Ramping is a way by which terms can be steady increased over some period of time in a simulation. This is most often done for model forcing terms, principally in order to avoid applying a shock to the model.
To understand why ramping of forcing terms is needed, consider tides. Starting a model with the water surface and velocity as zero everywhere, but with full tidal forcing, is analogous to having a planet at rest, then instantly putting a moon and sun in place, in motion with each other. This large, instantaneous change in conditions (forcing) is what creates the shock, which tends to result in spurious waves being formed. Ideally, one would initialize a tidal simulation with the water surface and velocity at all points matching what it should be at the point in time the model is initiated, given the phase of the tides, however this is not generally achievable. Gradually scaling up forcing terms helps avoid this problem.
NRAMP controls whether ramping is enabled, as well as how many ramping terms there are. This is to permit users to apply different ramping times for different forcing terms. All ramps in ADCIRC are applied as (truncated) hyperbolic tangent functions over a specified number of days. The various DRAMP* terms control the number of days over which the ramping is applied. Two other variables, FluxSettlingTime and DUnRampMete, affect the timing of the ramping terms.
- If NRAMP=1, then DRAMP is relative to coldstart time.
- If 1<NRAMP<8, then all DRAMP* terms are relative to coldstart time plus FluxSettlingTime
- If NRAMP=8, then all DRAMP* terms are as above, except DRAMPMete is relative to coldstart plus FluxSettlingTime plus DUnRampMete
FluxSettlingTIme is used because for large river systems such as the Mississippi River, it can take several days to initialize them and equilibrate; this may be partially relieved by using more advanced initialization schemes are used like the initial river elevation nodal attribute. DUnRampMete is used because often a simulation will be initiated without meteorological forcing, such as a tide-only simulation, and meteorological ramping is needed when this forcing is added at a later time.
There are also namelist controls for some ramping terms, such as water level offset forcing.