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

[Dennis Shea]

Hello,

The sample file you sent does not have the needed information. The
dpres_hybrid_ccm function requires the *hybrid coefficients*.

https://www.ncl.ucar.edu/Document/Functions/Built-in/dpres_hybrid_ccm.shtml

===
%> ncdump -h -v lev hus_6hrs_ERAI_historical_an-ml_20170301_20170302_00h.nc

dimensions:
        lon = 480 ;
        lat = 241 ;
        lev = 60 ;
        time = UNLIMITED ; // (1 currently)
variables:
 [snip]
        double lev(lev) ;
                lev:standard_name = "hybrid_sigma_pressure" ;
                lev:long_name = "hybrid level at layer midpoints" ;
                lev:units = "level" ;
                lev:positive = "down" ;
                lev:formula = "hyam hybm (mlev=hyam+hybm*aps)" ;
                lev:formula_terms = "ap: hyam b: hybm ps: aps" ;
[snip]
        float hus(time, lev, lat, lon) ;
[snip]
lev = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
20,
    21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38,
    39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56,
    57, 58, 59, 60
----------------------

Please resend a file with the needed information.

THX




On Thu, Jun 22, 2017 at 4:41 PM, Ghyslaine Boschat <
ghyslaine.boschat at unimelb.edu.au> wrote:

> 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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