Exact meaning of u and v variables?

[Jason Roberts]

Dear HYCOM folks,

 

As I understand it, the HYCOM GLBa0.08 grid utilizes three different coordinate systems, depending on latitude. The middle section uses Mercator, the northern section uses a tri-pole system, and the southern section uses an equirectangular system.

 

What is the exact meaning of the u and v variables? Do they mean:

 

1. Current speed in the east/west and north/south directions, regardless of where you are on the grid?

 

2. Current speed in the right/left and up/down directions, regardless of where you are on the grid?

 

I am pretty sure #1 is correct but I need to make sure. A colleague wants to use HYCOM currents to perform a hydrodynamic simulation at non-tropical latitudes. As input, the simulator requires currents defined according to #2. At tropical latitudes, we can use #1 currents as input to it without introducing much error. But farther and farther from the equator, more and more error will be introduced by assuming that currents flow right/left/up/down on the grid when they actually flow east/west/north/south, and we should transform the values appropriately. Do you agree?

 

Thanks,

Jason

[alan.wa...@hycom.org]

The native model velocity variables are x-wards and y-wards (your #2), but as part of the interpolation to fixed vertical levels we rotate them to eastwards and northwards (your #1).  These are the same south of 47N, where the grid is recti-linear, but not north of 47N, where the grid is curvi-linear in the Arctic bi-polar patch part of the tri-pole grid.

If you need velocities North of 47N you have two options:

a) Use the GLBu0.08 grid fields, which are rectilinear everywhere but do not go north of 80N.

b) Rotate the velocities back to x-wards y-wards.  The array pang in regional.grid can be used to do this.  The eastwards,northwards to x-wards,y-wards code is in ALL/force/src/wi.f:

              COSPANG  = COS(PANG(I,J))
              SINPANG  = SIN(PANG(I,J))
              TXM(I,J) = COSPANG*TXMIJ + SINPANG*TYMIJ
              TYM(I,J) = COSPANG*TYMIJ - SINPANG*TXMIJ

Alan.