[ncl-talk] plotting geopotential height

[Bill Ladwig]

Hi Lijun,

Try contouring at a larger interval, like 6 dm, instead of 2.5 dm.  That
should help a little with the issue over the mountains.

Hope this helps,

Bill

On Wed, Sep 7, 2016 at 4:09 AM, Guido Cioni <guidocioni at gmail.com> wrote:

> Hey,
>
> [1] it’s always preferable to use  wrfhelp at ucar.edu for WRF-related
> issues with NCL. We generally don’t know so well the WRF functions so that
> we can not help if something it’s going wrong when interpolating/reading
> from WRF output files. However it seems that your problem it’s not strictly
> WRF related so I’ll give you my 2 cents on what is going wrong.
>
> [2] when plotting any model variable @500 hPa you are getting the effect
> of the orography, especially with high mountains
>  peaks like the ones found in the Rockies. In that area you are exactly
> getting the orography since isobaric levels intersect the mountains so that
> geopotential height corresponds with geometric height. So, from that point
> of view, you are getting exactly what you should from a model.
>
> [3] you can mask the data over mountains with missing value so that NCL
> will automatically try to interpolate with values at lower altitudes. This
> can be easily achieved with the mask function and using the orography from
> the model output.
>
> [4] you can smooth the geopotential height field but I doubt that will
> give you the correct results, since you’ll need to use a really strong
> filter to mask the effect of the mountains. I personally tried multiple
> times to use the smoothing functions of NCL and never worked out, so that I
> cannot give you a lot of advices regarding this option…but you can start
> from here http://www.ncl.ucar.edu/Document/Functions/Built-in/
> wrf_smooth_2d.shtml
>
> Regards,
>
> Guido Cioni
> http://guidocioni.altervista.org
>
> On 07 Sep 2016, at 05:57, Lijun Diao <ljdiao at gmail.com> wrote:
>
> Hi, everyone:
>
> I am trying to plot 500 hpa geopotential height. I used wrf_user_getvar to
> get gh and wrf_user_intrp3d to interpolate to 500 hpa. Please verify I am
> doing right.
>
> gh             = wrf_user_getvar(b,"geopotential",-1)
> gh             = gh / 9.8 / 10.
> ; interpolation
> gh_plane       = wrf_user_intrp3d(gh, p, "h", 500, 0., False)
>
> For the contour lines, I noticed there are a bunch of small circles over
> the Rocky Mountain as shown in the attached f1.png. What I want is a
> smoother plot as in the black lines in f2.png. Those small gh line circles
> actually coincide with the topograph as shown in f3.png. Please let me how
> I can plot a smooth gh as in f2.
>
> thanks,
> ------------------------------------------------------------
> -----------------
> Lijun Diao
> ldiao at uh.edu
> Research Assistant
> Department of Earth & Atmospheric Sciences
> University of Houston
> 4800 Calhoun RD., Bldg. S&R1-312
> Houston, TX 77204-5007
> <f1.png><f2.png><f3.png>_______________________________________________
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