Eliassen-Palm Cross Sections for the Troposphere

Abstract “Eliassen-Palm (EP) cross sections” are meridional cross sections showing the Eliassen-Palm flux F by arrows and its divergence by contours. For large-scale, quasi-geostrophic motion F is defined to have φ and ρ components r0 cosφ[−u′v′, fv′θ′/θp] where φ is latitude, ρ pressure, θ potential temperature, r0 the radius of the earth, bars and primes denote zonal means and deviations and (u,v) is horizontal velocity. The theoretical reasons for using EP cross sections diagnostically are reviewed. The divergence of F reflects the magnitude of transient and irreversible eddy processes at each height and latitude, and is proportional to the northward flux of quasi-geostrophic (not Ertel's) potential vorticity. It is a direct measure of the total forcing of the zonal-mean state by the eddies. The direction of F indicates the relative importance of the principal eddy fluxes of heat and momentum. If the eddy dynamics is Rossby wavelike, then F is also a measure of net wave propagation from one height and ...