Developing and Bounding Ice Particle Mass- and Area-dimension Expressions for Use in Atmospheric Models and Remote Sensing

 

Authors

David L. Mitchell — Desert Research Institute
Ehsan Erfani — George Mason University

Category

Ice Physical and Radiative Properties

Description

Comparing one of the m-D curve fits based on SPARTICUS data (for indicated temperature regime) with ice particle m-D measurements corresponding to ice particle shapes found in cirrus clouds. These measurements were obtained during a cloud seeding program known as SCPP, conducted during the late 1980's, where ice particles were photographed through a microscope and then melted to form hemispherical drops from which their masses were calculated. The SCPP data have been grouped into size-bins; shown are the standard deviations (σ) in mass (m) and D for each size-bin. Mean values for m and D are shown by the intersection of the σ-bars. Also shown are comparisons with two other independent studies that derived m-D power laws from cirrus cloud field data. The Cotton et al. and Heymsfield et al. m-D expressions differ from the curve fit by less than 50% and 100%, respectively. The m-D power law for an ice sphere is shown for reference.
Ice particle mass- and projected area-dimension power law relationships are commonly used in the treatment of ice cloud microphysical and optical properties and the remote sensing of ice cloud properties. There is considerable uncertainty regarding their application and whether the ice particle mass (m) and projected area (A) dependence on maximum dimension D are appropriate for the conditions being modeled. At the last ASR annual meeting, talks were given stating that ice cloud property retrievals were virtually unbounded due the the high uncertainty regarding ice particle m-D and A-D power laws. This study develops m-D and A-D expressions from 2D-S probe measurements from many flights during SPARTICUS in synoptic and anvil ice clouds and finds that these relationships are not linear in log(m)-log(D) or log(A)-log(D) space when considering D ranging from 10 μm to several millimeters, although they are well described by 2nd order polynomial fits. The representativeness of these m-D expressions for cirrus clouds was tested by comparing them against accurate m-D field measurements (at ground level) of 827 ice particles having shapes characteristic of cirrus cloud ice particles. Favorable agreement here, as well as good agreement with m-D power laws developed from recent cirrus cloud field studies, improves confidence that these expressions are valid for cirrus clouds. These curve-fit expressions also appear representative for heavily rimed dendrites and for snowfall occurring over the Sierra Nevada mountains. A methodology for extracting and applying m-D and A-D power laws from the curve fit expressions was developed that preserves the conventional application of m-D and A-D power laws. Finally, by using these m-D field measurements, the impact of ice particle riming on the m-D power law was evaluated, providing guidance for modeling the riming process. In brief, riming affects the m-D prefactor but not the exponent. These m-D and A-D expressions were shown to be self-consistent, which is expected since they come from the same data set. They thus provide a means of making ice cloud microphysical and radiative processes self-consistent since these processes can be formulated in terms of m-D and A-D power laws. The ice microphysics and optics in CAM5 are formulated this way, and this new m-D/A-D treatment is being incorporated into CAM5 to make these processes self-consistent and more accurate.

Lead PI

David L. Mitchell — Desert Research Institute