Certain major model components, unlike more peripheral ones like bed deformation or calving, are “on” by default. They do not need to be turned on explicitly. For example, the SIA computation is so common that it would be a hassle to require an option to turn it on every time you need it.
But sometimes one wants to disable particular components, during model spin-up, for example. PISM has the following “off” switches:
-no_massdisables the mass-continuity (conservation of mass) step
-energy nonedisables the conservation of energy computation
-energy coldmakes PISM use temperature instead of enthalpy in the energy conservation code
-stress_balance nonedisables the stress balance computation (useful for testing surface mass balance inputs)
-dryessentially disables ocean models: ice is always considered to be grounded, the sub-shelf melt rate and temperature is not used, and the calving-front boundary condition is computed ignoring the water pressure exerted on the vertical face at a (possibly submerged) terminus.