Satellite Remote Sensing of the Dependence of Homogeneous Ice Nucleation on Latitude and Season

Abstract:

Cirrus clouds can be thought of as belonging to one of two categories: those formed through (1) homo- and (2) heterogeneous ice nucleation (henceforth hom and het) due to the very different microphysical and radiative properties associated with these two mechanisms. Hom cirrus will form only when atmospheric ice nuclei (IN) are sufficiently low in concentration, and studies suggest that mineral dust may account for most IN globally. Hence the occurrence of hom and het cirrus is likely to depend on latitude and season as mineral dust does, making satellite remote sensing the preferred method for characterizing this occurrence.

A new understanding of thermal absorption in two split-window channels renders a reinterpretation of a standard CALIPSO satellite retrieval; the effective absorption optical depth ratio or β_eff. Using earlier studies and aircraft measurements in cirrus clouds, β_eff is found to be tightly related to the ice particle number concentration/ice water content ratio, or N/IWC, and thresholds for hom cirrus are estimated in terms of N/IWC and β_eff. When applied to cold semi-transparent cirrus clouds, we find that (1) polar cirrus (T < -38 C) occur much more often during winter than summer and (2) hom cirrus prevail at high latitudes during winter, and during spring and fall over Antarctica. The figure shows estimates of the fraction of cirrus produced by hom (where β_eff > 1.15) during January and August, where green is ~ 50% and red ~ 90-100%. These high N/IWC values associated with hom cirrus occur in regions where mineral dust concentrations are predicted to be minimal. This high N/IWC condition during winter is likely to have a strong greenhouse effect that may increase high latitude temperatures by 2-5°K relative to conditions where het cirrus dominates (Storelvmo et al. 2014, Philos. Trans. A, Royal Soc.). Thus, the lack of mineral dust in the high latitudes during winter may result in a strong warming influence over these regions. Moreover, changes in dust concentrations due to a changing climate may significantly affect surface temperatures in the Polar Regions.

Other findings are that N/IWC is consistently higher over land than over oceans, and that hom cirrus are associated with steep orography (i.e. high cooling rates) where dust concentrations are expected to be low.