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Re: KI calculation with high elevations
Posted By: Bill Moninger In Response To: KI calculation with high elevations (Scott Dennstaedt)
Date: 23-October-08 1836Z
In Response To: KI calculation with high elevations (Scott Dennstaedt)
you asked why the K-index is not undefined on the GSD soundings pages when the surface has a pressure lower than 850 mb.
The code that calculates stability indices for us does the following when there is no 850 temperature/dewpoint in a sounding: it uses the temperature/dewpoint at the first level above 75 mb above the lowest point on the sounding (the surface for model soundings).
This effects the Showalter, Sweat, K-index, and any other thermodynamic indices that depend on the 850 mb temperature/dewpoint.
I can't see any justification for this in the AMS glossary, however, different groups apparently take a variety of approaches to calculating stability indices when the surface pressure is lower than 850 mb. A recent check of various upper air sites by us revealed the following:
Greg Thompson's (NCAR/UCAR) site ( http://www.rap.ucar.edu/weather/upper/ ) appears to calculate the K-Index in the manner that we do.
The U of Wyoming site ( http://weather.uwyo.edu/upperair/sounding.html ) does NOT calculate the K-Index (or any of the others that use 850 mb) if the elevation is too high (surface pressure below 850 mb).
The official NWS product that you sent ( http://weather.noaa.gov/pub/fax/QXUA00.TIF ) does NOT plot the K-Index if the sounding surface pressure is below 850 mb, as you pointed out.
But the weather workstation uses by NWS forecasters, AWIPS, does calculate a K-index for soundings from some sites for which the surface pressure is below 850 mb. We could not determine exactly what substitute for 850 mb is being used; it is not the same thing that we are doing.
So... I guess you can take your choice. If folks want to chime in about what they think ought to be done for our RUC and related soundings at high elevations, feel free.
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