Parameterization of ice fall speeds for reducing cloud uncertainties in climate models

Description

According to a study by Sanderson et al. (2008), the ice fall velocity is the second most important factor affecting the global feedback parameter in climate models. 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). 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 (or 2D-S) probe measures these three quantities from 10 to 1280 μm, using an ice particle projected area-mass relationship to estimate the size-resolved mass concentrations. The 2D-S 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 2D-S data along with data from other probes flown on the Learjet during SPARTICUS, a recent field campaign sampling midlatitude cirrus. These data are used to identify optimal cloud segments as per the guidelines given by SPEC Inc., Colorado. A segment begins when the extinction exceeds 0.1 Km-1. As each second of data is added to the segment, the maximum, mean, and minimum of the 1 Hz extinction and 1 Hz mean volume diameter (MVD) are found and compared. If both maximums do not exceed twice the mean and the minimums are not less than 0.4 times the mean, the segment passes and another second is added. A segment must reach 60 seconds in length to be kept, and when an acceptable segment reaches 120 seconds, the first half is cut off and kept as a segment and the second half continues adding seconds and comparing. 2D-S measurements of ice particle size, area and mass are used to calculate v(D) for each 2D-S 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, the 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. The Vm-temperature and Vm-effective diameter relationships obtained from SPARTICUS will be compared with corresponding relationships obtained from a Tropical campaign (TC4) and an Arctic campaign (ISDAC).