David L. Mitchell — Desert Research Institute
Anne Garnier — Laboratoire Atmosphères, Milieux, Observations Spatiales, U
Ehsan Erfani — George Mason University
Melody Avery — NASA Langley Research Center
Category
High-latitude clouds and aerosols
Description
CALIPSO IIR retrieval results for 2008 showing the fraction of cirrus clouds having N > 500 liter-1 for each season. Such cirrus clouds are most likely formed by homogeneous ice nucleation (Barahona & Nenes, 2009, ACP). Legend: winter, spring, summer & fall => DJF, MAM, JJA, & SON. Results are consistent with Cziczo et al. (2013, Science), they are consistent with the field campaign results reported in Krämer et al. (2009, ACP), and they have been partially validated in regards to the SPARTICUS, TC4 and ATTREX field campaigns.
A relatively new theoretical understanding of the absorption differences between split-window channels on satellite instruments has led to a new method for processing and interpreting split-window satellite measurements. Using the effective absorption optical depth retrieved from channels on the Imaging Infrared Radiometer (IIR) aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite, the ice particle number concentration (N), effective diameter (De) the ice water content (IWC), the ice water path (IWP) and the visible optical depth (OD) are retrieved from single-layer (no lower clouds) semi-transparent cirrus clouds having 0.3 < OD < 3.0 and cloud base temperature T < 235 K. Retrievals of these properties between 70N and 25S latitude are consistent with in situ measurements of these properties over the same latitude range, based on 5 cirrus cloud field campaigns. The success of this approach appears related to its sensitivity to the number concentration of small ice crystals (D < 60 μm).
These retrievals show a pronounced cirrus seasonal cycle in the N. Hemisphere over land north of 30°N latitude in terms of both cloud amount and microphysics, with greater cloud cover, higher N and smaller De during the winter season. We postulate that this is partially due to the seasonal cycle of deep convection that replenishes the supply of ice nuclei (IN) at cirrus levels, with homogeneous ice nucleation (henceforth hom) prevailing during boreal winter north of 30°N where deep convection is rare and snow often covers the ground, resulting in lower IN and higher N concentrations. In addition, hom cirrus tend to occur over mountainous terrain, possibly due to stronger, more sustained updrafts in orographic waves.
Over oceans, heterogeneous ice nucleation appears to prevail based on the lower N and higher De observed. Due to the relatively smooth ocean surface, lower amplitude atmospheric waves at cirrus cloud levels are expected.
Over pristine Antarctica, IN concentrations are expected to be minimal, allowing hom to dominate. Accordingly, over Antarctica cirrus clouds exhibit relatively high N and small De throughout the year.
Preliminary modeling of these cirrus variations in CAM5 and ECHAM6 suggest changes in cloud radiative forcing of 1 to 2 W m-2 are likely. The latitude distribution of hom and het in CAM5 is roughly opposite to their observed distribution in winter via CALIPSO.
