Representing the ice fall speed in climate models: Results from Tropical Composition, Cloud and Climate Coupling (TC4) and the Indirect and Semi-Direct Aerosol Campaign (ISDAC)



[1] Ice fall velocity has a strong impact on climate feedback, 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 on the basis of 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. These campaigns provide improved measurements of the ice particle size distribution (PSD) where the concentrations of artifact small ice particles (due to shattering of ice particles on the probe inlet tube) are greatly reduced. In addition to the PSD, the mass-weighted fall velocity (Vm) depends on the ice particle projected area and mass. The calculation of Vm was based on improved direct measurements of the PSD number and area concentration and improved estimates of ice particle mass. The effective diameter (De) was calculated in a similar way. The TC4 analysis has provided a diagnostic relationship that relates Vm to both cloud temperature (T) and ice water content (IWC) with an r2 of 0.78. A similar relationship for De was also obtained with an r2 of 0.82. The Vm relationship and associated Vm-IWC-T measurements were found to agree well with a Vm scheme based on T and cloud radar retrievals of Vm and IWC in tropical cirrus clouds. However, a critical climate-influencing 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. Finally, TC4 satellite retrievals of De and Vm are found to be consistent with corresponding observations.