Re: Reading NCAR/NCEP reanalysis grib files

Hi Chris,

Because GRIB files can contain U and V (or any other parameter) on
multiple grids or with different level types, NCL creates variable
names that encode other information about the data. So although the
names of the variables you want begin with "UGRD" and "VGRD", these
are not the full names that you need to use in your script. You
simply need to add the line:
print(infile)
after the 'addfile' call in order to see the actual names of the
variables (or you could use the supplied script ncl_filedump prior to
writing your own script).

2) Assuming the data is dimensioned time x lat x lon you can get an
array of all the data closest to a particular lat/lon location using,
e.g.

uloc = ugrd(:,{lat_loc},{lon_loc})

3) speed = sqrt(ugrd*ugrid + vgrd * vgrd)
     dir = atan2(u,v) ; radians -- see example 1 of the atan2
function for the details of this.

http://www.ncl.ucar.edu/Document/Functions/Built-in/atan2.shtml

On Jul 8, 2009, at 1:30 PM, Christopher G. Herbster wrote:

> Hi all,
>
> I have downloaded 10m winds (U and V) in grib format and I am having
> trouble reading the files.
>
> I have cat'ed both components into a single file. An ultimate goal is
> to extract wind information for an arbitrary location and use that
> information to build a wind rose.
>
> 1) When I try to read in the variables from the file I get an error
> that
> says they don't exist or there is a problem with the file.
>
> ;***************************************
> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/wind_rose.ncl"
> ;
> ; Need the next one for the read functions
> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
>
> ;***************************************
> begin
> ;
> fname = "10m_Winds/2008-12_10m-wind.grb"
> ;
> ;
> ; Open file to read
> infile = addfile(fname,"r") ; Open GRIB file to read
> ;
> ; Fill the U and V arrays
> ugrd = infile->UGRD
> vgrd = infile->VGRD
>
> usize = dimsizes(ugrd)
> print ("usize is " + usize)
>
>
> end
>
> -----------
>
> This yields:
>
> $ ncl read_reanalysis_windrose.ncl
> Copyright (C) 1995-2007 - All Rights Reserved
> University Corporation for Atmospheric Research
> NCAR Command Language Version 4.3.0
> The use of this software is governed by a License Agreement.
> See http://www.ncl.ucar.edu/ for more details.
> fatal:Either file (infile) isn't defined or variable (UGRD) is not a
> variable in the file
> fatal:Execute: Error occurred at or near line 26 in file
> read_reanalysis_windrose.ncl
>
> ----------
>
> wgrib says this is what is in the file:
>
> $ wgrib 10m_Winds/2008-12_10m-wind.grb -v
> 1:0:D=2008120100:UGRD:10 m above gnd:kpds=33,105,10:6hr fcst:"u
> wind [m/s]
> 2:20388:D=2008120106:UGRD:10 m above gnd:kpds=33,105,10:6hr fcst:"u
> wind
> [m/s]
> 3:40776:D=2008120112:UGRD:10 m above gnd:kpds=33,105,10:6hr fcst:"u
> wind
> [m/s]
> 4:61164:D=2008120118:UGRD:10 m above gnd:kpds=33,105,10:6hr fcst:"u
> wind
> [m/s]
> 5:81552:D=2008120200:UGRD:10 m above gnd:kpds=33,105,10:6hr fcst:"u
> wind
> [m/s]
> 6:101940:D=2008120206:UGRD:10 m above gnd:kpds=33,105,10:6hr fcst:"u
> wind [m/s]
> 7:122328:D=2008120212:UGRD:10 m above gnd:kpds=33,105,10:6hr fcst:"u
> wind [m/s]
> 8:142716:D=2008120218:UGRD:10 m above gnd:kpds=33,105,10:6hr fcst:"u
> wind [m/s]
> 9:163104:D=2008120300:UGRD:10 m above gnd:kpds=33,105,10:6hr fcst:"u
> wind [m/s]
> 10:183492:D=2008120306:UGRD:10 m above gnd:kpds=33,105,10:6hr fcst:"u
> wind [m/s]
> <... snip ...>
> 120:2428428:D=2008123018:UGRD:10 m above gnd:kpds=33,105,10:6hr
> fcst:"u
> wind [m/s]
> 121:2448816:D=2008123100:UGRD:10 m above gnd:kpds=33,105,10:6hr
> fcst:"u
> wind [m/s]
> 122:2469204:D=2008123106:UGRD:10 m above gnd:kpds=33,105,10:6hr
> fcst:"u
> wind [m/s]
> 123:2489592:D=2008123112:UGRD:10 m above gnd:kpds=33,105,10:6hr
> fcst:"u
> wind [m/s]
> 124:2509980:D=2008123118:UGRD:10 m above gnd:kpds=33,105,10:6hr
> fcst:"u
> wind [m/s]
> 125:2530368:D=2008120100:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 126:2550756:D=2008120106:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 127:2571144:D=2008120112:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 128:2591532:D=2008120118:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 129:2611920:D=2008120200:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 130:2632308:D=2008120206:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 131:2652696:D=2008120212:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 132:2673084:D=2008120218:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 133:2693472:D=2008120300:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 134:2713860:D=2008120306:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> <... snip ...>
> 244:4958796:D=2008123018:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 245:4979184:D=2008123100:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 246:4999572:D=2008123106:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 247:5019960:D=2008123112:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
> 248:5040348:D=2008123118:VGRD:10 m above gnd:kpds=34,105,10:6hr
> fcst:"v
> wind [m/s]
>
>
> ------------
>
> Any help in getting past this fundamental point is greatly
> appreciated.
>
>
> 2) If anyone could offer some code examples that would fill an
> array of
> values (over all [or some] times) from the horizontal grid that are
> valid for a given lat/lon that would be really helpful.
>
>
> 3) Are there any built-in functions (or contributed ones) that will
> convert my U and V components into Speed and Direction?
>
>
> Thanks in advance for any help!
>
> Cheers,
> Chris Herbster
>
> --
>
> Dr. Christopher G. Herbster
> Associate Professor
> Director of Science and Technology
> for the ERAU Weather Center
> Applied Aviation Sciences
> Embry-Riddle Aeronautical Univ.
> 600 S. Clyde Morris Blvd.
> Daytona Beach, FL 32114-3900
>
> 386.226.6444 Office
> 386.226.6446 Weather Center
> http://wx.erau.edu/
>
> Schedule at: http://wx.erau.edu/faculty/herbster/Schedules/
>
>
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Received on Wed Jul 08 2009 - 14:36:38 MDT