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Re: 20000928: TMWK - How is it calculated?

Mike,

Assuming:

THTA = TMPK * ( 1000 / PRES ) ** RKAPPA

then, is is your definition of Wetbulb potential temperature:

THTW = TMWK * ( 1000 / PRES ) ** RKAPPA 

If thats the case, then you can either use the gfunc THTA(tmpc,pres) as
thta(sub(tmwk,273),pres)
or
mul(tmwk,exp(quo(1000,pres),KAPPA))

Steve Chiswell
Unidata User Support




On Thu, 28 Sep 2000, Michael Brennan wrote:

> Steve,
> 
> Thanks for the info.  Now that I have that solved, one more question. 
> Is there any easy way to calculate Wetbulb Potential Temperature in
> GEMPAK or GARP?
> 
> Thanks again,
> 
> Mike
> 
> Steve Chiswell wrote:
> > 
> > Michael,
> > 
> > The Garp macro, eg $GARPHOME/fdf/scalar/default/HAS/WetbulbT_C
> > is attributed to:
> > ! GARP Field Description File (FDF) Template
> > !
> > ! Field: Wetbulb Temperature in Celsius.
> > !
> > ! Ronald Humble <address@hidden>
> > <snip>
> > function = sub(sub(TMPK,(mul(sub(quo(MIXR,quo(RELH,100)),MIXR),2490))),273)
> > 
> > The GEMPAK function TMWK (Wet Bulb Temperature in Kelvin)
> > is defined in the GEMPAK User Guide, appendix B, and in the
> > source code $GEMPAKHOME/source/gemlib/pr/prtmwb.f
> > 
> > C* This function computes wet bulb temperature from the temperature,    *
> > C* mixing ratio, and pressure.  The result is obtained by solving       *
> > C* for the temperature at which saturation occurs when the latent       *
> > C* heat required to vaporize the water is provided by a cooling of      *
> > C* the air.  The equation representing the process is                   *
> > C*                                                                      *
> > C*      ( TMPK - TMWB ) * CP - ( Rsat (TMWB) - RMIX ) * LVAP = 0        *
> > C*                                                                      *
> > C* This implicit equation is solved by Newton's method, since the       *
> > C* saturation mixing ratio Rsat is a transcendental function of         *
> > C* TMWB.                                                                *
> > C*                                                                      *
> > C* The expressions for the heat of vaporization (LVAP) and saturation   *
> > C* vapor pressure are equations (2) and (10) from Bolton (MWR, 1980).   *
> > 
> > Steve Chiswell
> > Unidata User Support
> > 
> > On Thu, 28 Sep 2000, Michael Brennan wrote:
> > 
> > > What is the difference between the way GEMPAK calculates TMWK and the
> > > GARP macro for Wetbulb Temperature?
> > >
> > > Thanks,
> > >
> > > Mike
> > >
>