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19990211: surface saturated mixing ratio calc

>From: Matt Fearon <address@hidden>
>Organization: Desert Research Institute
>Keywords: 199911020017.RAA21427

>I am trying to calculate a surface saturated mixing ratio
>for the entire grid, perhaps at hagl@2meters would be
>sufficient. Is there a way to perform or approximate
>this calculation using gddiag ? I noticed (smxs)
>in the gempak moisture parameters. However, I am
>not sure how or if this would work.
>
>Any advice would be greatly appreciated,
>Thank you.
>
>-Matt Fearon
>address@hidden
>
>

Matt, 

for saturated mixing ratio, you need the grids TMPC and PRES to
calculate MIXS. The variable SMXS is likewise calculated from TMPC and PALT-
which you would probably have if you were gridding surface data, but if you have
model data, then MIXS will be easier to come by.

If you have both TMPC and PRES on a grid level, then MIXS is one of the
parameters that can be computed automatically. Otherwise, you use the MIXR() 
function
with TMPC and PRES for saturation mixing ratio (DWPC and PRES for mixing ratio).
However, for many model grids (such as the eta grid 211 on IDD) as you mention 
above, 
you may have TMPC@2%HAGL and PRES@0%NONE. To calculate MIXS this way in gddiag, 
gdcntr, etc. 
you use the inline modifiers like:

GLEVEL = 0
GVCORD = NONE
GFUNC = MIXR(TMPC@2%HAGL,PRES)

Without modifiers, the PRES variable inherits the GLEVEL=0 and GVCORD=NONE.
With the inline modifiers, TMPC will use GLEVEL=2 and GVCORD=HAGL.

Using the 2 meter temperature should be a good measure for surface conditions.
You could also use TMPC@30:0%PAGL for example to estimate the lowest 30 
millibars
(for a lowest boundary layer) as well, depending on your needs.

Steve Chiswell