Gérard Brogniez

Observations of Saharan Aerosols: Results of ECLATS Field Experiment. Part II: Broadband Radiative Characteristics of the Aerosols and Vertical Radiative Flux Divergence

Abstract The results presented in this paper are a part of those obtained during the ECLATS experiment The broadband radiative characteristics of the Sahelian aerosol layer and the vertical radiative flux divergence within the dust layer were determined both from in situ measurements and Mie calculations. In situ measurements of the aerosol layers reflectances and transmittances of solar radiation led to aerosol single-scattering albedos close to ωA∼0.95. Measurements of the 8–14 μm radiances led to an optied depth by unit of volume of dust in a vertical column CA∼0.34 μm−1. Mie calculations assuming the aerosol refractive index published by Carlson and Benjamin for solar radiation and that measured by Volz for the atmospheric window, showed good agreement with observations. The ratio of infrared to visible optical thickness was δA(8–14 μm)/δA (0.55 μm)∼0.1, instead of 0.3 as calculated by Carlson and Benjamin. This discrepancy is attributable to differences in size distributions assumed. The radiative b...

Information Content of AVHRR Channels 4 and 5 with Respect to the Effective Radius of Cirrus Cloud Particles

Abstract This paper investigates the important difference in the relationship between brightness temperatures between the 11-μm and the 12-μn AVHRR data and the microphysical properties of the semitransparent cirrus clouds. In the nonscattering approximation, the emittance for channels 4 and 5 are related through the absorption coefficient ratio that is the key parameter giving access to the size of cloud particles. The observed mean value of this parameter corresponds to effective radius of 18 μm for polydisperse spheres and 12 μm for polydisperse infinitely long ice cylinders. Taking the multiple scattering into account, the brightness temperature difference enhances much more for cylinders than for spheres owing to the fact that the forward peak of scattering is less large for cylinders. To obtain the size of cloud particles, the method developed in the nonscattering case is still applicable if one makes use of the effective emittance that implicitly includes the effects of mattering. Thus, an effectiv...

Cloud thermodynamical phase classification from the POLDER spaceborne instrument

Cloud phase recognition is important for cloud studies. Ice crystals correspond to physical process and properties that differ from those of liquid water drops. The angular polarization signature is a good mean to discriminate between spherical and nonspherical particles (liquid and ice phase, respectively). POLDER (Polarization and Directionality of Earth Reflectances) has been launched on the Japanese ADEOS platform in August 1996. Because of its multidirectional, multispectral, and multipolarization capabilities this new radiometer gives useful information on clouds and their influence on radiation in the shortwave range. The POLDER bidirectional observation capability provides the polarization signatures within a large range of scattering angles in three spectral bands centered on 0.443, 0.670, and 0.865 μm with a spatial resolution of 6.2 km×6.2 km. These original features allow to obtain some information both on cloud thermodynamic phase and on cloud microphysics (size/shape). According to POLDER airborne observations, liquid cloud droplets exhibit very specific polarization features of a rainbow for scattering angles near 140°. Conversely, theoretical studies of scattering by various crystalline particles and also airborne measurements show that the rainbow characteristics disappear as soon as the particles depart from the spherical shape. In the paper the POLDER algorithm for cloud phase classification is presented, as well as the physical principle of this algorithm. Results derived from the POLDER spaceborne version are also presented and compared with lidar ground-based observations and satellite cloud classification. This cloud phase classification method is shown to be reliable. The major limitation appears when thin cirrus clouds overlap the liquid cloud layer. In this case, if the cirrus optical thickness is smaller than 2, the liquid phase may be retrieved. Otherwise, the ice phase is correctly detected as long as cloud detection works.

OBSERVATIONS OF HORIZONTALLY ORIENTED ICE CRYSTALS IN CIRRUS CLOUDS WITH POLDER-1/ADEOS-1

Abstract Optical and radiative properties of cirrus clouds need to be accurately described at global scale in order to correctly estimate the radiative impact of ice clouds. The orientation of ice crystals in cirrus is capable of having a strong impact on their radiative budget: a cirrus cloud composed of horizontally oriented ice crystals has a larger plane albedo than a cirrus cloud composed of randomly oriented particles. Until recently, space-borne sensors were not adapted to observe ice crystal orientations. The POLDER instrument onboard the ADEOS platform (October 1996–June 1997) enabled us to observe bidirectional polarized radiances. These data are useful to determine the frequency of occurrence of ice crystals horizontally oriented in space within cirrus clouds. This paper describes how the POLDER bidirectional polarized radiances have been analyzed to determine the fraction of ice crystals preferably horizontally oriented. This preferred orientation is identified by observing specular reflection phenomenon above thick ice clouds. Three different periods (January, March and June 1997) of ten days of POLDER/ADEOS data have been processed for this study, and show that at least 40% of the ice pixels exhibit specular reflection peaks that indicate preferred orientation of ice crystals. The intensity and the distribution of specular reflection peaks are presented and discussed as a function of different parameters (solar zenith angle, latitude, cloud reflectance).

Polarized light scattering by inhomogeneous hexagonal monocrystals: Validation with ADEOS‐POLDER measurements

Various in situ measurements of the light-scattering diagram in ice clouds were performed with a new nephelometer during several airborne campaigns. These measurements were favorably compared with a theoretical scattering model called Inhomogeneous Hexagonal Monocrystal (IHM) model. This model consists in computing the scattering of light by an ensemble of randomly oriented hexagonal ice crystals containing spherical impurities of soot and air bubbles. It is achieved by using a combination of ray tracing, Mie theory, and Monte Carlo techniques and enables to retrieve the six independent elements of the scattering matrix. This good agreement between nephelometer measurements and IHM model provides an opportunity to use this model in order to analyze ADEOS-POLDER total and polarized reflectance measurements over ice clouds. POLDER uses an original concept to observe ice cloud properties, enabling to measure reflectances and polarized reflectances, for a given scene, under several (up to 14) viewing directions. A first analysis of ice cloud spherical albedoes over the terrestrial globe for November 10, 1996, and April 23, 1997, shows a rather good agreement between measurements and modeling. Moreover, polarized reflectances are also calculated and show a satisfactory agreement with measurements.

Modeling of light scattering in cirrus clouds with inhomogeneous hexagonal monocrystals. Comparison with in-situ and ADEOS-POLDER measurements

An Inhhomogeneous Hexagonal Monocrystal (IHM) model is used to simulate light scattering by randomly oriented hexagonal ice crystals containing air bubbles. This model based on a combination of ray-tracing, Mie theory and Monte-Carlo techniques, allows to retrieve the scattering phase function. In-situ measurements of the light scattering diagram in natural cirrus clouds with an airborne nephelometer have been performed. The results given by the IHM model have been favorably adjusted with these measurements. This agreement provides an opportunity to use this model in order to analyze ADEOS-POLDER reflectance measurements over cirrus clouds. POLDER uses an original concept to measure, for a given scene, total and polarized reflectances under several viewing directions. A first analysis of cirrus cloud spherical albedoes for the 10th November 1996 shows a rather good agreement between measurements and calculations.

Sensitivity of retrieved POLDER directional cloud optical thickness to various ice particle models

The ”directional” values of cloud optical thickness (or cloud spherical albedo) retrieved from ADEOS-POLDER data constitute a strong constraint on the microphysical model used in the retrieval algorithm. In this paper, we focus on ice clouds. We quantify the departure of the directional values of spherical albedo from their averaged value. By so doing, we can assess the suitability of different ice particle scattering models (i.e., spheres, fractal polycrystal, inhomogeneous hexagonal monocrystal, or a synthesized phase function). The liquid water droplet and the ice fractal polycrystal models appear to be inappropriate since they induce a notable dependence of the ice cloud spherical albedo on scattering angle. On the contrary, much better agreement is achieved by using a synthesized phase function or an inhomegeneous hexagonal monocrystal model. Using these models, the retrieved ice cloud optical thickness is then found to be 40% smaller than using the droplet model.

Microphysical and optical properties of cirrus and contrails : cloud field study on 13 October 1989

Abstract During the intensive International Cirrus Experiment conducted over the North Sea during fall 1989, natural cirrus and contrail-induced cirrus were analyzed from in situ and remote sensing measurements (lidar and infrared radiometer). These two cloud types primarily formed at the same range of altitude (8200 m, −37°C). Analysis of the measurements depicts distinctive microphysical and optical properties in the two types of cirrus. Natural cirrus exhibits sheared fallstreaks of ice crystals up to 750 µm in size near the base level. From the top to the base of this cloud the mean values of ice water content and particle concentration increase from 15 to 50 mg m−3 and from 26 to 60 L−1, respectively. The corresponding visible optical depth is around 2.0. Greatest particle concentration and smallest ice crystals are measured at all levels in contrails leading to an optical depth of 0.8 in the denser cloud despite an ice water content that never exceeds 18 mg m−3. These results are consistent with rem...

CIRRUS CLOUDS' MICROPHYSICAL PROPERTIES DEDUCED FROM POLDER OBSERVATIONS

Abstract During the EUCREX’94 (EUropean Cloud Radiation EXperiment 1994) campaign, observations of bidirectional reflected solar light above cirrus clouds off Brittany coast were collected with an airborne version of the POLDER (POLarization and Directionality of the Earth’s Reflectances) instrument. This instrument was designed to measure the bidirectional total and polarized reflectances of the solar light scattered by a cloud. The polarization measurements provide a very good signature of the water phase (liquid or ice) in the observed cloud. A more precise analysis of the behaviour of the polarized reflectances shows that it is highly sensitive to the shape of the cirrus cloud ice crystals for scattering angles ranging between 60° and 110°. One sequence of the POLDER images collected during EUCREX’94 pointed out the presence of hexagonal plates with aspect ratio equal to 0.1, whereas the analysis of an another sequence of images collected during the same campaign clearly shows a specular reflection of the solar light corresponding to the presence of horizontally oriented prismatic ice crystals in the cloud.

Modeling total and polarized reflectances of ice clouds: evaluation by means of POLDER and ATSR-2 measurements

Four ice-crystal models are tested by use of ice-cloud reflectances derived from Along Track Scanning Radiometer-2 (ATSR-2) and Polarization and Directionality of Earths Reflectances (POLDER) radiance measurements. The analysis is based on dual-view ATSR-2 total reflectances of tropical cirrus and POLDER global-scale total and polarized reflectances of ice clouds at as many as 14 viewing directions. Adequate simulations of ATSR-2 total reflectances at 0.865 microm are obtained with model clouds consisting of moderately distorted imperfect hexagonal monocrystals (IMPs). The optically thickest clouds (tau > approximately 16) in the selected case tend to be better simulated by use of pure hexagonal monocrystals (PHMs). POLDER total reflectances at 0.670 microm are best simulated with columnar or platelike IMPs or columnar inhomogeneous hexagonal monocrystals (IHMs). Less-favorable simulations are obtained for platelike IHMs and polycrystals (POLYs). Inadequate simulations of POLDER total and polarized reflectances are obtained for model clouds consisting of PHMs. Better simulations of the POLDER polarized reflectances at 0.865 microm are obtained with IMPs, IHMs, or POLYs, although POLYs produce polarized reflectances that are systematically lower than most of the measurements. The best simulations of the polarized reflectance for the ice-crystal models assumed in this study are obtained for model clouds consisting of columnar IMPs or IHMs.