[gradsusr] Shift in the lattitude while plotting in grads mercator/latlon projction

[Masters.Steve]

I have plotted WRF data in grib format with a Mercator projection and it appears to plot correctly.  The grib2ctl.pl script that builds the ctl file constructs the YDEF line like

YDEF 120 LEVELS  -3.325 -3.305 -3.287 ….. <120 levels given>

It calculates and puts in the actual latitudes of the grid points for the particular Mercator grid that is defined in the file.  The levels in the YDEF (or XDEF) do not need to be at even intervals so this method appears to work for the Mercator-projection data.

For your application, you can pull out the latitudes of the grid points from one of the wrfout* files using something like “ncdump –v CLAT wrfout…” and build the YDEF line from these values.

Steve Masters    masters.steve at ensco.com<mailto:masters.steve at ensco.com>
ENSCO, Inc.
Melbourne, FL

From: gradsusr-bounces at gradsusr.org [mailto:gradsusr-bounces at gradsusr.org] On Behalf Of Yaqiang Wang
Sent: Friday, July 12, 2013 1:23 AM
To: GrADS Users Forum
Subject: Re: [gradsusr] Shift in the lattitude while plotting in grads mercator/latlon projction

Mercator projection is longitude equal-interval but not latitude equal-interval. The ctl without PDEF (Mercator is not supported in PDEF at present) may approximatively plot the data at low latitude area, but it's imposible plot the data at exactly position.

On Friday, July 12, 2013, itesh dash wrote:
The information actually were extracted from in the namelist.wps file using WRF2GrAGS which generate the control(ctl) file. For the model I only provided the grid resolution(9km), no of grid points in both north south (640) and east west(460)  and center latitude and longitude (18.0 and 91.0).

The XDEF,YDEF and other params were automatically calculated by WRF2Grads. But your guessing may be correct as i have changed the start lat from -3.1006 to -2.1006 and the shift is minimized. BUt is this correct way?

I am currently using the mercator projection. I will change to lat-lon projection and see if that makes the difference.

Thanks.



On Fri, Jul 12, 2013 at 12:40 AM, James T. Potemra <jimp at hawaii.edu<mailto:jimp at hawaii.edu>> wrote:
Itesh:

Your model output is 640 x 460, but your descriptor file has
xdef = 639 and ydef = 459.  These should be 640 and 460,
respectively.  Also, I think what Mohsen meant was to make
sure your starting lat/lon is the same in the descriptor file
and model grid, i.e., you have 61.1815 as the starting lon
and -3.1006 as the starting lat.  Are these the same as the
model grid?

Jim

On 7/11/13 7:31 AM, itesh dash wrote:
Dear Mohsen,

thanks for the response. But I am not even trying to use any script. Just by opening and plotting the ctl file.

Here is my ctl file

dset ^Jul1013_12.dat
undef 1.e35
xdef  639 linear   61.1815   0.0935
ydef  459 linear   -3.1006   0.0873
zdef    3 levels
  850.00000
  500.00000
  200.00000
tdef         8 linear 12z10jul2013  6hr
vars 1
HGT 0 0 Terrain Height
endvars

The wrf model ran with mercator projection 640 x 460 grid points 9 km resolution.

I could not figure out why the shift is there. Is there a way i can balance the shift.

regards,
itesh

On Thu, Jul 11, 2013 at 11:58 PM, Mohsen Soltani <soltani.clima at gmail.com<mailto:soltani.clima at gmail.com>> wrote:
itesh,

Make sure Lat/Lon in your .ctl file are the same as your script!

Mohsen

--
April showers bring May flowers!
--
Mohsen Soltani, M.Sc.
Climatology Grad Student,
Department of Climatology, Faculty of Geography,
University of Tehran, Tehran, Iran
voice: (+98) 9119772934
e-mail: soltani.clima at gmail.com<mailto:soltani.clima at gmail.com>

On Thu, Jul 11, 2013 at 9:05 AM, itesh dash <itesh at rimes.int<mailto:itesh at rimes.int>> wrote


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Dr. Yaqiang Wang
Chinese Academy of Meteorological Sciences (CAMS)
46, Zhong-Guan-Cun South Avenue
Beijing, 100081
China
　　　　　　　　　　　　　
yaqiang.wang at gmail.com<mailto:yaqiang.wang at gmail.com>

http://www.meteothinker.com
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