Re: Masking MSLP-WRF MSLP issues

From: Sam Hawkins <s.hawkins_at_nyahnyahspammersnyahnyah>
Date: Wed Mar 02 2011 - 10:08:10 MST


I had a very quick look into this some time ago when I needed to write
an equivalent reduction to mean sea level pressure in Python, to use
with PyNGL. I think the default wrf_get_var method is based on this paper:

Benjamin, Stanley G., Patricia A. Miller, 1990:
An Alternative Sea Level Pressure Reduction and a Statistical Comparison
of Geostrophic Wind Estimates with Observed Surface Winds.
Mon. Wea. Rev., 118, 20992116.


On 02/03/2011 16:01, Mary Haley wrote:
> 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)?
>> 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 10:07:59 2011

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