Transforming eta to Z levels

Miscellaneous questions you have about anything related to graphically displaying WRF output

Transforming eta to Z levels

Postby Weather_Pirate » Thu Jul 01, 2010 6:27 pm

Hi there,

I'm working with some data from a WRF output in Matlab. And for visualisation/interpretation of the data, it would be useful to have my data in Z levels instead of the eta levels...
I found a way, after doing some maths to convert it, but I can't find any where the eta value of the level (between 1 and 0)... does any body know how I can get it or if there's another way of doing this.

thank you all
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Re: Transforming eta to Z levels

Postby dbh409 » Thu Jul 01, 2010 7:39 pm

I'm not sure with the second part of your post if you are having trouble finding the equation for eta, or the values themselves from the wrfout files. If it is the former, then here you are:
Image

Note that Ps is surface pressure, Pt is pressure at the top, and T is the column temperature. This is derived from the definition of eta and the hypsometric equation, in case you were curious.

If you are having problems finding eta values in the wrfout files, it isn't stored there. Look in your namelist.input file under the variable "eta_levels".
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Re: Transforming eta to Z levels

Postby Weather_Pirate » Sat Jul 03, 2010 6:05 pm

Yes, that's the way I was going.... but its problematic, because I need the mean temperature of the atmospheric column. I thought about the Geopotential z = [(PH+PHB)/g] + HGT... maybe it's easy this way
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Re: Transforming eta to Z levels

Postby dbh409 » Mon Jul 05, 2010 7:17 pm

The way around calculating a column temperature is simply to calculate delta z instead of z with the equation I provided. Then, Tbar can be calculated fairly easily by just taking the average of the two temperatures around the eta level of interest - the temperature at the eta level and the one before. So what you'll have is a vector of delta z, the first would be zero, then just sum up all the previous delta zs to get the total height for any given level.
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