The FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A band)
algorithm is a simple, fast and robust algorithm used to retrieve cloud
information in operational satellite data processing. It ...has been applied to
GOME-1 (Global Ozone Monitoring Experiment), SCIAMACHY (Scanning Imaging
Absorption Spectrometer for Atmospheric Chartography), GOME-2 and more
recently to TROPOMI (Tropospheric Monitoring Instrument). FRESCO retrieves
effective cloud fraction and cloud pressure from measurements in the oxygen A
band around 761 nm. In this paper, we propose a new version of the
algorithm, called FRESCO-B, which is based on measurements in the oxygen B
band around 687 nm. Such a method is interesting for vegetated
surfaces where the surface albedo is much lower in the B band than in the A
band, which limits the ground contribution to the top-of-atmosphere
reflectances. In this study we first perform retrieval simulations. These
show that the retrieved cloud pressures from FRESCO-B and FRESCO differ only
between −10 and +10 hPa, except for high, thin clouds over
vegetation where the difference is larger (about +15 to +30 hPa),
with FRESCO-B yielding higher pressure. Next, inter-comparison between
FRESCO-B and FRESCO retrievals over 1 month of GOME-2B data reveals that the
effective cloud fractions retrieved in the O2 A and B bands are
very similar (mean difference of 0.003), while the cloud pressures show a
mean difference of 11.5 hPa, with FRESCO-B retrieving higher
pressures than FRESCO. This agrees with the simulations and is partly due to
deeper photon penetrations of the O2 B band in clouds compared to
the O2 A-band photons and partly due to the surface albedo bias in
FRESCO. Finally, validation with ground-based measurements shows that the
FRESCO-B cloud pressure represents an altitude within the cloud boundaries
for clouds that are not too far from the Lambertian reflector model, which
occurs in about 50 % of the cases.
Information content analyses on cloud top altitude (CTOP) and geometrical thickness (CGT) from multi-angular A-band measurements in the case of monolayer homogeneous clouds are conducted. In the ...framework of future multi-angular radiometer development, we compared the potential performances of the 3MI (Multi-viewing, Multi-channel and Multi-polarization Imaging) instrument developed by EUMETSAT, which is an extension of POLDER/PARASOL instrument and MSPI (Multiangle SpectroPolarimetric Imager) developed by NASA's Jet Propulsion Laboratory. Quantitative information content estimates were realized for thin, moderately opaque and opaque clouds for different surface albedo and viewing geometry configurations. Analyses show that retrieval of CTOP is possible with a high accuracy in most of the cases investigated. Retrieval of CGT is also possible for optically thick clouds above a black surface, at least when CGT > 1–2 km and for thin clouds for CGT > 2–3 km. However, for intermediate optical thicknesses (COT ≃ 4), we show that the retrieval of CGT is not simultaneously possible with CTOP. A comparison between 3MI and MSPI shows a higher information content for MSPI's measurements, traceable to a thinner filter inside the oxygen A-band, yielding higher signal-to-noise ratio for absorption estimation. Cases of cloud scenes above bright surfaces are more complex but it is shown that the retrieval of CTOP remains possible in almost all situations while the information content on CGT appears to be insufficient in many cases, particularly for COT < 4 and CGT < 2–3 km.
Les nuages sont les principaux modulateurs du bilan radiatif terrestre et ils jouent un rôle prépondérant sur le climat de la Terre. Depuis plusieurs années, il est admis que la représentation des ...propriétés nuageuses est l'une des principales sources d’incertitude dans les modèles de prévision du climat et du temps météorologique. Dans ce travail de thèse, nous nous sommes concentrés sur les propriétés macrophysiques des atmosphères nuageuses et plus particulièrement sur leur structure verticale (caractère monocouche/multicouche, altitude et extension verticale des couches nuageuses). Pour cela, nous nous sommes basés sur les mesures multiangulaires effectuées dans la bande A de l’oxygène par le radiomètre POLDER embarqué sur la plateforme satellitaire PARASOL. Dans un premier temps, nous avons cherché à caractériser puis à distinguer de manière statistique les situations nuageuses monocouches et multicouches en construisant un arbre de décision. Ensuite, nous avons construit des paramétrisations permettant d'estimer les pressions de sommet et de milieu des nuages monocouches, ainsi que leur extension verticale. Enfin, des analyses statistiques réalisées sur cinq ans de données, complétées par des cas d'étude variés, ont permis de vérifier la validité de ces nouveaux produits géophysiques et d'en déterminer leurs limites. Cette étude a été rendue possible par la richesse des informations colocalisées de POLDER avec celles de deux instruments de télédétection active de la constellation de satellites A-Train, qui fournissent des informations précises, mais avec une faible couverture spatiale, sur le profil vertical de l'atmosphère.
Clouds are key modulators of the Earth's radiative balance and they play a major role in the climate of Earth. It has been recognized for several years that the representation of cloud properties is one of the main sources of uncertainty in climate and meteorological prediction models. In this thesis, we focused on the macrophysical properties of cloudy atmospheres and particularly on their vertical structure (single /multi-layered character, altitude and vertical extension of cloud layers). For this purpose, we relied on the multiangular measurements in the oxygen A band made by the radiometer POLDER on board PARASOL satellite platform. As a first step, we intended to characterize and to statistically distinguish monolayer and multilayer cloudy situations by designing a decision tree. Then, we built parameterizations in order to estimate top and middle pressures of monolayers clouds as well as their vertical extension. Finally, statistical analyses performed on five years of data, completed by various case studies, allowed us to check the validity of these new geophysical products and to determine their limits. This study was made possible by the richness of POLDER information collocated with two active sensors of the satellite constellation A- Train, which provide accurate information on the vertical profile of the atmosphere, but with a low spatial coverage.
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