From: Xin Xi <xxi_at_nyahnyahspammersnyahnyah>

Date: Tue Nov 12 2013 - 13:40:40 MST

Received on Tue Nov 12 13:41:01 2013

Date: Tue Nov 12 2013 - 13:40:40 MST

Mary, thank you very much!

On Tue, Nov 12, 2013 at 3:31 PM, Mary Haley <haley@ucar.edu> wrote:

*> Hi Xin,
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*>
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*> When you use "where", you will get all the lat/lon points that fall within
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*> the region you've defined.
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*>
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*> When you use wrf_user_ll_to_ij, it's just giving you the location of the
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*> two min points, and the location of the two max points in the array. When
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*> you subscript the array with these indexes, the points in this range don't
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*> necessarily represent every point in the range, but rather a rectangular
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*> box. And, in fact, this rectangular box may give you values outside your
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*> range.
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*>
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*> A graphic is the best way to show this. The attached graphic shows a full
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*> WRF lat/lon grid on the top, and then the two bottom plots show the two
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*> different subsetting methods, where the navy blue points represent the
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*> min/max lat and lon.
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*>
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*> --Mary
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*>
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*>
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*> On Nov 11, 2013, at 9:27 PM, Xin Xi <xxi@gatech.edu> wrote:
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*>
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*> Hello,
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*> I have a 2D field X from WRF with coordinates lat2d and lon2d. I want to
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*> calculate the sum of subset of X within lat/lon bounds of (min_lat,
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*> max_lat) and (min_lon, max_lon). I tried two different ways, which I
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*> expected the same results but they turn out to be different. Which is the
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*> correct way?
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*>
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*> method 1:
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*> X=where(lat2d.lt.min_lat .or. lat2d.gt.max_lat .or. \
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*> lon2d.lt.min_lon.or.lon2d.gt.max_lon, 0, X)
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*> sum_X=sum(X)
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*> (in fact, the original X is missing value outside the lat/lon bounds, I
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*> use the where function just for precaution).
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*>
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*> method 2:
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*> opt=True
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*> loc=wrf_user_ll_to_ij(geo_file,(/min_lon,max_lon/),(/min_lat,max_lat/),opt)
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*> loc=loc-1
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*> subsetX=X(loc(1,0):loc(1,1) , loc(0,0):loc(0,1))
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*> sum_X=sum(subsetX)
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*>
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*> The sum_X from method 2 is a lot smaller than method 1. Is it because
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*> wrf_user_ll_to_ij loses some grids at the bounds?
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*>
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*>
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*>
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*>
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-- *Xin XiSchool of Earth and Atmospheric SciencesGeorgia Institute of Technology311 Ferst Drive, Atlanta, GA 30332-0340*

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