epflux

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

Available in version 6.4.0 and later.

Prototype

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.

Arguments

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.

  Description

  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.
  

  References:

   EP-Flux computations
    Guide: http://www.met.reading.ac.uk/~pn904784/snap/ep_flux_calculations.html
    Test:  http://www.met.reading.ac.uk/~pn904784/snap/ssw20122013.html
  
    Edmon,H.J., B.J. Hoskins,and M.E. McIntyre,1980:
    Eliassen-Palm cross sections for the troposphere.
    J. Atmos. Sci., 37:2600-2616
    doi: http://dx.doi.org/10.1175/1520-0469(1980)037<2600:EPCSFT>2.0.CO;2
  

  Acknowledgement:

  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:
     http://www.esrl.noaa.gov/psd/data/epflux/epflux.2010.ncl
  
  
  
  See Also
  
  static_stability, pot_temp
  
  Examples
  
  Example 1: Click for an Example 
  which uses NCEP Reanalysis data.