[ncl-talk] EOF analysis with NARR data (LON, LAT values)

[Jothi Ganesh Shanmuga Sundaram]

Thanks a lot Dave. It's helpful. I tried running without any coordinate
script, it is running and I was able to generate the EOF and EOF-time
series netcdf files for the entire domain. I am interested in coordinate
subscripting as in the EOF example 1.

When I tried to enter the lon lat ranges, the script does not run. It says
lon is not a dimension or variable. It accepts x or y, but x and y are some
strange values in the data. If I type ncl_filedump file -v lon, I see the
values.

Here is the ncl_filedump summary.

short air ( time, y, x )
..
float lat ( y, x )
....
float lon ( y, x )

1) Is there a way to do the subscripting in the EOF code by defining lon
and lat instead of x and y.

2) Also, the output netcdf file of EOF does not have the lon, lat meta
data. Instead it has x and y, which cannot does not overlay when I tried
with NARR plot.

Here is my script.

;Read the NARR data (Air temperature at 2m)
f = addfile ("air.2m.mon.mean.nc", "r")

;Define the parameter, convert the format short to float
 p = short2flt(f->air(::12,:,:))

;Calculate the weights
w = sqrt(cos(0.01745329*p&y))  ;  p&y works, but if i enter p&lat, it does
not work.
wp = p*conform(p, w, 1)
copy_VarCoords(p, wp)

t = wp(y|:,x|:,time|:)            ; x and y works, but if i enter lon or
lat, it does not work.

;Number of EOF to be calculated
neof = 3

;NCL EOF functions
eof = eofunc_Wrap(t, neof, False)        ;EOF (3modesXlonXlat)
eof_ts = eofunc_ts_Wrap (t, eof, False)  ;EOF (3modesXtime)

;Export EOF spatial data as netcdfile
system ("/bin/rm -f EOFnarr.nc")
fout = addfile ("EOFnarr.nc", "c")
fout at title = "EOFs of pre 1971-2013"
fout->EOF = eof

Regards
Jothiganesh




On Tue, Jun 16, 2015 at 9:58 PM, Dave Allured - NOAA Affiliate <
dave.allured at noaa.gov> wrote:

> Jothiganesh,
>
> Yes you can directly read NARR data and compute EOF's without regridding.
> The underlying EOF functions eofunc and eofunc_ts do not use any spatial
> coordinates.  They operate purely on numeric arrays.
>
> The first EOF example uses coordinate subscripting and the lonFlip
> function.  If you remove both of these methods from the example, then you
> can compute EOF's over the entire NARR grid.
>
> However, to generate valid plots, you will need to retain the NARR
> coordinate metadata, and modify the plot section to use one of the plotting
> methods shown on the NARR examples page.
>
> I recommend one of the projection methods in NARR example 4, because they
> use the retained 2-D coordinate grids to directly align the data with the
> underlying map.  After you get that working, you can switch back to Lambert
> or some other projection of your choice.
>
> --Dave
>
>
> On Tue, Jun 16, 2015 at 6:39 PM, Jothi Ganesh Shanmuga Sundaram <
> joshanmugasundaram at mix.wvu.edu> wrote:
>
>> Hello,
>>
>> I use NARR gridded data (349x277 Lambert Conformal Conic grid) in netcdf
>> format in NCL. I am able to plot graphics without any problem using the
>> existing script in NCL website.
>>
>> I want to do EOF analysis with NARR data. When I tried to use the EOF
>> script available from NCL website to analyze NARR data, it is not working.
>> When I checked the data using ncl_file dump the lon, lat formats are very
>> different. My question is
>>
>> 1) Whether I can do EOF analysis in NCL by directly reading NARR data? If
>> so, I cannot define the LON LAT value, as NARR has a projected coordinates
>> value?
>>
>> 2) Do I need to regrid the NARR curvilinear to rectlinear data before
>> doing EOF? or Is there a way to convert the format of LON, LAT in grid
>> without regridding inorder to do EOF analysis in NCL
>>
>> Thanks for your help.
>>
>> Regards
>> Jothiganesh
>>
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