COBOLD or COBOLD is the short form of ``COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions with l=2,3''.
It is used to model solar and stellar surface convection. For solar-type stars only a small fraction of the stellar surface layers are included in the computational domain. In the case of red supergiants the computational box contains the entire star.
COBOLD solves the coupled non-linear equations of compressible hydrodynamics in an external gravity field together with non-local frequency-dependent radiation transport. Operator splitting is applied to solve the equations of hydrodynamics (including gravity), the radiative energy transfer (with a long-characteristics or a short-characteristics ray scheme), and possibly additional 3D (turbulent) diffusion in individual sub steps. The 3D hydrodynamics step is further simplified with directional splitting. The 1D sub steps are performed with a Roe solver, accounting for an external gravity field and an arbitrary equation of state from a table.
The radiation transport is computed with either one of three modules:
MSradmodule: It uses long characteristics. The lateral boundaries have to be periodic. Top and bottom can be closed or open (``solar module'').
LHDradmodule: It uses long characteristics and is restricted to an equidistant grid and open boundaries at all surfaces (old ``supergiant module'').
SHORTradmodule: It uses short characteristics and is restricted to an equidistant grid and open boundaries at all surfaces (new ``supergiant module'').
To get a brief overview you might want to look into the ``Quickstart Sections'' ``How to compile COBOLD'' (Sect. 3.1), ``Introduction to UIO'' (Sect. 4.1), ``How to make a proper Parameter File'' (Sect. 5.3.1), ``How to run COBOLD'' (Sect. 6.1).