A New Approach for Computing Single Scattering Properties of Ice Clouds Using a Size-Shape Distribution of Spheroidal Particles
Eide, H.A., and Stamnes, K., University of Alaska, Fairbanks; Stamnes, J.J., University of Bergen, Norway; Schulz, F.M., University of Rochester
Ninth Atmospheric Radiation Measurement (ARM) Science Team Meeting
Clouds are of paramount importance for the global energy balance and hence for our climate. In global circulation models (GCMs), designed to predict future climate, the effects of clouds are commonly based on the scattering and absorption properties of spherical particles. At high latitudes as well as at high enough altitudes anywhere on our planet, clouds frequently consist of ice particles that are far from spherical in shape. Ice particles usually have elongated needle-like shapes or flat, disk-like shapes. The radiative effects of ice clouds are not accounted for accurately in GCMs, and it is therefore reasonable to question their ability to predict the evolution of our climate in a realistic manner. We have developed a new method for computing the single-scattering properties of spheroidal particles using the Separation of Variables Method (SVM). The single-scattering properties are needed for every particle shape that we want to include in a GCM. The advantage of the spheroidal particle model is that by varying its aspect ratio, the spheroid can easily be changed from a near-spherical to an elongated needle-like (prolate) particle or a flat disk-like (oblate) particle. This makes it relatively easy to mimic the influence of particle shape on the scattering and absorption of radiation by polar and high altitude ice clouds. An important part of the single-scattering solution for spheroidal particles is the computation of the expansion coefficients that are needed in the angular and radial spheroidal functions. Problems that for several decades have hampered previous numerical schemes to compute these coefficients have been overcome with our new method, so that we can now handle realistic sizes and shapes and particle absorption in an effective manner. Our recent development of a method that employs the SVM to compute the T-matrix for spheroidal particles of arbitrary aspect ratio, makes it possible to construct realistic and efficient models for scattering of light by ice clouds. This new development takes advantage of the fact that analytical averaging over size-shape distributions of cloud particles (using the T-matrix) allows for efficient computation of scattering/absorption cross sections and phase functions for an ensemble of ice particles of different shapes and sizes.
Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).
