AERI-Lidar Retrievals of Ice Cloud Physical Properties, Including the First Estimates of Photon Tunneling Contributions to Absorption
Mitchell, D.L.(a) and DeSlover, D.H.(b), Desert Research Institute (a), Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin (b)
Twelfth Atmospheric Radiation Measurement (ARM) Science Team Meeting
The Atmospheric Emitted Radiance Interferometer (AERI) and the High Spectral Resolution Lidar (HSRL) have been used to determine the spectral dependence of alpha in the window region (8.5-12.5 micron wavelength), where alpha is the ratio of optical depth at a visible wavelength to infrared absorption optical depth for a cirrus cloud. Using alpha and cloud emissivity measurements, it is generally possible to retrieve effective diameter Deff, ice water path (IWP) and estimate the contribution of photon tunneling to absorption in ice clouds. Photon tunneling accounts for absorption of radiation beyond a particle's physical cross-section, and may contribute around 20-45% of the absorption from 8-100 micron wavelength. Tunneling contributions can be expressed through a tunneling factor TF, which gives the fraction of tunneled radiation absorbed relative to that predicted for ice spheres. While TF was recently measured for hexagonal columns in a laboratory to be about 0.6, this value may not be representative of natural cirrus, since TF depends on particle shape. This study reveals the first estimates of TF from natural cirrus. In the wavelength region 11-12.5 microns, essentially all radiation incident to a particle's physical cross-section is absorbed at the ice surface, and absorption is area dependent. Without tunneling, the absorption efficiency Qabs generally equals 1.0 in this region, and alpha equals 2.0, since alpha = Qext/Qabs, where Qext = visible extinction efficiency = 2. But due to tunneling, Qabs > 1.0 and alpha < 2.0. This behavior makes it possible to retrieve TF in this region. In the wavelength region 8.5-10.0 microns, absorption partially depends on ice particle volume or mass (as well as area), allowing Deff to be retrieved from alpha. Cloud emissivity E may also be obtained from the AERI-HSRL measurements. Knowing E, Deff and alpha (i.e. Qabs), IWP can easily be calculated. Of the 3 cirrus case studies analyzed thus far, TF = 0.8 for 2 cases and TF = 0.4 for the other. This indicates TF may vary considerably depending on what ice particle shapes are dominant.
Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).
