Representing the ice fall speed in climate models: results from TC4 and ISDAC
Authors
David L. Mitchell — Desert Research Institute
Paul Lawson — SPEC, Inc.
Subhashree Mishra — DOE - SunShot Initiative, AAAS S&T Policy Fellow
Category
Cloud Properties
Description
Relationship between De and Vm for all TC4 cirrus cloud types and some NAMMA in situ cirrus (solid line). The dashed curve is the best fit curve obtained by Heymsfield et al. (2003) for anvil cirrus sampled during the TRMM project. The long-dashed curve estimates Vm in the Stokes flow regime.
Ice fall-velocity is a critical climate feedback parameter influencing cirrus cloud coverage and radiative forcing as well as upper troposphere relative humidity. This study aims to provide the atmospheric modeling community with better parameterizations of the ice fall-speed in cirrus clouds based on aircraft measurements from recent field campaigns, especially the Tropical Composition, Cloud and Climate Coupling (TC4) campaign in 2007 and the Indirect and Semi-Direct Aerosol Campaign (ISDAC) in 2008. Historical measurements of the ice particle size distribution (PSD) have been flawed by ice artifacts produced by natural ice particles colliding with the inlet tube of various measurement probes. Data processing techniques used in conjunction with new probes used in recent field campaigns appear to have significantly reduced the artifact concentration of small ice particles.
The mass-weighted fall velocity (Vm) depends on the PSD and ice particle shapes. The characterization of ice particle projected area and mass (i.e. a representation of ice particle shape) was also improved based on data collected during at least one of the field campaigns, allowing realistic estimates of Vm to be obtained. The calculation of Vm was based on improved direct measurements of the PSD, ice particle area, and estimated mass. The effective diameter (De) was calculated in a similar way.
The TC4 analysis has provided diagnostic relationships that relate Vm to (1) both cloud temperature and ice water content (IWC) with an r2 of 0.75 and (2) to temperature alone with r2 = 0.72. Similar relationships for De were also obtained. However, a critical climate feedback parameter like the ice fall-speed needs to be coupled with the cloud microphysics and radiation in climate models. This is made possible through strong correlations between De and Vm regarding TC4 and ISDAC cirrus, and it ensures that Vm is consistent with the cloud microphysical and optical properties. Finally, TC4 satellite retrievals of De and Vm are found to be consistent with corresponding observations.
