Masking MSLP-WRF MSLP issues

From: Peter Q. Olsson <olsson_at_nyahnyahspammersnyahnyah>
Date: Mon Feb 28 2011 - 10:03:31 MST

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).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 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)".


   | 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 Mon Feb 28 10:03:38 2011

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