Thursday, 13 February 2003
Multispectral Thermal Retrievals of Size Distribution Shape, Effective Size, Ice Water Path, Optical Depth and Photon Tunneling Contriubtion
Satellite and ground-based retrievals of cirrus ice water path (IWP) and effective particle size (Deff) are needed to describe quantitatively the radiative forcing role of cirrus in cloud resolving models and GCMs, a long-standing goal of climate research. Unfortunately, Deff and IWP are not sufficient for completely describing the effects of cirrus in the radiative balance. Size distribution shape must also be known since cirrus absorption optical thickness can vary by as much as 40% for a cloud with otherwise the same IWP and Deff values. Additionally, a process called photon tunneling can contribute between 15 and 40% of cirrus absorption at thermal infrared (TIR) wavelengths. Photon tunneling describes the attenuation of radiation beyond the physical cross-section of a particle, where radiation is either absorbed or scattered away from the forward diffraction peak. Tunneling is an edge-effect process that is strongest at wavelengths where the refractive index of ice particles is high (most notably in the TIR region between 7 and 15 um), and when Deff and wavelength are comparable.
In this study we exploit satellite multispectral radiance observations for ice clouds and modified anomalous diffraction theory (ADA), which accurately parameterizes photon tunneling, to retrieve a tunneling estimate. The tunneling estimate is dependent on particle shape (the "tunneling factor" for water or ice spheres is 1.0, but it can be as small as 0). We will present the first estimates of tunneling found in natural cirrus clouds.
Our retrieval methodology also provides a relatively accurate (± 20%) means for estimating IWP based on the second (area) and third (mass) moments of the size distribution, whereby Deff is also retrieved. Using cirrus multispectral radiance observations in the 3.9, 7, 8.5, and 10-13-um regions provides cirrus absorption information that spans the range between size distribution area weighted (longer wavelengths) and mass-weighted (shorter wavelength) extremes. The enhanced accuracy derives from the fact that mass weighted absorption is relatively insensitive to particle size.