Large-scale analysis of cirrus clouds from AVHRR data : Assessment of both a microphysical index and the cloud-top temperature
Jean-Claude BURIEZ, Y. FOUQUART, V. GIRAUD, Frédéric PAROL et G. Sèze
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An algorithm that allows an automatic analysis of cirrus properties from Advanced Very High Resolution Radiometer (AVHRR) observations is presented. Further investigations of the information content and physical meaning of the brightness temperature differences (BTD) between channels 4 (11 μm) and 5 (12 μm) of the radiometer have led to the development of an automatic procedure to provide global estimates both of the cirrus cloud temperature and of the ratio of the equivalent absorption coefficients in the two channels, accounting for scattering effects. The ratio is useful since its variations are related to differences in microphysical properties. Assuming that cirrus clouds are composed of ice spheres, the effective diameter of the particle size distribution can be deduced from this microphysical index. The automatic procedure includes first, a cloud classification and a selection of the pixels corresponding to the envelope of the BTD diagram observed at a scale of typically 100 X 100 pixels. The classification, which uses dynamic cluster analysis, takes into account spectral and spatial properties of the AVHRR pixels. The selection is made through a series of tests, which also guarantees that the BTD diagram contains the necessary information, such as the presence of both cirrus-free pixels and pixels totally covered by opaque cirrus in the same area. Finally, the cloud temperature and the equivalent absorption coefficient ratio are found by fitting the envelope of the BTD diagram with a theoretical curve. Note that the method leads to the retrieval of the maximum value of the equivalent absorption coefficient ratio in the scene under consideration. This, in turn, corresponds to the minimum value of the effective diameter of the size distribution of equivalent Mie particles. The automatic analysis has been applied to a series of 21 AVHRR images acquired during the International Cirrus Experiment (ICE'89). Although the dataset is obviously much too limited to draw any conclusion at the global scale, it is large enough to permit derivation of cirrus properties that are statistically representative of the cirrus systems contained therein. The authors found that on average, the maximum equivalent absorption coefficient ratio increases with the cloud-top temperature with a jump between 235 and 240 K. More precisely, for cloud temperatures warmer than 235 K, the retrieved equivalent absorption coefficient ratio sometimes corresponds to very small equivalent spheres (diameter smaller than 20 μm). This is never observed for lower cloud temperatures. This change in cirrus microphysical properties points out that ice crystal habits may vary from one temperature regime to another. It may be attributed to a modification of the size and/or shape of the particles.
Cirrus ; Physical parameter ; Automatic analysis ; Algorithm ; Image analysis ; Absorptance ; Brightness temperature ; Temperature difference ; Classification ; Particle size ;
Remote sensing of cirrus radiative parameters during EUCREX'94. Case study of 17 April 1994. Part II : Microphysical models
Gérard BROGNIEZ, H. Chepfer, P. H. FLAMANT, Philippe Flamant, J. PELON, L. SAUVAGE, L. Sauvage et V. TROUILLET
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In this paper, a quantitative analysis of in situ and radiative measurements concerning cirrus clouds is presented. These measurements were performed during the European Cloud and Radiative Experiment 1994 (EUCREX'94) as discussed in an earlier paper (Part I). The analyses are expressed in terms of cirrus microphysics structure. The complex microphysical structure of cirrus cloud is approximated by simple hexagonal monocrystalline particles (columns and plates) and by polyerystalline particles (randomized triadic Koch fractals of second generation) both arbitrarily oriented in space (3D). The authors have also considered hexagonal plates randomly oriented in horizontal planes with a tilted angle of 15° (2D). Radiative properties of cirrus cloud are analyzed. assuming that the cloud is composed of 3D ice crystals, by way of an adding-doubling code. For the hypothesis of 2D ice crystals, a modified successive order of scattering code has been used. The first order of scattering is calculated exactly using the scattering phase function of 2D crystals; for the higher orders, it is assumed that the same particles are 3D oriented. To explain the whole dataset, the most appropriate microphysics, in terms of radiative properties of cirrus clouds, is that of the 2D hexagonal plates whose aspect ratio (length divided by diameter) is 0.05.
Cirrus ; Radiative properties ; Optical properties ; Satellite observation ; Aircraft observation ; Ice crystals ; Polycrystalline ice ; Hexagonal crystals ; Case study ; France ; East Atlantic ; Europe ;
Atmospheric water vapor estimate by a differential absorption technique with the polarisation and directionality of the Earth reflectances (POLDER) instrument
S. BOUFFIES, S. BOUFFIES, F.-M. BREON, Francois-Marie Breon, P. DUBUISSON et Didier TANRE
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The polarization and directionality of the Earth reflectances (POLDER) instrument, to be launched in 1996 on the Japanese ADEOS (advanced Earth observing satellite) platform includes a channel which covers the 910 nm water vapor absorption band (near IR), as well as a channel centered at 865 nm. An estimate of the total atmospheric water vapor content can be derived from the ratio of the two reflectance measurements. In addition to the spectroscopic parameterization, the major uncertainties of 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. An airborne version of the POLDER instrument has been developed and flown over various targets. These targets include semiarid surfaces (Sahel), bog, coniferous and deciduous forest (Boreal forest), and the ocean in the glint direction. Water vapor measurements from radiosondes, concomitant with the POLDER measurements, are available for the method validation. Over water bodies with glint present, an overestimate is found, which is still not understood at present. For other surfaces the comparisons are, in general, in good agreement with the uncertainty analysis.
Radiative transfer ; Vapor ; Water ; Uncertainty ; Ground surface ; Reflectance ; Aerosols ; Vertical profile ; Airborne instruments ;
Ice crystal shapes in cirrus clouds derived from POLDER/ADEOS-1
H. CHEPFER, J. F. DE HAAN, P. H. FLAMANT, J. W. HOVENIER, Goloub PHILLIPPE et Jérome RIEDI
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This paper discusses the retrieval of ice crystal shapes of cirrus clouds on a global scale using observations collected with POLDER-1 (POLarization and Directionality of the Earth Reflectance) onboard the ADEOS-1 platform. The retrieval is based on polarized bidirectional observations made by POLDER. First, normalized polarized radiances are simulated for cirrus clouds composed of ice crystals that differ in shape and are randomly oriented in space. Different values of cloud optical depths, viewing geometries and solar zenith angles are used in the simulations. This sensitivity study shows that the normalized polarized radiance is highly sensitive to the shape of the scatterers for specific viewing geometries, and that it saturates after a few scattering events, which makes it rapidly independent of the optical depth of the cirrus clouds. Next, normalized polarized radiance observations obtained by POLDER have been selected, based on suitable viewing geometries and on the occurrence of thick cirrus clouds composed of particles randomly oriented in space. For various ice crystal shapes these observations are compared with calculated values pertaining to the same geometry, in order to determine the shape that best reproduces the measurements. The method is tested fully for the POLDER data collected on January 12, 1997. Thereafter, it is applied to six periods of 6 days of observations obtained in January, February, March, April, May, and June 1997. This study shows that the particle shape is highly variable with location and season, and that polycrystals and hexagonal columns are dominant at low latitudes, whereas hexagonal plates occur more frequently at high latitudes.
Ice crystals ; Crystal form ; Cirrus ; Ice cloud ; Polarization ; Reflectance ; Radiance ; Optical thickness ; Solar zenithal distance ; Sensitivity analysis ; Particle shape ; Polycrystal ; Hexagonal crystals ;
European Stratospheric Monitoring Stations in the ALPS II
Forecasting, Environmental Protection, Measurement Methods, Meteorology
Laboratoire d'Optique Atmosphèrique
Belgian Institute for Space Aeronomy
Norwegian Institute for Air Research
Centre National de la Recherche Scientifique (CNRS) FRANCE
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE FRANCE
FRAUNHOFER-GESELLSCHAFT ZUR FOERDERUNG DER ANGEWANDTEN FORSCHUNG E.V. DEUTSCHLAND
UNIVERSITY OF BERNE SCHWEIZ/SUISSE/SVIZZERA
UNIVERSITÀ DEGLI STUDI - L'AQUILA ITALIA
UNIVERSITE DE LIEGE BELGIQUE-BELGIË
To expand the intensive co-ordinated ground-based observations to provide climatological measurements of ozone total amounts and vertical profiles, aerosol vertical distributions, ClO vertical profiles, NO2, NO, reservoirs and long-lived tracers total amounts; - to study the coherence of the ozone vertical profiles database recorded since the mid-eighties. The problem of the possible meteorological bias in the ozone profile retrieved from lidars in the low stratosphere will be addressed; - to identify the processes which drive the variability of ozone over the Alps in the low and middle stratosphere by coupling the ozone vertical profiles obtained at different sites and using dynamical tracers (trajectories, potential temperatures and vorticies); - to study the nitrogen and chlorine budgets with ground-based and satellite measurements to test and validate the stratospheric models; - to assess the impact of heterogeneous chemistry processes on the partitioning of stratospheric species by using ground-based and satellite data combined with 2-D and 3-D modelling studies; - to compare the time series of stratospheric species now available from the Alpine stations with long-term simulation over the last decade; - to contribute to stratospheric studies on the connections of mid-latitude changes with high and low latitudes; - to validate satellite measurements of stratospheric species performed by GOME and possibly ADEOS; - to finalise intercomparisons for aerosol and ClO retrieval methods.
AVIRIS measurements of spectral reflectance characteristics of whitecaps
Pierre-Yves Deschamps, Robert J. FROUIN, Robert J. Frouin, Hiroshi KAWAMURA et Motoaki KISHINO
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The spectral reflectance of oceanic whitecaps in the visible and near infrared was investigated using high-altitude, 20 m resolution AVIRIS measurements off the Southern California coast. The whitecap effect on surface reflectance was expressed as a function of the difference between the reflectance of pixels contaminated by whitecaps and of adjacent pixels free of whitecaps. Whitecap reflectance was found to decrease substantially in the near infrared, by about 40% at 850 nm and 80% at 1,600 nm, in agreement with previous measurements in the coastal zone and the open ocean. The spectral dependence of whitecap reflectance appears to be fairly independent of environmental conditions, making it easy to take into account the resulting -and significant- effects in ocean color and aerosol remote sensing algorithms.
color ; reflectance ; Space remote sensing ; data processing ; atmospheric correction ; surface water ; coastal environment ; Southern California ; algorithms ; aerosols ; California ; United States ; North America ;