Date: Wed, 19 Aug 1998 16:03:23 -0500 (CDT) From: Scott Overpeck To: schultz@nssl.noaa.gov Subject: Gempak... MIME-Version: 1.0 Dr. Schultz, After finding the functions in the Gempak5.2 handbook, I modified the equation for Pot. Vort. so that one can define the wind (geo, ageo, etc.), and theta. The equation is ugly and certainly not the recommended way to go. I also checked around here for new versions of Gempak, and sure enough no one mentioned that we have 5.4 version. So I compared the output from each version and found that the equation that I created for 5.2 comes close to 5.4, but does not get the magnitudes exactly right. This is what I used, and you are welcome to test it and modify it. gfunc=mul(-9.80616;quo(mul(avor(vlav(wnd));ldf(thts));mul(100;ldf(pres)))) scale=6 cint=.1/-4/0 few things to note: the avor uses the vector layer average of the wind or geostophic wind; ldf(thts) can be changed for thte; the factor of 100 accounts for conversion of mb to Pa. I also found that version 5.2 PVOR calculation does NOT include gravity. I used this with GDCROSS, and will see how it does with GDPLOT. In the long run, if you have Gempak5.4, use it. Thanks again for your help and time. ******************************************************************************* Date: Wed, 19 Aug 1998 16:34:22 -0500 (CDT) From: Scott Overpeck To: David Schultz Subject: Re: Gempak... MIME-Version: 1.0 Dr. Schultz, I don't have a problem you referencing the e-mail on the web. I did use the equation in version 5.4 and got the same thing as for version 5.2. There is some subtle difference in the way 5.4 does pvor(thes,geo) and the equation I have. It would be nice to get the equation that I've come up with so far closer to the results obtained with using pvor(thes,geo). Anyway, that is what I have found, and like I said, people can test it and modify things as they wish. People need to know that the equation is not perfect. One other note is that version 5.2 uses thts for sat. thetaE whereas 5.4 uses thes. That's all that I have found for now. Thanks again. ******************************************************************************* Date: Wed, 19 Aug 1998 17:29:18 -0500 (CDT) From: Scott Overpeck To: schultz@nssl.noaa.gov Subject: Final update... MIME-Version: 1.0 Dr. Schultz, I found that when gempak5.4 calculates pvor in pres coordinates it applies a correction. In the case of pres coordinates, the vorticity of the layer averaged wind is corrected by the addition of : (k cross partial(wind vect) w/res to sat. theta-e) dot grad sat. theta-E Since the equation I used does not account for this, the values will be off a bit in magnitude, and to some degree in space. I don't know how to make gempak do the above equation, and so the users need to use gempak5.4 to get correct calculations, or understand that 5.2 will allow them to analyze for dry SI only. The other option would be to use theta coordinates in which gempak will not make the above correction. That is the disclaimer so far with the equation I gave you. +-------------------------------+ |Scott Overpeck | |Graduate Research Assistant | |Department of Meteorology | |Texas A&M University | |E-mail: scott-o@tamu.edu | |Office: O&M 1113 409-845-1679 | +-------------------------------+