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Compute quasi-geostrophic Eliassen-Palm fluxes at isobaric levels using NCEP Reanalysis.

Available in version 6.4.0 and later.


load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"  ; This library is automatically loaded
                                                             ; from NCL V6.2.0 onward.
                                                             ; No need for user to explicitly load.

	function epflux (
		u        : numeric,  ; float, double, integer only
		v        : numeric,  
		t        : numeric,  
		plvl [*] : numeric,  
		lat  [*] : numeric,  
		opt  [1] : logical   

	return_val  :  Array of type list containing four quantities. See below.



Arrays containing zonal (u) and meridional (v) wind components (m/s) and temperatures (K). The arrays must be three-dimensional (plvl,lat,lon) or, more commonly, four-dimensional (time,plvl,lat,lon). Note: The grid structure must be rectilinear.


Array containing isobaric pressure levels (Pa or hPa [mb]).


A one-dimensional array specifying the latitudes associated with the rectilinear grid.


If opt=False, default options will always be used. If opt=True, assorted options are activated.

  • opt@raw ; Default is opt@raw=False. If opt@raw=True, return variables prior to any scaling.
  • opt@scale_sqrt_p ; Default is opt@scale_sqrt_p=True. Scale by sqrt(P0/plvl).
  • opt@magf ; Magnify (scale) Fphi and Fp at plvl.le.100 hPa; Default is no magnification.
  • opt@print_var_info; Print each variables summary.

      Return value

      A variable of type list containing four (4) variables with shape (plvl,nlat). The type is the same as u. The variables are the meridional (Fphi) and vertical (Fp) components of the EP-flux; the EP flux divergence (EPdiv) and the acceleration (dudt) from EP flux divergence. NOTE: The variables within the list can be accessed directly via NCL's list syntax. However, many users find it clearer to explicitly extract the variables from the list and subsequently delete the returned list variable. See examples.


      NOTE: This function has been verified for NCEP Reanalysis data only. In principle, this should work for other suitable data sets.

      The Glossary of Meteorology defines the Eliassen-palm_flux as follows:

       A vector quantity with nonzero components in the latitude-height plane,
       the direction and magnitude of which determine the relative importance 
       of eddy heat flux and momentum flux.
       When the Eliassen-Palm flux (EP) vector points upward, the meridional heat flux 
       dominates; when the EPF vector points in the meridional direction, the meridional 
       flux of zonal momentum dominates. The divergence of the Eliassen-Palm flux is more 
       frequently used as a diagnostic tool, as it is proportional to the eddy potential 
       vorticity flux. In the special case when the EP divergence is zero, as for steady, 
       frictionless, linear waves, thermal wind balance is maintained in an idealized 
       zonally symmetric atmosphere through the induction of a mean meridional circulation, 
       even though the waves produce significant eddy heat and momentum fluxes.


       EP-Flux computations
        Edmon,H.J., B.J. Hoskins,and M.E. McIntyre,1980:
        Eliassen-Palm cross sections for the troposphere.
        J. Atmos. Sci., 37:2600-2616


      The NCL function was based upon code developed by J. Barsugli (NOAA/PSD) and
      adapted by C. Smith (NOAA/PSD).  The original NCL script is at:

      See Also

      static_stability, pot_temp


      Example 1: Click for an Example which uses NCEP Reanalysis data.