MERIS, since the spectral reflectance ratio is more sensitive. Second, the methodology is applied to MERIS and POLDER imagery. Estimates of aerosol altitude are compared with lidar profiles of backscattering coefficient acquired during the AOPEX-2004 experiment. Retrievals are consistent with measurements and theory. These comparisons demonstrate the potential of the differential absorption methodology for obtaining information on aerosol vertical distribution.
Ocean-color remote sensing from space is currently limited to cloud-free areas. Consequently, the daily ocean coverage is 15-20%, and weekly products show no information in many areas. This limits considerably the utility of satellite ocean color observations for operational oceanography. Global coverage is required every three to five days in the open ocean and at least every day in the coastal zone. In view of the requirements for spatial coverage, and of the effects of clouds on observations of ocean color, an algorithm is proposed to estimate marine reflectance in the presence of a thin or broken cloud layer. The algorithm's theoretical basis is that cloud reflectance at some near-infrared wavelength may be accurately extrapolated to shorter wavelengths, whatever the cloud geometry, without any additional information. The interaction between cloud droplets and molecules, in particular, follows a λ-4 law. On the contrary, estimating aerosol scattering requires at least a measurement of its spectral dependence. Applying the algorithm to actual satellite ocean color imagery, a substantial gain in ocean coverage is obtained. The oceanic features retrieved below the clouds exhibit continuity with the adjacent features in clear areas. The daily ocean coverage is expected to be increased to up to 50% with the proposed algorithm, allowing one to resolve better phytoplankton blooms in the open ocean and "events" linked to wind forcing in the coastal zone. This could lead to important new information about the temporal variability of biological processes.
The vertical distribution of absorbing aerosols affects significantly the reflectance of the ocean-atmosphere system. The effect, due to the coupling between molecule scattering and aerosol absorption, is important in the visible, especially in the blue, and becomes negligible in the near-infrared. Differences between top-of-atmosphere reflectance obtained with distinct vertical distributions increase with the sun, and view zenith angle, and the aerosol optical thickness, and with decreasing scattering albedo, but are practically independent of wind speed. In atmospheric correction algorithms, these differences are directly translated into errors on the retrieved water reflectance. They may reach large values even for small aerosol optical thickness, preventing accurate retrieval of chlorophyll concentration. A method has been developed to estimate aerosol altitude from data in the oxygen A-band of the MERIS, and POLDER sensors. The method is sufficiently sensitive to improve retrievals of water reflectance and chlorophyll concentration in the presence of absorbing aerosols.
We present the results of the first in-flight spectral calibration of the ENVISAT/MERIS instrument. An operational algorithm using the high sensitivity of absorption to the spectral location in the O2 bands is briefly presented. We show that an accuracy of ± 0.02 nm can be reached with a careful analysis of the whole dataset. This method was successfully compared to other techniques and proved to have the lowest noise. Consequences for the MERIS Surface Pressure product are investigated and optimization of the O2 bands setting is proposed.
Operational MERIS (MEdium Resolution Imaging Spectrometer) level 2 processing uses auxiliary data generated by two radiative transfer tools. These two codes simulate upwelling radiances within a coupled 'Atmosphere Land' system, using different approaches based on the matrix operator method (FUB), the discrete ordinate method and the successive orders technique (LISE). Intervalidation of these two radiative transfer tools was performed in order to implement them in the MERIS level 2 processing.. An extensive exercise was conducted for cases without gaseous absorption. The scattering processes both by the molecules and the aerosols were retrieved within few tenths of a percent. Nevertheless, some substantial discrepancies occurred if the polarization is not taken into account mainly in the Rayleigh scattering computations. Errors on the aerosol optical depth reach up to 30 percent in some geometries as observed in the SeaWiFS (Sea viewing Wide Field of view Sensor) images. The parameterization of the water vapor absorption defined for each of these two codes leads to a well agreement not only for the MERIS bands with residual absorption but also in the MERIS band centred at 900nm which is used for the water vapor retrieval. As for the strong oxygen absorption at the 760.625 nm MERIS wavelength, its parameterization varies between the two codes. Nevertheless, the systematic biases in the two codes will be removed thanks to the use of a differential method between two MERIS adjacent bands. For the oxygen absorption at 760.625 nm, a more exhaustive study need to be achieved.
The POLDER (polarization and directionality of the earth reflectances) instrument, to be launched in 1996 on the Japanese ADEOS platform, includes a channel which covers the 910 nm water-vapor absorption band (near IR), as well as a channel at 865 nm. It is expected that the ratio of the two reflectance measurements can yield an estimate of the total atmospheric water vapor content. The major uncertainties on this estimate result from (1) the surface reflectance spectral signature, (2) scattering by atmospheric aerosol, and (3) the water-vapor vertical profile. A radiative transfer model has been developed in order to quantify these uncertainties. From radiative transfer simulations, an uncertainty on the order of 10% is expected on the total water vapor amount. Moreover, an airborne version of the POLDER instrument has been developed and flown over various targets. These targets include semi arid surfaces (Sahel), bog, coniferous and deciduous forest (Boreal forest) and the ocean. Water vapor measurements from radiosondes, concomitant with the POLDER measurements, are available. These data are used for the method validation.
The Polar Ozone and Aerosol Measurements II (POAM II) has been developed by the Naval Research Laboratory and launched end of September 1993 on the french satellite SPOT 3. The instrument observes solar occultations at 9 wavelength channels. Two channels are devoted to the retrieval of the water vapor profile, one at 935 nm in the water vapor absorption band, and a nearby channel at 920 nm, almost free of water vapor absorption. The two channels are used in a differential mode to separate the aerosol extinction from the water vapor absorption. The major difficulty to retrieve a water vapor profile from the transmission data, is due to the line structure of the absorption spectrum. Line-by-line models are used as basic benchmarks, and the GEISA and HITRAN models are compared. However the line-by-line models are much too complex to be run at each step in an inversion algorithm; a simple parameterization has been sought, following the method used for SAGE II. Results are presented.
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