[gradsusr] Total Snow Accumulation with GFS .25 Degree

[Jeff Duda]

Jeff,
'gxout fwrite' writes to a standard binary file, not a GRIB file. You will
not be able to use the wgrib products on such a file. Is there a reason you
need to write the snow data to a separate file? You can perform math on
arrays from a GRIB2 file. See this helpful write-up on wgrib2 tricks for
inspiration: https://ftp.cpc.ncep.noaa.gov/wd51we/wgrib2/tricks.wgrib2

Jeff Duda

On Thu, Dec 30, 2021 at 2:48 PM Jeff Chabot <jsc219 at gmail.com> wrote:

> 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
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> gradsusr mailing list
>>>>> gradsusr at gradsusr.org
>>>>> http://gradsusr.org/mailman/listinfo/gradsusr
>>>>>
>>>>
>>>>
>>>> --
>>>> 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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>>
>>
>> --
>> 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
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