Re: Masking MSLP-WRF MSLP issues

On Feb 28, 2011, at 10:03 AM, Peter Q. Olsson wrote:

> Hello all-
>
> I would like some advice on what might be a common problem.
>
> I am plotting MSLP in a domain that is about 50/50 ocean and land. As most of us know,
> MSLP is well-defined at sea level, but involves several assumptions over elevated
> terrain, which are seldom met. The result is that terrain -> large, MSLP contours become
> all about terrain, w/ no relation to pressure distributions.
>
> What I would like to do is to minimize this issue by plotting MSLP contour lines only
> where they relate to pressure patterns in a reasonable way. One way I have come up with
> is to somehow mask off the contours where the terrain height exceeds an arbitrary limit:
> sat for example's sake 200 meters. Now, I can see how this would be done with color-fill,
> essentially setting MSLP values to missing-value at i,j points where terrain exceeds the
> threshold (though I am not sure how to do this adjustment in the most elegant NCL
> way).

Hi Peter,

Maybe I'm misunderstanding the question, but have you looked at the "where" function for
setting values you don't want to missing (_FillValue attribute)?

http://www.ncl.ucar.edu/Document/Functions/Built-in/where.shtml

> However, I am not sure how to implement this for contour lines, since they seem to
> be informed by values in adjoining cells. I must confess I have never understood how
> NCARG did contour lines... I guess I am most concerned on how contours would be drawn in
> fjords where topo is ~few meters at one point and 400 meters at an adjacent point.
>

I think if you set your data to missing using the "where" function, then hopefully this shouldn't
be an issue.

> I would appreciate a nudge in the right direction.
>
> Finally, there seem to be a few algorithms for computing MSL in NCL/NCARG land. I would
> be interested in hearing others' thoughts on the comparative value of these methods. I
> currently use "wrf_user_getvar(nc_file,"slp",time_dim)".

This function will only work for WRF-ARW data. IF that's what you have, then this is
probably the best way to get this quantity.

If you don't have WRF-ARW data, then maybe Dennis will have something to add about the
differences between pslec, pslhyp, and pslhor.

--Mary

>
> PQO
>
> ___________________________________________________________
> |
> | Dr. Peter Q. Olsson,
> | Alaska State Climatologist, Alaska State Climate Center
> | Chief Scientist, Alaska Experimental Forecast Facility
> | University of Alaska Anchorage
> | 2811 Merrill Field Drive
> | Anchorage, AK 99501
> | voice: (907) 786-7234, fax (907) 786-7237|
>
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Received on Wed Mar 2 09:01:26 2011