[gradsusr] Total Snow Accumulation with GFS .25 Degree

[Jeff Chabot]

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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