Question calculating transport of passive tracer

[agaul]

Hi everyone,

I am trying to calculate how much of a passive tracer crosses from offshore to onshore of the shelf-break, and I have calculated this using two different methods. These methods give different answers by a factor of 3, which shouldn't be the case (I think), so I am wondering if I am doing something blatantly wrong or if there is a reason that these two methods actually shouldn't be expected to agree.

Method A)
1) Take a slice of Hvom_dye_01 (from roms_avg.nc) in the alongshore, at the shelf-break. This will have dimensions (Nx,1,Nz,timesteps), where timesteps are from the beginning to end of the simulation. And it has units [m^3/s*kg/m^3=kg/s]
2) Sum the slice in space to get the total flux of passive tracer at each timestep [kg/s].
3) Integrate in time to get the total transport of passive tracer to onshore of the shelf-break [kg].

Method B)
1) Use the dye_01 concentration from roms_his.nc at the final model timestep [units kg/m^3]
2) Integrate this field in space over the region onshore of the shelf-break to get the total mass of tracer that is onshore of the shelf-break [kg].

Thank you in advance,
Alan
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My model is along-shore in x, cross-shore in y, and has periodic east & west boundaries. The passive tracer is set to 1 in regions offshore of the shelf-break as an initial condition.

[rduran]

For flux and transport, you need a velocity. I don't see that in A. By including the velocity you account for stuff leaving, not just stuff entering.

[wilkin]

What you say is, in principle, correct. Generally speaking, in method B you need to subtract the mass of tracer at the start, but that is zero for this specific set up, right?

How are you calculating dz in the volume integral? It is easy to get that wrong if you are coding from scratch.

Replying to rduran: The v-velocity is the "v" in Hvom_dye_01

[agaul]

Hi John, thanks for your reply.

Generally speaking, in method B you need to subtract the mass of tracer at the start, but that is zero for this specific set up, right?
Correct, in this setup the starting mass of tracer is zero in the region I am summing over, which is everywhere onshore of the shelfbreak

How are you calculating dz in the volume integral? It is easy to get that wrong if you are coding from scratch.
I am using the roms_zint.m function in your roms matlab tools package.