Compute quasi-geostrophic Eliassen-Palm fluxes at isobaric levels.
Available in version 6.4.0 and later.
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl" function epflux ( u : numeric, ; float, double, integer only v : numeric, t : numeric, plvl [*] : numeric, lat [*] : numeric, opt  : 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.plvl
Array containing isobaric pressure levels (Pa or hPa [mb]).lat
A one-dimensional array specifying the latitudes associated with the rectilinear grid.opt
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.
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 shoud work for any 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 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
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:
Example 1: Click for an Example which uses NCEP Reanalysis data.