### sea_surface_wave_mean_from_direction

accepted
Created: Nov. 6, 2023

Proposer: Dawn Petraitis

Proposed Date: 2023-09-13

Change Date: Nov. 6, 2023, 3:25 p.m.

Term: **sea_surface_wave_spectral_mean_direction**

Unit: degrees

Unit ref: UAAA

AMIP:

GRIB:

Change Date: Nov. 6, 2023, 3:26 p.m.

Term: **sea_surface_wave_spectral_mean_direction**

Unit: degrees

Unit ref: UAAA

AMIP:

GRIB:

The wave direction in each frequency band, calculated from the first-order components of the wave directional spectrum.

Change Date: Feb. 22, 2024, 1:49 p.m.

Term: **sea_surface_wave_principal_from_direction**

Unit: degrees

Unit ref: UAAA

AMIP:

GRIB:

The wave direction in each frequency band, calculated from the first-order components of the wave directional spectrum. The full directional wave spectrum is described as a Fourier series: S = a0/2 + a1cos(theta) + b1sin(theta) + a2cos(2theta) + b2sin(2theta). The Fourier coefficients a1, b1, a2, & b2 can be converted to polar coordinates as follows: R1 = (SQRT(a1a1+b1b1))/a0, R2 = (SQRT(a2a2+b2b2))/a0, ALPHA1 = 270.0-ARCTAN(b1,a1), ALPHA2 = 270.0-(0.5*ARCTAN(b2,a2)+{0 or 180, whichever minimizes the difference between ALPHA1 and ALPHA2}). ALPHA1 is the mean wave direction, which is determined from the first-order Fourier coefficients. This spectral parameter is a separate quantity from the bulk parameter (MWDIR), which has the standard name sea_surface_wave_from_direction_at_variance_spectral_density_maximum. The phrase "from_direction" is used in the construction X_from_direction and indicates the direction from which the velocity vector of X is coming. The direction is a bearing in the usual geographical sense, measured positive clockwise from due north.

Change Date: May 17, 2024, 3:47 p.m.

Term: **sea_surface_wave_mean_from_direction**

Unit: degrees

Unit ref: UAAA

AMIP:

GRIB:

The wave direction in each frequency band, calculated from the first-order components of the wave directional spectrum. The full directional wave spectrum is described as a Fourier series: S = a0/2 + a1cos(theta) + b1sin(theta) + a2cos(2theta) + b2sin(2theta). The Fourier coefficients a1, b1, a2, & b2 can be converted to polar coordinates as follows: R1 = (SQRT(a1a1+b1b1))/a0, R2 = (SQRT(a2a2+b2b2))/a0, ALPHA1 = 270.0-ARCTAN(b1,a1), ALPHA2 = 270.0-(0.5*ARCTAN(b2,a2)+{0 or 180, whichever minimizes the difference between ALPHA1 and ALPHA2}). ALPHA1 is the mean wave direction, which is determined from the first-order Fourier coefficients. This spectral parameter is a separate quantity from the bulk parameter (MWDIR), which has the standard name sea_surface_wave_from_direction_at_variance_spectral_density_maximum. The phrase "from_direction" is used in the construction X_from_direction and indicates the direction from which the velocity vector of X is coming. The direction is a bearing in the usual geographical sense, measured positive clockwise from due north.