3 or 6 hourly precip from girb files...

[Iain Russell]

Hi Dan

Different model grib files store accumulated precip in different ways. Some give you the accumulation over the period from the last timestep to the current one (like what I think you want) .Others do it a bit differently , for example;

T+6 (6hr accum)
T+12 (12 hr accum)
T+18 (6hr accum)
T+24(12 hr accum)

Other model grib files are different yet again ; some give you an accumulation as a running total through the model forecast time periods (so accum-precip at 48 hr is in fact from 0-48hr).

This can introduce some quirks into how you deal with the data. In my case, I always pre-process the data so that I have the precip in _my_ desired temporal resolution before I try to display it , depending on what I'm trying to do. e.g. in some cases I want to show 24 hours amounts, so first I break it down into accumulation over the interval (prev-step to current-step) like you want, then add 'em all up to give me the 24 hour amounts at whichever timestep I want. 

You can get an idea of how your model grib files are storing the precip accumulation by doing a wgrib -v on the file and having a look at the output.

Unfortunately I don' t know if Grads has any built-in capability to handle the accumulated precip  in a standard way.

Hope this helps

Regards
Iain
  ----- Original Message ----- 
  From: Dan Leins 
  To: GRADSUSR at LIST.CINECA.IT 
  Sent: Sunday, November 13, 2005 7:06 PM
  Subject: 3 or 6 hourly precip from girb files...


  To all,

  I've been trying to use GrADS to plot 3 or 6 hourly precip plots (depending on what the model timesteps are), but been uncussessful thus far.  I've used grib2ctl.pl to dump my model grib files to a control file and gone from there, with the only precip options being... 

  acpcpsfc (convective precip), apcpsfc (surface total precip), and ncpcpsfc (large scale precip).  Another one deals with precip rate, but I'm not interested in that.

  I'm simply interested in how much precip fell between the previous model timestep and the one I'm working with, but using each of these options simply increments the precip through all 48 hours - like a running total.  So if it rains from hours 03-09 - and is dry after that, I still see precip being output for hours 12-48 even if it's not going to rain.  Does anyone have any idea how to plot the precip that fell ONLY during the time valid in the grib file?  

  I don't know if it helps or anything, but when creating the idx file - I use the -0 and -v options - maybe that makes a difference...

  Thanks,
  Dan Leins
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