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20040518: IDV equiv. pot. temp.

>From: "Fingerhut, William A" <address@hidden>
>Organization: Lyndon State College
>Keywords: 200405181456.i4IEuttK019619 IDV theta-e mixing ratio

Hi Bill-

re: Equivalent Potential Temperature (theta-e) calculation
>Could it involve units ?  (;-)

Geez, I should hire you and save myself some time. ;-)  I just
figured this out, but finding that the units were wrong was
quite involved and not obvious.

here are the gory details:

We calculate theta-e from temperature, pressure and mixing
ratio.  The latter is derived from temperature and pressure
and relative humidity.

When we were calculating mixing ratio (w) from saturation
mixing ratio (ws) and relative humidity (f) we were doing:

w = ws * f/100

(rearranging equation 6-13 of Byers).

What we should have been doing is:

w = ws*f/100%

(left off the units of the 100), so in effect we had
w in units of g/kg * %.  So, the mixing ratio was off
by a factor of 100.

I spent most of my time looking at the formula that the
IDV uses for calculating theta-e instead of looking at the
units.

As a side effect, if you plot mixing ratio as contours, the
values will be correct now also.

Thanks for finding this bug.  The fix will be in the next
release.

Don Murray
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