Parameterization of Ice Fall Speeds for Reducing Cloud Uncertainties in Climate Models

Global Climate Models (GCMs) are highly sensitive to the representation of clouds and their feedbacks. As stated by Soden and Held (2006) the inter-model differences in cloud feedback are the largest source of uncertainty in current predictions of climate sensitivity. Sanderson et al. (2008) performed a principal component analysis across an ensemble of GCM runs and found that 70% of the ensemble variance in the global feedback parameter was due to two leading factors, the entrainment coefficient and the ice fall velocity (Vf). In spite of its importance, the ice fall velocity in climate models is highly uncertain due in part to its dependence on the ice particle size distribution (PSD) which has been plagued with measurement uncertainties associated with ice particle shattering on probe inlets. The focus of this research is to improve the parameterization of ice mass sedimentation rates in GCMs which is the product of the ice water content (IWC) and the mass weighted fall speed (Vm). Ice sedimentation rates strongly influence the life cycle of cirrus clouds and hence the earth’s radiation budget. To accurately estimate ice mass sedimentation rates, accurate measurements of the PSD regarding ice particle number, projected area and mass are needed. A relatively new probe called the 2D-Stereo (2DS) probe measures these 3 quantities using an ice particle projected area-mass relationship to estimate size-resolved mass concentrations. The 2DS probe also employs better inlet design and particle detection algorithm to minimize shattering artifacts. The 2DS estimates of ice water content (IWC), based on PSD integrations using the area-mass relationship, generally agree well (within ~ 20%) with CVI measurements of IWC during the TC4 campaign. This study uses 2DS data from SPARTICUS, a recent field campaign sampling midlatitude cirrus. In a measurement context, the mass weighted fall speed (Vm) is given as: Vm = ∑ v(D)m(D)N(D)ΔD / ∑ m(D)N(D)ΔD , where v(D) = ice particle fall velocity, m(D) = ice particle mass, N(D) = size distribution, D = ice particle maximum dimension at bin midpoint and ΔD = bin width. 2DS measurements are used to calculate v(D) for each 2DS size-bin using the methodology of Heymsfield and Westbrook (2010), which differs from the approach of Mitchell and Heymsfield (2005) by ~ 6% on average. Using this equation, measured PSD of size-resolved number, area and mass concentration can be used to solve for Vm. Standard temperature and pressure are assumed when calculating v(D) and Vm, and a prefactor is applied to Vm to adjust it to the desired temperature and pressure. In order to evaluate the degree of improvement in fall speed measurement by the use of newer 2DS probe, a comparison of fall speeds calculated from the use of (a) 2DS data alone (as described here) and (b) 2DS data combined with FSSP data, will be presented at the conference. In addition, the Vm-temperature relationships obtained from SPARTICUS (separate relationships for synoptic and anvil cirrus) will be compared with the same relationship obtained from tropical anvil cirrus in the TC4 campaign and from Arctic cirrus in the ISDAC field campaign.