Snow in GraDS

[James Kinter]

Dear Patrick - WEASD in GFS represents water equivalent snow depth
accumulated to date.  The change in accumulated snow between two
times is snowfall minus snowmelt, not simply snowfall. If you
consider only positive values of the 6-hourly difference of WEASD,
you will get a pretty good approximation of snowfall since it is
unlikely (but not impossible) to have snowmelt occur in the same 6-hr
period as snowfall. For example:
sum(maskout(weasd-weasd(t-1),weasd-weasd(t-1)),t=2,t=13)
should give you a good approximation of the total snowfall in the
first 72 hours a forecast, whereas,
weasd(t=13)-weasd(t=1)
will not, because it may include periods of both snowfall and snowmelt.

The WEASD unit is the mass of all the snow accumulated in a square
meter, which is the same mass if all the snow were melted to liquid
(water equivalent). If WEASD = 1, that is a 1 kg/m^2 mass of water
equivalent, which represents a depth of 1 mm (liquid). That is 1 cm
of snow only if you assume a 10:1 ratio of snow volume to liquid
water volume. This ratio varies from 4:1 to 100:1 in nature, but 10:1
works well on average. Hope that helps. -- Jim



On 27Jan2007, at 3:28 PM, Patrick Reuter wrote:

> Hi Folks using Grads !
>
> Can someone tell me how to calculate the 6h accumulated snowfall
> frm the
> GFS grib entry ? I saw the WEASD entry in kg/m*m, but I dont know
> how to
> interpret this data. It seems to be some sort of accumulation, but
> from
> when ? To get the snowfall for one 6h interval, do I have to take the
> difference between two entries ?
>
> Example : GFS Data calculated at 00Z the 20070127. I want the snowfall
> from 6h to 12h. Do I have to take the difference between the two
> values
> (WEASD+12 - WEASD+6) ?
>
> Hope this is clear ... Any help is greatly appreciated!!
>
> BTW : 1 kg/m*m is about 1cm snow ?
>
>   Patrick
>

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