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Vector Subtraction

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Greg Fishel
Posts: 13
Joined: Mon Jul 05, 2004 5:02 pm

Vector Subtraction

Post by Greg Fishel »

I was wondering if there is a way to display a storm relative isentropic analysis. Many papers have indicated that simply doing pressure advection on an isentropic surface is not terribly helpful, especially if the system has a significant translational speed, which most winter systems do. I would like to be able to display pressure advection by the storm relative wind as opposed to the true wind. The help guide shows a command VADC, where you enter a vector and a constant. I have tried command strings, such as WIND 2320, but to no avail. If anyone has any ideas, please let me know. Thanks!
Robert Dewey
Junior Member
Posts: 2
Joined: Fri Jan 07, 2005 3:36 am
Location: Detroit, MI

Post by Robert Dewey »

This is a very good question that I would like to know as well.

My main weather analysis tool is NAWIPS/GEMPAK, but I am also a hard core PCGRIDDS fan.

Allowing for a storm relative analysis of isentropic data/theta surfaces, wind speeds, frontogenesis, and vertical motion helps greatly when trying to determine the area of heavy banded snows. The difference between ground relative and storm relative can be greater than 100 miles in some cases, which can obviously have an effect on a forecast.
Senior Member
Posts: 300
Joined: Fri Jun 25, 2004 11:42 pm

Post by jkrob »


This is something I have been researching for the past few months. I have read Dr. James Moore's isentropic related papers and references he makes to incorporating system motion into omega calculations. The more I think about the concept and incorporating a diagnostic command into PCGRIDDS32, the more involved and complex the problem becomes. In short, (from my perspective) you have to compare the motion of individual contours (pressure surfaces?) between two forecast hours...which is easier said than done.

If anyone is willing to contribute an algorithm, I'd be willing to use it ;-) Does AWIPS, NAWIPS, GEMPAK, etc. have a macro or script to calculate this? It can be done (it's only software)...the question is, how?

Robert Dewey
Junior Member
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Joined: Fri Jan 07, 2005 3:36 am
Location: Detroit, MI

Post by Robert Dewey »


If anyone is willing to contribute an algorithm, I'd be willing to use it ]

I have been trying to figure out a way in GEMPAK as well, and have yet to come across any scripts. I am with you in thinking that developing a script or macro would probably be pretty hard for PCGRIDDS, and maybe a bit easier in GEMPAK, since it handles multiple data times better. I would like to do case studies with regards to mean synoptic motion and it's affects on meteorological parameters, particularly on the data I saved from the big Ohio snowstorm (12/23/04), where a couple places got nearly 3 feet of snow from a nice banded event...
Posts: 46
Joined: Thu Jul 15, 2004 3:42 pm
Location: Kansas

Post by geophi »

I have a program I developed many years ago in QuickBASIC, now ported to Visual Basic that you can run outside of PCGRIDDS32 and it will write a command file. It is at:

Put it in the MACROS directory and run it, change the dir and speed for the system of interest, and a SYST.CMD file will be produced that will display the system relative wind vectors, pressure on that isentropic surface, and resulting vertical motion on that isentropic surface.

Depending on your OS and what you have installed on it, you might need the MS VB6 runtime files available here

At the time, I was supplied a macro from another person, that purports to do the same within PCGRIDDS only for a layer (i.e. you need to SLYR i295 i300, or whatever), but I could never get it to work. That macro is below if anyone wants to see if they can get it to work.
TXT2 Enter STORM DIRECTION (degrees) at the prompt for a constant to add.
TXT3 Enter STORM SPEED at the prompt for constant to multiply by.
smlc 1+03 ((advt (pres lave,last)) grtn 0. clr3)/
smlc 1+03 ((advt (pres lave,last)) lstn 0. clr4 dash)/
pres ci50 clr5 lave/
TXTP --- Adiabatic Vertical Motion (microbars/sec) ---
TXTQ --- Brown is Pressure (mb) ---
TXTR --- Green indicates UPWARD ---
TXTS --- Yellow indicates DOWNWARD ---
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