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ocean-eddy-cpt / gcm-filters-paper

Manuscript on spatial filtering method
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EKE analysis: filter native model velocities, then rotate #12

Closed NoraLoose closed 3 years ago

NoraLoose commented 3 years ago

UVEL and VVEL are velocities with respect to the model grid, which is locally rotated by ANGLE radians with respect to lat/lon directions: zonVEL = cos(ANGLE) UVEL - sin(ANGLE) VVEL merVEL = sin(ANGLE) UVEL + cos(ANGLE) VVEL

Previously, I had followed the approach: 1) rotate the native model velocities (UVEL,VVEL) onto zonal/meridional velocities (zonVEL,merVEL) 2) filter (zonVEL,merVEL)

This pull request changes the approach to the correct one: 1) filter native model velocities (UVEL,VVEL) 2) rotate filtered native velocities onto zonal/meridional velocities

The difference between the two approaches is pretty small (probably because ANGLE(x,y) is of large scale).

NoraLoose commented 3 years ago

Actually, for computing $EKE = \bar{|u|^2} - |\bar{u}|^2$, no velocity rotation at all is necessary, since (UVEL,VVEL) are in a locally orthogonal basis. The above outline (first filter, then rotate) is however the correct one when you for instance want to plot filtered zonal velocities.