Sensitivity of cirrus cloud radiative properties to ice crystal size and shape in general circulation model simulations

Conference · Sat Apr 01 04:00:00 EST 1995

OSTI ID:83190

Mitchell, D L ^[1]; Kristjansson, J E ^[2]; Newman, M J ^[3]

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1. Desert Research Institute, Reno, NV (United States)
2. Univ. of Oslo (Norway)
3. Los Alamos National Lab., NM (United States)

Recent research has shown that the radiative properties of cirrus clouds (i.e., optical depth, albedo, emissivity) depend on the shapes and sizes of ice crystals. For instance, the cloud albedo may vary by a factor of two, depending on whether hexagonal columns or bullet rosette ice crystals are assumed for a given ice water path (IWP). This variance occurs primarily because, at sizes characteristic of cirrus clouds, bullet rosettes have less mass than columns of the same size. However, their projected areas may be comparable. Thus, for a given IWP and mean cloud ice particle size, the optical depth will be considerably greater for rosettes, since many more rosettes are required to account for the IWP than are columns. The same could be said of hexagonal plates and columns, with plates exhibiting the greater optical depth. Satellite information suggests that the albedos of tropical cirrus clouds are greater than those of midlatitude cirrus, with albedos as high as 60%-80%. The reasons for this are not understood, but might be attributed in part to differences in ice particle size and shape. For instance, in the tropical western Pacific, ice crystal size distributions in cirrus near the tropopause exhibited median mass dimensions (D{sub m} around 30 {mu}m) and contained planar polycrystals. Very small ice crystals (typically 10 {mu}m, often ranging from about 2 {mu}m - 100 {mu}m) of indeterminate shape were sampled in anvil cirrus by an ice particle replicator in this region during the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE). If fewer columnar ice crystals were present in tropical versus midlatitude cirrus and/or sizes were smaller, tropical cirrus should exhibit greater size distribution projected area, producing greater optical depth, albedo, and emissivity for the same IWP. Smaller crystal sizes would also promote higher albedos via enhanced backscattering.

• Research Organization: USDOE Office of Energy Research, Washington, DC (United States). Environmental Sciences Div.
• OSTI ID: 83190
• Report Number(s): CONF-940277--; ON: DE95009951; CNN: Grant NA36GPO278
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
ALBEDO
BACKSCATTERING
CLOUDS
CRYSTALS
DISTRIBUTION
FORCING FUNCTIONS
GENERAL CIRCULATION MODELS
ICE
PARTICLE SIZE
POLYCRYSTALS
SENSITIVITY
TROPOPAUSE