ModelE comes with a full complement of tracer code that follows all mass transports in the model. All tracer fields are computed in mass units (not concentration) and thus are perfectly conserved (with one exception explained below). There is a library of pre-coded physics for some common tracers that have been used historically with the GISS models. For the simplest applications, the relevant tracer code needs to be turned on using pre-processor directives in the rundeck (details below), and then by editing TRACER_COM.f the specific tracer can be coded for. Some generic code is allowed for any tracers (such as radioactive decay, diagnostics etc.). Some specific tracers require special physics and these require some extra code (for instance water isotopes or chemistry).
The tracers break naturally into five groups: atmospheric tracers that follow air mass, tracers that are either purely water based or have a soluble component, reactive gas or aerosol tracers that may interact with the hydrologic cycle, tracers that are purely oceanic, and special tracers. These are dealt with in turn below.