# Mapping vertical levels to physical altitude

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#### mfryba

##### New member
Hello, I'm not a weather person per se (physicist) and am trying to do some basic WRF runs to generate "realistic" radar returns for a separate simulation. I did find the vapor_wrf Python code to compute dBZ. I ran the basic em_quarter_ss case to get something to start with. I understand the X and Y grids are Cartesian and DX and DY give the spacing. The Z coordinate is a bit tougher since I'm not well versed in pressure level jargon. To generate simulated 3-D radar returns I need to map those Z coordinates to real altitude. It appears some algorithm using DZBOT, DZSTRETCH_? and perhaps some other variables is required. Any tips? Thanks!

Hi,
DZBOT, DZSTRETCH etc. are designed for setting vertical eta levels. For ideal cases, vertical levels are calculate based on ZTOP, which corresponds to real attitude.
In wrfout, you can find variables PH and PHB, (PH + PHB)/9.8 will give you the real altitude of each model level. You can do vertical interpolation to put the variables in any levels you want for dBZ calculation.

Ming Chen said:
Hi,
DZBOT, DZSTRETCH etc. are designed for setting vertical eta levels. For ideal cases, vertical levels are calculate based on ZTOP, which corresponds to real attitude.
In wrfout, you can find variables PH and PHB, (PH + PHB)/9.8 will give you the real altitude of each model level. You can do vertical interpolation to put the variables in any levels you want for dBZ calculation.

Thanks! I started reviewing the Technical Note that has some details; appreciate the quick summary because it is clear that once I potentially get away from an ideal case the mapping of eta levels to altitude is non-trivial, and even the specification of eta levels and how that maps to nominal dry pressure is a bit hairy. Curious as to units; I see PH and PHB have units m2 s-2; what is the units of that 9.8 factor and how does that relate to physical constants? Oh, I just found on page 5.142 of the Users Guide that same equation, and the 9.81 is clearly just acceleration due to gravity g, which has units of m s-2, so that makes sense that the result is in meters.

PH and PHB are geopotential, which is a physical variable defined in geoscience. You can easily find its meaning in the website.