ITC Colloquium - Kevin Heng (CSH)


Thursday, April 27, 2017, 11:00am to 12:00pm




“Two-Stream Radiative Transfer in Exoplanetary Atmospheres”

Abstract:  “The two-stream treatment of radiative transfer approximates the passage of radiation through an atmosphere as a pair of outgoing and incoming rays.  Mathematically, it circumvents the need to evaluate an integral in the radiative transfer equation by solving for the moments of the intensity, rather than the intensity itself.  This simplification results in the need to specify values for constants known as Eddington coefficients.  Two-stream radiative transfer has been known to perform poorly in the presence of medium and large aerosols, which is a concern since the study of clouds/hazes is a frontier topic in (exo)planetary atmospheres.  For example, successfully attributing the presence of liquid water on the surface of early Mars to the scattering greenhouse effect, mediated by carbon-dioxide ice clouds, depends on whether two- or multi-stream radiative transfer is performed.  In this talk, I will review the two-stream treatment of radiative transfer, focusing on historical developments and highlights.  I will discuss how the specification of the Eddington coefficients may be generalised to drastically improve the accuracy of the two-stream method.  By comparing our calculations to brute-force, 32-stream calculations, I show that our improved two-stream method out-performs previous methods at the order-of-magnitude level in terms of accuracy.  In particular, it out-performs the two-stream source function method originally developed by Toon et al. (1989), which is widely used in the radiative transfer codes of Marley, Fortney, etc.  I will show an example of reflectivity calculations of carbon-dioxide ice clouds using the improved two-stream method.  Finally, I will argue that the simplicity and accuracy of the improved two-stream method allows it to be widely adopted in both radiative transfer codes and three-dimensional general circulation models.”




See also: Colloquium, 2016-17