All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 120 58 6
PDF Downloads 1716 52 8

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

F. ParolLaboratoire d'Optique Atmosphérique, Université des Sciences et Techniques de Lille, Villeneuve d'Ascq, France

Search for other papers by F. Parol in
Current site
Google Scholar
PubMed
Close
,
J. C. BuriezLaboratoire d'Optique Atmosphérique, Université des Sciences et Techniques de Lille, Villeneuve d'Ascq, France

Search for other papers by J. C. Buriez in
Current site
Google Scholar
PubMed
Close
,
G. BrogniezLaboratoire d'Optique Atmosphérique, Université des Sciences et Techniques de Lille, Villeneuve d'Ascq, France

Search for other papers by G. Brogniez in
Current site
Google Scholar
PubMed
Close
, and
Y. FouquartLaboratoire d'Optique Atmosphérique, Université des Sciences et Techniques de Lille, Villeneuve d'Ascq, France

Search for other papers by Y. Fouquart in
Current site
Google Scholar
PubMed
Close
Restricted access

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.

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.

Save