Regner's main field of expertise is 3D, hydrodynamics simulations of convection in deep stellar atmospheres. Realizing that such simulations can never be better than the atomic physics they employ, he has worked on compiling the highest quality and most complete set of absorption coefficients for summation to atmospheric opacities. Along the same vein, Regner also is working on updates to the Mihalas-Hummer-Dappen equation of state, including many new physical processes, as well as hundreds of molecules. Radiative transfer in 3D, time-dependent simulations, is very expensive compared to in conventional 1D stellar atmospheres. The monochromatic opacities, at 100,000s wavelengths, are therefore binned into a dozen or so bins, making the problem tractable. Regner improved the formulation of binned radiative transfer, ensuring convergence to the monochromatic case and, for the first time, reproducing solar observations of limb-darkening, Hydrogen lines and flux spectra, by the same fully theoretical model.

A grid of such simulations have, so far, been used for a calibration of the mixing-length formulation of convection, temporal spectra of granulation, a newand consistent formulation for using full atmosphere calculations as outer boundary conditions for 1D stellar structure models, calculation of the p-mode frequency shift from the convective expansion of stellar atmosphere, and the direct interactions between convection and p modes (incl. excitation/damping).

Regner rides his bike to work and enjoys hiking in the mountains with his wife, Charlotte, and daughter, Annelise. At their South Boulder home, the cat Ottimo, provides calm when the work piles up too high. Archery, scotch, Mike Oldfield and Tolkien provide welcome distractions from work.