[gradsusr] Total Snow Accumulation with GFS .25 Degree

[Jeff Duda]

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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