ZERO POTENTIAL VORTICITY ENVELOPES FOR THE ZONAL-MEAN VELOCITY OF THEVENUS TITAN ATMOSPHERES

The diagnostic analysis of numerical simulations of the Venus/Titan wi nd regime reveals an overlooked constraint upon the latitudinal struct ure of their zonal-mean angular momentum. The numerical experiments, a s well as the limited planetary observations, are approximately consis tent with the hypothesis that within the latitudes bounded by the wind maxima the total Ertel potential vorticity associated with the zonal- mean motion is approximately well mixed with respect to the neutral eq uatorial value for a stable circulation. The implied latitudinal profi le of angular momentum is of the form M less-than-or-equal-to M(e)(cos lambda)2/Ri, where lambda is the latitude and Ri the local Richardson number, generally intermediate between the two extremes of uniform an gular momentum (Ri --> infinity) and uniform angular velocity (Ri = 1) . The full range of angular momentum profile variation appears to be r ealized within the observed meridional-vertical structure of the Venus atmosphere, at least crudely approaching the implied relationship bet ween stratification and zonal velocity there. While not itself indicat ive of a particular eddy mechanism or specific to atmospheric superrot ation, the zero potential vorticity (ZPV) constraint represents a limi ting bound for the eddy-mean flow adjustment of a neutrally stable bar oclinic circulation and may be usefully applied to the diagnostic anal ysis of future remote sounding and in situ measurements from planetary spacecraft.