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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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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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