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19990715: PV cross sections

John,

What is your scale value shown in the cross section options 
(eg the more>> button)? Have you taken that into account
after factoring by -g? Can you provide the the modified equation
you are using?

Steve Chiswell
Unidata User Support

On Thu, 15 Jul 1999, John Knox wrote:

> Dear GEMPAK/GARP users,
> 
> A question regarding PV cross sections.  
> 
> We're studying a tropopause fold event in NCEP reanalysis 
> data and we're trying to recreate the classic cross-section 
> plot where PV values of > 2 PV units are tonguing down 
> into the lower troposphere.  
> 
> Problems: 
> 
> 1) The PV in the GARP cross-section menu was 
> lacking a -g factor and so gave wrong sign and magnitude.  
> Fixed.
> 
> 2) Once that's fixed, the PV values are still too *high* by 
> a factor of 2 or 3.  For example, lower-troposphere 
> PV values across the continent are never below 1 and 
> usually are = 2, 3, 4 PV units.  I don't think the 
> stratosphere came down to the surface across the whole U.S. 
> in this event!!
> 
> I suspect that the problem lies in the depth of the layer 
> across which the PV is calculated.  However, in the case of 
> the cross-section plotting it's not immediately clear 
> how to alter this depth [in the plan view, you just change 
> the pressure levels].
> 
> The bottom line is, I'm sure someone out there has made 
> beautiful PV cross sections with values that correspond 
> to what's in the published literature, and I'd like to know 
> how to do it too.
> 
> Thanks for your help!
> 
> John
> 
> ------------------------------------------------
> Dr. John A. Knox
> Assistant Professor of Geography and Meteorology
> Room 15, Mueller Hall
> Valparaiso University
> Valparaiso, IN 46383
> E-mail: address@hidden
> Office Phone: 219-464-6003
> 
>