Vector arrows for primary wave direction

[Arlindo da Silva]

On Tue, Nov 3, 2009 at 11:15 AM, Nathan Cool <nathan at nathancool.com> wrote:

> 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.
>
>
Given the direction you should be able to construct a vector of magnitude 1
 pointing the direction of the wave, say

ga-> display cos(0.01745*WVDIRsfc);sin(0.01745*WVDIRsfc)

The factor 0.01745 is the conversion of deg to radians. (This assumes that
when the wave direction is 90 deg, the wave is going north).

   Arlindo

--
Arlindo da Silva
dasilva at alum.mit.edu
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