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

[Dennis Shea]

Specifically, needed:

One file containing the surface pressure from:
ps_6hrs_ERAI_historical_an-sfc_"+years(i)+""+monthstr(k)+"*.nc

The file containing the hybrid coefficients.

ERA-Interim_coordvars.nc
======

                ; Hybrid coordinates - a and b coeffs: in ERAI p=a+b*Ps
but NCL assumes  p = a(k)*p0 + b(k)*ps   ---> A(ncl)=A(ERAI)/p0

                in =
addfile("/short/w48/gab563/work/ERA-Interim_coordvars.nc","r")
                hyai = in->a_interface                             ; [Pa]
model "interface" coefficients (*top to bottom*)
                hybi = in->b_interface
                hyai = hyai/p0
                hyam = in->a_model_ave                             ; [Pa]
model level coefficients obtained by averaging model interface coefficients
(*top to bottom*)
                hybm = in->b_model_ave
                hyam = hyam/p0



On Fri, Jun 23, 2017 at 5:15 PM, Dennis Shea <shea at ucar.edu> wrote:

> 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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>
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