# [McIDAS #QSH-852428]: AMRC MD files and Altimeter setting

• Subject: [McIDAS #QSH-852428]: AMRC MD files and Altimeter setting
• Date: Mon, 03 Nov 2008 15:25:09 -0700

```Hi Robert,

I apologize for letting this one slip through the cracks!

re:
> I have a non-political, non OS question finally for you.

:-)

> I assume you are at least familiar with the SSEC AWS sites in Antarctica
> and the MD files that are produced by Matt?

Familiar yes, but I am no expert.

> My question is can McIDAS use the station elevation that is given in the
> MD files and calculate altimeter setting from the reported station pressures
> and then plot/contour it?

Since PTDISP and PTCON both have the ability to create new parameters as a
function
of existing ones, I would say yes.  The McIDAS-X Users Guide section on PTDISP
has
an example of how to calculate thickness between two different levels:

PTDISP RTPTSRC/UPPERMAND SEL='DAY #Y;TIME 12' MAP=USA
PARAM='THIK[M]=(287/9.8)*((P1+P2)/2)*LOG(1000/500)' P1=T[K] 'P 1000' P2=T[K] 'P
500'

This entry plots 1000-500 mb thickness in meters at 12 UTC today on a map of
the United States. The
thickness values are calculated using the equation in the PARAM keyword,
known as the Hypsometric Equation.

> If so could you help me out a bit on the PTDISP required to do it?  I was
> always a bit fuzzy with
> PTDISP and am even fuzzier on it these days.

First, you would need to come up with the formula needed to calculate altimeter
setting using
the elevation and station pressure.  Then you need to instantiate the formula
in PTDISP/PTCON
using the PARAM= keyword.

It seems like you would need to use a "standard" atmosphere that is valid for
the region(s) in
Antarctica where the AWS stations are located.  Perhaps Dr. Matthew Lazzara can
help there :-)
It could be that AMRC has some "rules of thumb" relationships between pressure
and altimeter
setting that could be easily incorporated into PTDISP/PTCON invocations.

After some digging around, I was reminded that McIDAS has a function called
STNPREA that returns
station pressure given surface pressure and station elevation:

McScienceUtils.c::

/*
*\$ Name:
*\$      STNPREA - Station Pressure interface for Equation Anal.
*\$
*\$ Interface:
*\$
*\$      double function
*\$      STNPREA(double pressure, double elevation)
*\$
*\$ Input:
*\$      pressure   - Sea Level Pressure in units of HPA
*\$      elevation  - Station Elevation in units of M
*\$
*\$ Input and Output:
*\$      none
*\$
*\$ Output:
*\$      none
*\$
*\$ Return values:
*\$      Computed station pressure or Missing data value
*\$
*\$ Remarks:
*\$      Note that input values must be in the correct units.
*\$
*\$ Categories:
*\$      graphic
*/

STNPREA is used in the definition of PSTN.ALTIM in SFCPLOT.CORE (~mcidas/data):

PSTN.ALTIM   UNIT=HPA \
FORMAT=F6.1 \
MATH='STNPREA(P1,P2)'\
ISFC='P1=PSL[HPA]' 'P2=ZS[M]' \
SVCA=na \
SYN= na \
ISHP=na

I would start with this to see if it gives you the values you are looking for:

SFCPLOT PSTN.ALTIM ANTARCTICA ...

> Thanks,

No worries.

Again, I apologize for the incredibly slow response.  This one just got
overlooked!

Cheers,

Tom
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