Re: wrf_user_ll_to_ij function

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

-- 
*Xin XiSchool of Earth and Atmospheric SciencesGeorgia Institute of
Technology311 Ferst Drive, Atlanta, GA 30332-0340*

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Received on Tue Nov 12 13:41:01 2013

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