Weather and Climate Systems

Parameterization of Ice Fall Speeds in Mid-latitude cirrus for Global Climate Models

Subhashree Mishra
Postdoctoral Research Associate
Cooperative Institute for Mesoscale Meteorological Studies (CIMMS)

13 November 2013, 1:00 PM

National Weather Center, Room 1313
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

The climate sensitivity of global climate models (GCMs) can be sensitive to the treatment of the ice particle fall velocity. Sanderson et al. (2008) used the Hadley Centre atmospheric model (HadAM3) and performed a linear analysis using a multi-thousand member perturbed physics ensemble to relate climate sensitivity S to atmospheric GCM model parameters. Based on their study, the physical processes that accounted for 70% of the variance in the global feedback parameter λ (λ = 1/S) were due to two leading factors: the entrainment coefficient and the ice fall velocity (Vi). In this study, the mass weighted ice fall speed (Vm) in mid-latitude cirrus clouds is computed from in-situ measurements of ice particle area and number concentration made by the 2-dimensional Stereo (2D-S) probe during the Small Particles In Cirrus (SPartICus) field campaign. Vm in mid-latitude cirrus is parameterized in terms of cloud temperature (T) and ice water content (IWC), and also by relating Vm to the ice particle size distribution effective diameter (De). 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 (MMCR) measurements. Results also show a strong correlation between De and Vm owing to their similar mathematical formulations. Estimating Vm from De makes Vm a function of IWC and projected area, realistically coupling Vm with both the cloud microphysics and radiative properties.

Speaker bio

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