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20041027: Time interpolation?

On Wed, 2004-10-27 at 12:11, Christian Pagé wrote:
> Hi everyone,
> 
> Is there a way to do time interpolation in gempak with a grid file?
> I have 3-hourly data model data that I would like to interpolate to
> 1-hourly data. Is it possible?
> 
> Thanks

Christian,

You can use GDDIAG and interpolate a field using the formula:
Xi = X0 + ( X1 - X0)/(t1 - t0) * (ti - t0)
where ti is a time in the range t0 to t1.


As example, here is a script that would interpolate the 3 hourly 500mb
heights in the eta212 to the f+1 and f+2 times for a a starting forcast
time, and looped for each 3 hourly forecast interval. The desired
function could also be looped though the desired parameters as well.
Note that gddiag is runs twice for each 3 hour forecast interval,
the first for t+1 hour and the second for t+2 hours.

Also, you can use the ddt(s) function in place of the sub(s1,s2) and
use the GDATTIM=time1:time2 time interval.

Steve Chiswell
Unidata User Support


#!/bin/csh -f

# set starting forecast hour and forecast increment
@ START = 0
@ INC = 3

# set model run time
set MODTIME="041027/1200"

# initialize forecast time array
set FTIM=("" "" "" "")

while ( $START < 84 )
   @ STOP = $START + $INC

   @ TN1 = $START + 1
   @ TN2 = $START + 2

   # create forecast fxxx strings
   set TARR=(${START} $TN1 $TN2 $STOP)
   @ CNT = 1
   while ( $CNT <= 4 )
      if ( $TARR[$CNT] < 10 ) then
         set FTIM[$CNT] = f00${TARR[$CNT]}
      else
         set FTIM[$CNT] = f0${TARR[$CNT]}
      endif
      echo look $CNT $TARR[$CNT] ftim $FTIM[$CNT]
      @ CNT = $CNT + 1
   end

   gddiag << EOF
   GDFILE  = eta212|${MODTIME}
   GDOUTF  = eta212|${MODTIME}
   GFUNC   = add(hght,mul(quo(sub(hght^${FTIM[4]},hght),${INC}),1)
   GDATTIM = $FTIM[1]
   GLEVEL  = 500
   GVCORD  = pres
   GRDNAM  = hght^${MODTIME}${FTIM[2]}
   GPACK   = grib      
   GRDHDR  =
   r

   GFUNC   = add(hght,mul(quo(sub(hght^${FTIM[4]},hght),${INC}),2)
   GRDNAM  = hght^${MODTIME}${FTIM[3]}
   r

   e
EOF

   @ START = $START + $INC
end