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C. Fravalo, Y. Fouquart, and R. Rosset

Abstract

The low stratiform cloud model presented here appears as a generalization of Lilly's model (1968). Its main new apsects lie 1) in a detailed vertical computation of the longwave and the shortwave radiative flux profiles and 2) in a formulation of the entrainment rate and the turbulent flux profiles which takes into account the nonlinear vertical structure of the radiative fluxes. Furthermore, the radiative divergence is no longer externally prescribed, like in other models; it is determined as function of the mixed-layer and cloud characteristics. All these features allow a more complete coupling between the turbulent mixing and the radiative fluxes. Within the cloud, the turbulent flux profiles of the moist static energy and the virtual dry static energy are nonlinear functions of height, due to the radiative divergence. This nonlinear structure results in a realistic negative entrainment flux at cloud top.

In the sensitivity tests, the stress has been put on the variability of the radiative and turbulent fluxes as functions of the cloud microphysics. The result is that the integrated liquid water content (liquid water path) is the predominant factor in fixing the radiative and turbulent fluxes, with a secondary but non-negligible role played by the drop size distribution.

For optically thin clouds where the shortwave absorption is negligible, the infrared cooling is distributed throughout the whole cloud and is highly sensitive to the liquid water path; thus the turbulent fluxes and the entrainment rate also depend strongly on the liquid water path.

When increasing the liquid water path, the longwave cooling becomes saturated and localized in a layer of progressively reduced thickness at cloud top. Thus for thick clouds, with no solar flux, our model gives results which are similar to those of Lilly-type models.

The solar heating does not saturate as the liquid water path increases. Moreover, since it is distributed throughout the cloud deck, it not only reduces the radiative divergence and the turbulent kinetic energy production at cloud top, but it also acts as a source of this latter energy component near the cloud base. The overall result is a noticeable reduction of the entrainment rate. This suggests a strong diurnal cycle for thick stratocumulus decks.

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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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M. Legrand, J. J. Bertrand, M. Desbois, L. Menenger, and Y. Fouquart

Abstract

Optical depth of Saharan dust derived from photometric measurements made during the dry season at a Sahelian site (Niamey, Republic of Niger) is compared with METEOSAT-2 radiance in the 10.5–12.5 μm channel for different times of the daily cycle. The ability of retrieving dual optical depth using the outgoing radiance of infrared atmospheric window is clearly demonstrated for the middle of the day. Results obtained with nighttime data through a relation between dust optical depth and visibility are also discussed. The major causes of error are identified and quantitatively estimated.

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Y. Fouquart, B. Bonnel, M. Chaoui Roquai, R. Santer, and A. Cerf

Abstract

A series of ground-based and airborne observations of desert aerosols, the ECLATS experiment was carried out in December 1980 in the vicinity of Niamey (Niger). This paper deals with aerosol optical thicknesses and size distributions derived from (i) in situ measurements using singe particle optical counters (a Kratel and a Knollenberg FSSP), (ii) a ground-based cascade impactor, and (iii) ground-based measurements of the spectral variation of the sober extinction.

During the experiment, aerosol optical thicknesses (at 550 nm) varied from 0.20 on very clear days to 1.5 during a so-called “dry haze” episode.

Comparisons between size distributions derived from in situ measurements from ground-based cascade impactor, and from inversion of the spectral optical thicknesses, showed that the optical counters drastically underestimated the concentration of small (r<0.5 μm) particles It was shown that the occurrence of a “dry haze” episode was characterized by a large increase (an order of magnitude in this particular case) of the intermediate particles (r≅0.5 μm), whereas the concentration in very (r<0.2 μm) and large (r>1 μm) particles remained roughly constant.

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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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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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H. W. Barker, G. L. Stephens, P. T. Partain, J. W. Bergman, B. Bonnel, K. Campana, E. E. Clothiaux, S. Clough, S. Cusack, J. Delamere, J. Edwards, K. F. Evans, Y. Fouquart, S. Freidenreich, V. Galin, Y. Hou, S. Kato, J. Li, E. Mlawer, J.-J. Morcrette, W. O'Hirok, P. Räisänen, V. Ramaswamy, B. Ritter, E. Rozanov, M. Schlesinger, K. Shibata, P. Sporyshev, Z. Sun, M. Wendisch, N. Wood, and F. Yang

Abstract

The primary purpose of this study is to assess the performance of 1D solar radiative transfer codes that are used currently both for research and in weather and climate models. Emphasis is on interpretation and handling of unresolved clouds. Answers are sought to the following questions: (i) How well do 1D solar codes interpret and handle columns of information pertaining to partly cloudy atmospheres? (ii) Regardless of the adequacy of their assumptions about unresolved clouds, do 1D solar codes perform as intended?

One clear-sky and two plane-parallel, homogeneous (PPH) overcast cloud cases serve to elucidate 1D model differences due to varying treatments of gaseous transmittances, cloud optical properties, and basic radiative transfer. The remaining four cases involve 3D distributions of cloud water and water vapor as simulated by cloud-resolving models. Results for 25 1D codes, which included two line-by-line (LBL) models (clear and overcast only) and four 3D Monte Carlo (MC) photon transport algorithms, were submitted by 22 groups. Benchmark, domain-averaged irradiance profiles were computed by the MC codes. For the clear and overcast cases, all MC estimates of top-of-atmosphere albedo, atmospheric absorptance, and surface absorptance agree with one of the LBL codes to within ±2%. Most 1D codes underestimate atmospheric absorptance by typically 15–25 W m–2 at overhead sun for the standard tropical atmosphere regardless of clouds.

Depending on assumptions about unresolved clouds, the 1D codes were partitioned into four genres: (i) horizontal variability, (ii) exact overlap of PPH clouds, (iii) maximum/random overlap of PPH clouds, and (iv) random overlap of PPH clouds. A single MC code was used to establish conditional benchmarks applicable to each genre, and all MC codes were used to establish the full 3D benchmarks. There is a tendency for 1D codes to cluster near their respective conditional benchmarks, though intragenre variances typically exceed those for the clear and overcast cases. The majority of 1D codes fall into the extreme category of maximum/random overlap of PPH clouds and thus generally disagree with full 3D benchmark values. Given the fairly limited scope of these tests and the inability of any one code to perform extremely well for all cases begs the question that a paradigm shift is due for modeling 1D solar fluxes for cloudy atmospheres.

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