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Project Title: Toward Exascale Simulations of 3D Radiative Transfer for Cloudy Atmospheres

Project originators:

  • Howard Barker, Environment Canada
  • Jacon Cole, Environment Canada
  • Philip Austin, UBC

SHARCNET programming support:

  • Pawel Pomorski
  • Alex Razoumov

The goal of this project is to modify an existing radiative transfer code (one of the components of climate and weather modelling used by the EC group) to make it fully parallel. The code uses a Monte Carlo approach to inject photons into the domain studied and compute their propagation.

The approach of the initial code is to supply each processor with the entire spatial domain and inject N photons per each parallel processor. This approach does result in a parallel speedup but suffers from memory limitations. Thus the first improvement to the code will be to decompose the spatial domain into subdomains which will be assigned to individual processors. This will require adding code to handle photons transferring between domains as they cross the boundary.

Once that is done, the second stage of the project will be to improve the above approach to ensure good load balancing between processors.

Physics assumptions that we don't need to worry about.

  1. Radiation propagates at a finite speed "c", so strictly speaking the equation of radiative transfer should include a time-dependent advection term (differential transfer equation) or the "retarded" energy (integral transfer equation). The working (and pretty safe) assumption is that "c" is so large that all changes in the radiation field can be assumed instantaneous, and those "c"-dependent terms can be dropped.
  2. The conservation equations of radiation hydrodynamics describe conservation of mass, momentum, energy. Ideally, they should be solved in one (perhaps, implicit) update. Operator-splitting them into two steps (hydrodynamics and transfer) is good enough for this problem, but it's worth keeping in mind that it's an approximation.