Vector arrows for primary wave direction

[Nathan Cool]

Good morning everyone,

I'm generating wave analysis models, but I'm having a tough time showing the
mean-wave (or primary) wave direction arrows.

The vars in the Grib are:
-------------------------------------------------
DIRPWsfc  0 107,1,1  ** surface Primary wave direction [deg]
DIRSWsfc  0 109,1,1  ** surface Secondary wave direction [deg]
HTSGWsfc  0 100,1,1  ** surface Sig height of wind waves and swell [m]
PERPWsfc  0 108,1,1  ** surface Primary wave mean period [s]
PERSWsfc  0 110,1,1  ** surface Secondary wave mean period [s]
UGRDsfc  0 33,1,1  ** surface u wind [m/s]
VGRDsfc  0 34,1,1  ** surface v wind [m/s]
WDIRsfc  0 31,1,1  ** surface Wind direction [deg]
WINDsfc  0 32,1,1  ** surface Wind speed [m/s]
WVDIRsfc  0 101,1,1  ** surface Direction of wind waves [deg]
WVPERsfc  0 103,1,1  ** surface Mean period of wind waves [s]

For displaying wind vector arrows it's straightforward as there are U and V
variables (UGRDsfc and VGRDsfc) for the zonal and meridional components.
Displaying primary wave direction though seems more elusive, yet I see it
done on many models such as the following:
https://www.fnmoc.navy.mil/ww3_cgi/dynamic/ww3.w.npac.sig_wav_ht.000.gif

...which was generated using GrADS by FNMOC.

Any help you can provide would be greatly appreciated, and thank you for
your time.

--
Nathan Cool
nathan at nathancool.com
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