RDF Potential Vorticity

Ertel's Potential Voticity (EPV) is plotted in the ATTREX region
at 47 kft, 53 kft and 58 kft (approximately 150, 100, and 70 mb).
EPV is roughly conserved following the motion. Since its mean structure
is a positive gradient from the south to the north at any given theta
surface (or altitude), a map of EPV is an indicator of the origin of
the air mass.

Since EPV is a second derivative quantity of traditional observables
(such as wind), narrow intrusions of midlatitude air into the tropics 
generated by nonlinear and transient wind fields will be smoothed out
by direct calculation of EPV from analysis data. RDF (Reverse Domain
Fill) EPV means that we display EPV at a given point that is
advected by isentropic back trajectories over a period of 14 days.
This allows the wind fields to continually advect and sharpen the field.
Both analysis and RDF EPV are shown. Winds and EPV are based on 6-hourly
analyses and forecasts from the NCEP global Forecast System.