Vector arrows for primary wave direction
Matthias Fripp
matthias.fripp at ECI.OX.AC.UK
Wed Nov 4 13:18:49 EST 2009
That's right -- I just checked this page ( http://polar.ncep.noaa.gov/waves/products.html
), which reports that the datasets use the "oceanographical
convention", and this page ( http://www.aims.gov.au/pages/facilities/oceanographic-buoys/ob-1.html
), which says that the oceanographic convention is to show "the
direction the current is flowing towards, measured in degrees
clockwise from North".
If you replace DIRPWsfc with (90 - DIRPWsfc), you can convert between
the two conventions. So that would give you
set gxout vector
display cos((90-DIRPWsfc) * 3.14159/180); sin((90-DIRPWsfc) *
3.14159/180)
Sorry about that -- I'm so used to the physicists' approach to angles
that I forgot about the real world!
Matthias
On Nov 4, 2009, at 6:10 PM, James T. Potemra wrote:
> 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.
>>>
>>>
>>
>>
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