Ice crystal habits and solar radiation

Authors
Citation
K. Wyser, Ice crystal habits and solar radiation, TELLUS A, 51(5), 1999, pp. 937-950
Citations number
32
Categorie Soggetti
Earth Sciences
Journal title
TELLUS SERIES A-DYNAMIC METEOROLOGY AND OCEANOGRAPHY
ISSN journal
02806495 → ACNP
Volume
51
Issue
5
Year of publication
1999
Pages
937 - 950
Database
ISI
SICI code
0280-6495(199910)51:5<937:ICHASR>2.0.ZU;2-9
Abstract
The single-scattering properties of cirrus clouds depend not only, upon the ice water path and the size of the crystals, but also upon the crystal sha pe. A novel parameterization has been used to explore the differences in br oadband short-wave radiation due to various crystal habits. The differences in the net-fluxes are found to be appreciable, both at the tropopause and at the surface. They reach values up to the order of several 10 W m(-2) for mid-latitude and up to some 100 W m(-2) for tropical cirrus. The largest d ifferences are found for an intermediate range of optical depths and the op tical thickness of natural cirrus happen to be in this intermediate range. The absorptivity of the cirrus cloud is independent of the crystal habit, n evertheless, the change in the cloud's reflectivity has an impact on the tr opospheric absorption and, thus, the energy absorbed by the troposphere dep ends on the crystal habit. The sensitivity of the results with respect to s urface albedo and solar zenith angle has been investigated. A lower zenith angle enhances the differences found for the various habits, and this effec t is even amplified by a high amount of water vapour as it may be found in the tropics. A comparison with observations from ICE'89 reveals that the la rge differences in the modelled fluxes are comparable to the uncertainty in the measurements. Despite this observational difficulty, the large differe nces in SW fluxes emphasise the importance of the crystal habit for radiati on schemes.