[gradsusr] Total Snow Accumulation with GFS .25 Degree

Thu Dec 30 16:43:59 EST 2021

Hi Jeff,

I have been manipulating your pseudo-code to try to get it to work and I am
getting much closer. I will try this next: fhr = (t - 1)*3 in my
equations.  I was actually trying (t+1)*3 and that didn't really help.  I
also tried changing the start time in the sum line to t=2 (instead of t=1)
to get past issues of apcpsfc at t=1.

I am using the NOMADS server for the data, for example:

https://nomads.ncep.noaa.gov/dods/gfs_0p25/gfs20211230/gfs_0p25_06z

I use fwrite, display variables, and create a dat file.

For example:
ttime        = 1
TIME_STEPS   = 129
'set fwrite gfs_hd'.dat'
'set gxout fwrite'
while(1)
'set t 'ttime''
'set x 1 1440'
'set y 1 721'
'set lev 1000'
'd const(apcpsfc,0,-u)'
'd csnowsfc'
ttime = ttime + 1

if ttime > TIME_STEPS ;
  break;
endif
endwhile
'disable fwrite'

My ctl file has an "hr" in it:
TDEF 129 linear 06Z30DEC2021 3hr

The above method works great, producing other products.  I download other
variables and plot them just fine.

But, wgrib2 and g2ctl don't work with my grads generated dat file.  I tried
both scripts, wgrib2 doesn't give me any output and g2ctl says it is not a
girb file.

Thanks again,

Jeff C

On Thu, Dec 30, 2021 at 12:48 PM Jeff Duda <jeffduda319 at gmail.com> wrote:

> Jeff,
> My previous response included some pseudo-code...it was not meant to be
> used verbatim. You will need to make adjustments for your specific
> application.
>
> What is your source for the GFS data? Can you post output from wgrib2 -v
> or g2ctl ?
>
> As far as I've ever known, the time index in Grads always starts at 1,
> regardless of the actual forecast length. The time axis is defined in the
> control file...in the TDEF entry. So in reality, a formula converting the
> time index in Grads to the valid forecast hour is fhr = (t - 1)*3 instead
> of what I previously said...unless of course your TDEF entry did not
> include the 00-hr output from GFS.
>
> Remember, a modulus in math is simply a remainder. So (3 mod 6) = 3 even
> though the value of 3/6 is 0.5.
>
> This method does not depend on the initialization time.
>
> Jeff Duda
>
> On Thu, Dec 30, 2021 at 12:36 AM Jeff Chabot <jsc219 at gmail.com> wrote:
>
>> Hi Jeff,
>>
>> The code below looks good and I thought it would solve my issue, but it
>> didn't quite work.  Does it assume that t=1 is at the initial model run
>> time such as 0Z, 6Z, 12Z, and 18Z?  Also, I had issues at t=1 with
>> apcpsfc.  I had a lot of values there at: -39330708 based on a grid check.
>> I believe that I corrected the issue by using const(apcpsfc,0,-u) when I
>> download the .dat file to set all of those large negative numbers to 0.
>>
>> I am finding that I still need the following if statement or t=1 will
>> error out and t=2 will show no data:
>>
>> if (t=1|t=2)
>>  'define snow = apcpsfc * csnowsfc'
>> else
>> ...
>>
>> Can you tell me if I am using the math_fmod properly?
>>
>> For example, let's say I'm using the 18Z model run, t=1 (18Z), t=2 (21Z)
>> and t=3 (0Z), etc...  At t = 3, then fhr = 3 * 3 = 9 and 9/6 is 1 with a
>> remainder of 3 so skip to the if (mod = 3).  Am I using that function
>> correctly?  So, t =1 (say at 18Z), would be 3/6 = 0.5 which is not defined
>> unless I use my if statement above.
>>
>> Thanks again,
>>
>> Jeff C
>>
>>
>> On Wed, Dec 29, 2021 at 2:03 PM Jeff Duda <jeffduda319 at gmail.com> wrote:
>>
>>> csnowsfc represents a snow-mask only for falling precip in that time
>>> step. It is not a fixed mask. Therefore, you will not be able to used your
>>> proposed explicit formula for computing fallen snow at each time step. You
>>> will need to sum the time-step snowfall to get run-total snowfall
>>> accumulation using something like the following:
>>>
>>> fhr = t * 3
>>> mod = math_fmod(fhr,6)
>>> if (mod = 0)
>>>  'define snow = sum(apcpsfc*csnowsfc,t=1,t-0,2)'
>>> else
>>>  if (mod = 3)
>>>   'define snow = sum(apcpsfc*csnowsfc,t=1,t-1,2) + apcpsfc*csnowsfc'
>>>  endif
>>> endif
>>>
>>> 'd 10*snow/25.4'
>>>
>>> the ",2)" in the sum command refers to skipping one time slice each, to
>>> avoid double counting those 3-hour increments. The second part of the
>>> if-statement is to add the final 3-hour accumulated snowfall.
>>>
>>> There is a different way to do this, since those data files also contain
>>> snowdepth as snodsfc. You could simply display 39.97*(snowdsfc -
>>> snowdsfc(t-1)) as the difference in snow depth between 3-hour outputs
>>> (scaled to inches from meters). Yes, there will be some negatives due to
>>> snow melt, but you can easily mask those out. I would imagine this method
>>> would be quite similar to the above method, as it is unlikely that there
>>> would be much snow melt or removal at a grid point during which snow was
>>> actively falling during a 3-hour window. And this latter formula is
>>> certainly simpler and less computationally intensive.
>>>
>>> Jeff Duda
>>>
>>> On Tue, Dec 28, 2021 at 10:03 PM Jeff Chabot <jsc219 at gmail.com> wrote:
>>>
>>>> Hello GrADS Users,
>>>>
>>>> I have been struggling to plot total snow accumulations with GFS .25
>>>> degree for the past week or so.  I know GFS does the following with precip
>>>> accum: 0-3hr;0-6hr,6-9hr; 6-12hr, etc and I am trying to workaround that
>>>> alternating precip totals.
>>>>
>>>> I was able to get this equation to work for total liquid precip, but I
>>>> can't get it to work with snow:
>>>>
>>>> if t=1|t=2
>>>> 'display 'precip'
>>>> else
>>>> 'display sum(precip,t-1,t+0) - sum(precip,t-1,t-1)'
>>>> endif
>>>>
>>>> Where:
>>>> precip = apcpsfc/25.4 (for inches) and t = time steps from 1 to 129.
>>>>
>>>> When I try to convert this to snow, I either get snow totals off the
>>>> charts 100+ inches or I get rolling precip that moves and does not
>>>> accumulate.  I am using a 10:1 ratio with csnowsfc to detect snow where 1
>>>> is snow, 0 is no snow.
>>>>
>>>> I started with the equation above:
>>>>
>>>> if t=1|t=2
>>>> 'display 'asnow'
>>>> else
>>>> 'define preciptot = sum(precip,t-1,t+0) - sum(precip,t-1,t-1)'
>>>> 'display preciptot * 10'
>>>> endif
>>>>
>>>> Where:
>>>> asnow = apcpsfc * csnowsfc * 10.
>>>>
>>>> The above equation shows a 10:1 ratio for all of the precip.  Once I
>>>> attempt to multiply by * csnowsfc (in the else section), I lose total
>>>> precip and instead see moving precip over time.  I tried it in the sum
>>>> equations multiplying precip * csnowsfc; then just in the display preciptot
>>>> * csnowsfc * 10.  I even tried to use the same sum equation for precip
>>>> substituting precip for csnowsfc, but none of these attempts worked to show
>>>> total snow accumulations from t=1 to 129. I just can't get it to work with
>>>> GFS.  All other models like nam, hrrr, use a nice simple sum equation:
>>>> 'display sum(asnow,t=1,t='t')'.
>>>>
>>>> Any help here would be really appreciative.
>>>>
>>>> Sincerely,
>>>>
>>>> Jeff C
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
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>>>>
>>>
>>>
>>> --
>>> Jeff Duda, Research Scientist
>>> University of Colorado Boulder
>>> Cooperative Institute for Research in Environmental Sciences
>>> NOAA/OAR/ESRL/Global Systems Laboratory
>>> Boulder, CO
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>>>
>> _______________________________________________
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>
>
> --
> Jeff Duda, Research Scientist
> University of Colorado Boulder
> Cooperative Institute for Research in Environmental Sciences
> NOAA/OAR/ESRL/Global Systems Laboratory
> Boulder, CO
> _______________________________________________
> gradsusr mailing list
> gradsusr at gradsusr.org
> http://gradsusr.org/mailman/listinfo/gradsusr
>
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