# Re: EOF and weight

From: Dennis Shea <shea_at_nyahnyahspammersnyahnyah>
Date: Wed Mar 30 2011 - 10:47:43 MDT

The EOFs are derived from a covariance matrix.
The weighting results in an area weighted covariance.

On 03/29/2011 10:57 AM, Mark Chan wrote:
> Hi NCL users,
>
> Below is a piece of NCL script from "EOF_1.ncl", in which the "weight" is
> calculated as:
>
> wSLP = SLP*conform(SLP, clat, 1)The weight is understandable for "area
> weighted average", but for EOF analysis, the weight application here is just
> making the value in high latitude relatively smaller, instead of reducing the
> number of grids accordingly (the typical way for EOF analysis). I may
> misunderstand it. Can anyone explain a bit more about the EOF and weight?
>
> Thanks a lot!
> Mark
>
>
>
> ; =================================================================
> ; create weights: sqrt(cos(lat)) [or sqrt(gw) ]
> ; =================================================================
> rad = 4.*atan(1.)/180.
> clat = f->lat
> clat = sqrt( cos(rad*clat) ) ; gw for gaussian grid
>
> ; =================================================================
> ; weight all observations
> ; =================================================================
> wSLP = SLP ; copy meta data
> wSLP = SLP*conform(SLP, clat, 1)
> wSLP@long_name = "Wgt: "+wSLP@long_name
>
> ; =================================================================
> ; Reorder (lat,lon,time) the *weighted* input data
> ; Access the area of interest via coordinate subscripting
> ; =================================================================
> x = wSLP({lat|latS:latN},{lon|lonL:lonR},time|:)
>
> eof = eofunc_Wrap(x, neof, optEOF)
>
>
>
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Received on Wed Mar 30 10:47:49 2011

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