In terms of climate models, what is the best way to decribe what 'boundary conditions' are?

Question

In terms of climate models, what is the best way to decribe what 'boundary conditions' are?

Answer 1

Conditions beyond which systems change dramatically. Like a cliff is a boundary conditions for walking at a certain altitude.

Answer 2

These are mostly related to conservation of mass, momentum, and energy at control surfaces in the model.  For instance, if you are running a coupled atmosphere-ocean general circulation model, you want it to conserve momentum across the atmosphere-ocean boundary.  You also want the energy flux out of the top of the atmosphere to equal the incident solar energy flux.  You want the freshwater flux in the ocean to balance so that net salinity is conserved and there are energy fluxes across the ocean that have to balance.  There are also physical boundary conditions related to fluid velocities going to zero at solid surfaces (at the bottom of oceans, along coasts, and in the case of the atmosphere, at the bottom of the planetary boundary layer).