RDF Potential Vorticity

Ertel's Potential Voticity (EPV) is plotted in the ATTREX region
at 43 kft, 48 kft, 53 kft, and 58 kft.
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 10 days.
This allows the wind fields to continually advect and sharpen the field.
Both analysis and RDF EPV and latitude of origin are shown. Winds and 
EPV are based on 6-hourly analyses and forecasts from the NCEP global 
Forecast System.