WRF-ARW potential vorticity (PV) on the 322-K isentropic surface. Key features include: Encirclement of Sandy inner-core by 322-K polar stratospheric PV. Radial contraction of low-level PV ring concurrent with downward-penetration of 322-K PV prio to landfall. Colored according to potential vorticity units (PVU). Discussed in Mel Shapiro's presentation at the AMS 2013 Townhall "A research-community perspective of the life cycle of Hurricane Sandy".
