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F. Parol, J. C. Buriez, G. Brogniez, and Y. Fouquart

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 effective absorption coefficient ratio is defined and we derive a direct relationship between this ratio and the optical properties of the cloud particles. The mean value of the effective absorption coefficient ratio corresponds to ice spheres of effective radius of 26 μm or a bit less in the case of water spheres (supercooled droplets), but no agreement can be obtained for fully randomly oriented cylinders.

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Y. Fouquart, B. Bonnel, G. Brogniez, J. C. Buriez, L. Smith, J. J. Morcrette, and A. Cerf

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 layer's 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 C A∼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 budget of the Sahelian aerosol layer was determined for clear and dusty conditions. The additional aerosol shortwave heating was as much as 5 K day−1 for δA(0.55 μm) = 1.5 and with the sun overhead, whereas the additional cooling was close to 1 K day−1. As a consequence of the large temperature discontinuity at the surface, important infrared heating at the surface layer was observed.

The rather large differences between the aerosol optical properties reported here and those previously reported in the literature are due to different aerosol size distributions; therefore the present paper stresses the importance of careful determination of the size distributions.

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G. Brogniez, J. C. Buriez, V. Giraud, F. Parol, and C. Vanbauce

Abstract

Ground-based observations and satellite data have been compared for the 18 October 1989 case study of the International Cirrus Experiment (ICE) field campaign. They correspond to thin cirrus clouds with infrared emittances in the range 0–0.3. Good correspondence was obtained when comparing the time variability of the effective downward beam emittance of the cirrus clouds observed at Nordholz (53.8°N, 8.3°E) to the spatial variability of the effective upward beam emittance derived from NOAA-11 Advanced Very High Resolution Radiometer (AVHRR) data acquired at 1225 UTC. A simple model of cirrus cloud particles was found to satisfy both the ground-based observations of the angular dependence of the scattered solar radiation at 0.85 µm and the satellite observations of the brightness temperatures in channel 4 (11 µm) and channel 5 (12 µm) of NOAA-11 AVHRR. The best fit was obtained for fully randomly oriented hexagonal ice plates with a thickness of 10–20 µm and a diameter of 200–500 µm. Although actual cloud ice crystals are probably not all hexagonal plates, our simple model of randomly oriented ice plates allows us to appropriately simulate the optical properties of the observed cirrus in which particles surely present a large variety of shapes. The equivalent radius of the retrieved ice plates (i.e., the radius of spheres of the same volume) is 50–80 µm. However, ice spheres do not simulate the halo of cirrus clouds observed from the aureolemeter measurements. Moreover, assuming spherical particles to explain brightness temperature measurements in AVHRR channels 4 and 5 leads to an effective radius of 27 µm, which is noticeably smaller than the one obtained with the hypothesis of hexagonal plates.

On the other hand, analysis of AVHRR data also highlights the important difference between natural thin cirrus and jet contrail microphysics. Contrails are revealed to be composed of smaller equivalent spherical particles with an effective radius of about 4.5 µm.

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R. W. Saunders, G. Brogniez, J. C. Buriez, R. Meerkötter, and P. Wendling

Abstract

During the 1989 intensive field campaign of the International Cirrus Experiment (ICE) over the North Sea, broadband radiative fluxes were measured in, above, and below cirrus cloud by a number of European meteorological research aircraft. One mission during the campaign was an intercomparison flight in clear air with no cloud above in order to compare, among other things, radiative flux measurements made by the U.K. C-130, the French Merlin, and the German Falcon aircraft. All three aircraft measured shortwave flux (0.3–3 µm) with standard Eppley pyranometers above and below the fuselage. The intercomparison showed agreement between the three aircraft of within 2% for both the upwelling and downwelling shortwave flux components. Using a coincident temperature and humidity radiosonde profile, the downward clear-sky fluxes at the level of the aircraft were also calculated using a variety of different radiation models. Modeled shortwave fluxes were all higher (between 2% and 4%) than the measured values. In addition to shortwave fluxes the C-130 and Merlin also measured near-infrared fluxes (0.7–3 µm) by having additional Eppley pyranometers mounted with red domes over the thermopiles. The near-infrared fluxes measured by the Merlin and C-130 were different because slightly different red-dome filters were used; model calculations show the difference between the measured fluxes was consistent with the different pass band of the filters. Infrared fluxes (4–40 µm) were measured using standard Eppley pyrgeometers on the Falcon and pyrgeometers developed at the Meteorological Research Flight on the C-130; comparisons show no significant differences for the downwelling fluxes but the Falcon upwelling fluxes were 7% higher than the corresponding C-130 values. This latter difference is higher than would be expected for these instruments. The modeled infrared fluxes were up to 9% lower than the C-130 and Falcon measurements.

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V. Giraud, J. C. Buriez, Y. Fouquart, F. Parol, and G. Seze

Abstract

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 × 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 toanother. It may be attributed to a modification of the size and/or shape of the particles.

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