Vector arrows for primary wave direction

[James T. Potemra]

Hi Nathan:

Matthias suggestion is correct, but I think you need to be careful with
the direction.  His calculation
presumes that the direction is measured counterclockwise from the east
(so a direction of zero would
be a wave headed due east, direction 90 would be headed north, and so
on).  The direction from the
model is probably compass heading (0 is north).  So, you might have to
add an extra step to convert
the direction angle.

Jim

Nathan Cool wrote:
> Thanks Matthias. I sure do appreciate the help. That produced some
> interesting results, but it didn't seem accurate (arrows pointing in
> some rather bizarre directions). Perhaps though this is a data
> problem, and I need to grab different GRIB files? Currently, I'm using
> GRIBs from the NCEP server:
> ftp://polar.ncep.noaa.gov/pub/waves/latest_run/
>
> -Nathan
>
>
> On Wed, Nov 4, 2009 at 7:53 AM, Matthias Fripp
> <matthias.fripp at eci.ox.ac.uk <mailto:matthias.fripp at eci.ox.ac.uk>> wrote:
>
>     How about this:
>
>     set gxout vector
>     display cos(DIRPWsfc * 3.14159/180); sin(DIRPWsfc * 3.14159/180)
>
>     This converts the wave direction from degrees to radians, and then
>     calculates its u and v components.
>
>     Matthias
>
>     On Nov 4, 2009, at 2:38 PM, Nathan Cool wrote:
>
>>     Thanks, but I'm not trying to display "wind" vector arrows;
>>     instead, I'm trying to display the primary wave direction.
>>
>>     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]
>>
>>     ...and I can see that others have been able to show the primary
>>     wave direction (DIRPWsfc) using vector arrows, as is the case on
>>     the FNMOC models like this one:
>>     https://www.fnmoc.navy.mil/ww3_cgi/dynamic/ww3.w.npac.sig_wav_ht.000.gif
>>
>>     Do you know how this can be done?
>>
>>     Thanks,
>>
>>     -Nathan
>>
>>
>>     On Tue, Nov 3, 2009 at 9:58 PM, sushant puranik
>>     <sushantpuranik at gmail.com <mailto:sushantpuranik at gmail.com>> wrote:
>>
>>         Hi
>>         I am able to generate wind direction. try his
>>         'set arrscl 1 30'
>>         'set cthick 5'
>>         'set arrowhead 0.10'
>>         'set clevs 3 5 10 15 20 25'
>>         'set ccols 0 4 1 2 3 5 12'
>>         'set gxout vector'
>>         'd
>>         skip(UGRDsfc,2,2);skip(VGRDsfc,2,2);skip(sqrt(UGRDsfc*UGRDsfc+VGRDsfc*VGRDsfc),2,2)'
>>         'run cbar.gs <http://cbar.gs>'
>>         'printim wind(m/s)'
>>
>>         with the help of such script i am able to generate output
>>         which is attached with this mail.
>>
>>         all the best.
>>
>>
>>         On Tue, Nov 3, 2009 at 9:45 PM, Nathan Cool
>>         <nathan at nathancool.com <mailto: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.
>>
>>             --
>>             Nathan Cool
>>             nathan at nathancool.com <mailto:nathan at nathancool.com>
>>
>>
>>
>>
>>         --
>>         Sushant Puranik
>>         Junior Research Fellow
>>         Dept. of Atmospheric & Space Sciences,
>>         University of Pune,
>>         Pune-07,
>>         India.
>>
>>
>
>