Parameterization of ice fall speeds in midlatitude cirrus: Results from SPartICus

The climate sensitivity predicted in general circulation models can be sensitive to the treatment of the ice particle fall velocity. In this study, the mass-weighted ice fall speed (Vm) and the number concentration ice fall speed (Vn) in midlatitude cirrus clouds are computed from in situ measurements of ice particle area and number concentration made by the two-dimensional stereo probe during the Small Particles In Cirrus field campaign. For single-moment ice microphysical schemes, Vm and the ice particle size distribution effective diameter De were parameterized in terms of cloud temperature (T) and ice water content (IWC). For two-moment schemes, Vm and Vn were related to De and the mean maximum dimension¯D, respectively. For single-moment schemes, although the correlations of Vm and De with T were higher than the correlations of Vm and De with IWC, it is demonstrated that Vm and De are better predicted by using both T and IWC. The parameterization relating Vm to T and IWC is compared with another scheme relating Vm to T and IWC, with the latter based on millimeter cloud radar measurements. Regarding two-moment ice microphysical schemes, a strong correlation was found between De and Vm and between¯D and Vn owing to their similar weightings by ice particle mass and number concentration, respectively. Estimating Vm from De makes Vm a function of IWC and projected area, realistically coupling Vm with both the cloud microphysics and radiative properties.