Re: Center fix mapping of the "fix" folder in the ATCF system
Posted by
Chris in Tampa on 10/21/2010, 7:13 pm
I took a look at theta-e calculations. I was able to come up with a few equations but I think I'll hold off on adding anything. I was looking at a whole lot of different sources, with a focus on these:
http://journals.ametsoc.org/doi/abs/10.1175/1520-0493(1980)108%3C1046:TCOEPT%3E2.0.CO%3B2
http://amsglossary.allenpress.com/glossary/search?id=equivalent-potential-temperatur1
http://hurri.kean.edu/~yoh/calculations/diagnostic/diagnostic.html
http://www.srh.noaa.gov/epz/?n=wxcalc
http://www.ofcm.gov/fmh3/pdf/12-app-d.pdf
There are a few problems I see.
One, there are a lot of different equations. (from the first link especially, but the paper does seem well cited since) Some have larger errors than others. I'm just not sure which is best. I really couldn't find any great official reference to use. I could for the mixing ratio and vapor pressure, but throughout there are issues. Specific heat capacity is variable and I didn't get into the equation to that because it seemed even more complex. I saw some different values used, 1005.6 and around 1004. I settled on 1004.64 because the last link references R_d / c_pd being equal to exactly 2 / 7. I solved for 1004.64 and then searched for it online and did find some official use of that number.
I used the second and third links a lot. I never could get the second link equation to work. Or maybe I did. It was several Joules off from the third links equation and calculator though, which was way too large of an error.
I would like to see this articles which has some updated equations but it was published too recently to access:
http://journals.ametsoc.org/doi/abs/10.1175/2009MWR2774.1
Some constants I was using.
gas constant for dry air
R_d=287.04
gas constant for water vapor
R_v=461.5
heat capacity at constant pressure of dry air
c_pd=1004.64
latent heat of vaporization
L_v=2501000
Two, there are some instances it would seem where the equations are not valid, like at really cold temperatures. While I could not display it at certain temps, I'm not sure how good the equations are to that point. I see some various temperatures not to use certain equations at. I would not want to pick a temperature not to display it at arbitrarily. It also seems like there are some assumptions made and that theta-e is not always useful in every circumstance. I don't know enough about it though to show or not show it on that basis.
Because of the issues involved, I would have only displayed theta-e on request, which would mean it would not appear on my live decoder and would only on the manual one, which is rarely used. So for now, I'll hold off on it.
I might revisit again in the future because it along with CAPE do seem really important, but since the only people looking at these kinds of values would be people who know it really well, I would need to make sure it was absolutely as correct as possible. But I don't think I would ever have it by default in my live system. It would add a lot of data to the dropsonde tables and since a lot of people would not need to see that, I would not want to have it always displayed. There are a lot of others things I could calculate from looking at all the stuff I looked at and specifically using just theta-e would stress more important over any other things I could calculate and while it may be true, I'm not specifically sure. |
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