[ncl-talk] Question regarding the hybrid base pressure in NCL functions

[Ghyslaine Boschat]

Dear NCL users,

I am using NCL version 6.4.0 to calculate vertically integrated meridional energy transports in the atmosphere using ERA-Interim. I am specifically using the NCL functions cz2ccm to calculate the geopotential height in hybrid coordinates and then dpres_hybrid_ccm to perform vertical integration of my data on model levels.

Both these NCL functions require as input the hybrid base pressure p0 and hybrid coefficients a and b. p0 is usually equal to 100000Pa but is not available for ERA-Interim and I am not sure what to use…

To my understanding, NCL assumes the coefficients are of the form:
                                     p = a(k)*p0 + b(k)*ps
whereas the equivalent ERA-Interim pressure equation is
                                     p = A(k) + B(k)*ps

Therefore, for ERA-Interim data do I need to just use p0=1 or else keep p0 = 100000 with the provided hybrid coefficients becoming hyam= hyam/p0 and hyai= hyai/p0 (i.e. A(k)=a(k)/p0)?

My results are slightly different depending on these 2 options.

Thank you for your help with this.

Regards,

Ghyslaine
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