Melody Avery — NASA Langley Research Center
David L. Mitchell — Desert Research Institute
Anne Garnier — Laboratoire Atmosphères, Milieux, Observations Spatiales, U
Category
Ice Nucleation and Cloud Phase
Description
Preliminary results showing the seasonal dependence of the effective absorption optical depth ratio (directly related to ice particle number concentration/ice water content or N/IWC), with higher values indicating higher N/IWC. N/IWC differences appear to depend on mineral dust concentration, orography (i.e. wave cloud cirrus), the height of deep convective clouds (reaching colder temperatures in the tropical western Pacific) and possibly whether deep convenction is over land or water (i.e. higher updrafts over land).
In 2013 a paper published in Science by Cziczo et al., based on 4 field campaigns in the tropics (3) and mid-latitudes (1), found that heterogeneous ice nucleation occurred in 94% of the clouds sampled. Another group reported at a conference in July of 2014 that of 13 field campaigns studied in Europe and America, heterogeneous ice nucleation dominated in most of them, although homogeneous ice nucleation dominated in several field campaigns. All the above field campaigns were conducted in the northern hemisphere below 60°N latitude where mineral dust concentrations are appreciable (mineral dust appears to be the primary ice nuclei for heterogeneous ice nucleation). With different field campaigns yielding different assessments of the dominate ice nucleation mode, it became evident that only through satellite remote sensing could it be determined which ice nucleation mechanism dominated as a function of latitude and season.
A new theoretical understanding of the 11 and 12 μm split-window channels (aboard several satellites) and the analysis of cirrus cloud in situ measurements from the SPARTICUS and TC4 field campaigns have revealed a tight relationship between the effective 12/11 μm absorption optical depth ratio, or βeff, and the ice particle number concentration/ice water content ratio, or N/IWC. Since βeff is an operational product of the imaging infrared radiometer aboard CALIPSO, it was possible to quickly obtain global coverage of βeff and N/IWC.
This new βeff study targeting cold semi-transparent cirrus clouds has shown that (1) polar cold cirrus (T < -38 C) occur much more often during winter than summer and (2) the ice particle number concentration/ice water content ratio, or N/IWC, is relatively high at high latitudes during winter, suggesting that homogeneous nucleation occurs frequently there. Homogeneous nucleation is further supported by the fact that high N/IWC values tend to coincide with regions of low mineral dust concentrations, as simulated by CAM5 (Storelvmo and Herger, 2014, JGR). Based on CAM5 simulations, this high N/IWC during winter (and probably from Dec. – April) is likely to have a strong greenhouse effect that may increase high latitude temperatures by 2-5°K relative to cirrus conditions where heterogeneous nucleation dominates (Storelvmo et al. 2014, Philos. Trans. A, Royal Soc.). Thus, the lack of mineral dust in the Polar Regions during winter may result in a strong warming influence over these regions.
