Potential vorticity asymmetries and tropical cyclone motion

A set of nine synoptic-flow cases, incorporating Omega dropwindsonde observ ations for six tropical storms and hurricanes, is used to deduce the three- dimensional distribution of potential vorticity (PV) that contributed to th e deep-layer mean (DLM) wind that steered the cyclones. A piecewise inversi on technique, the same as that previously applied by Shapiro to Hurricane G loria of 1985, is used to derive the DLM wind induced by pieces of anomalou s PV restricted to cylinders of different radii centered on each cyclone. T he cylinder of PV that induces a DLM wind that best matches the observed DL M wind near the center of each cyclone is evaluated. It is found that the results can be loosely placed into two categories desc ribing the spatial scale of the PV anomalies that influenced the cyclone's motion. Four of the cases, including Hurricane Gloria, had "local" control, with a good match (to within similar to 40%) between the observed DLM wind near the cyclone center and the DLM wind attributable to a cylinder of PV with a given radius less than or equal to 1500 km. Further decomposition of the PV anomaly into upper (400 mb and above) and lower levels (500 mb and below) indicates the dominance of upper-level features in steering two of t he cyclones (Hurricanes Gloria of 1985 and Andrew of 1992), while Hurricane Debby of 1982 was steered by more barotropic features. These results suppl ement those found in other studies. Five of the cases, by contrast, had "large-scale" control; with no cylinder of radius less than or equal to 2000 km having a good match between the in duced and observed DLM wind. Hurricanes Emily of 1987 and 1993 fell into th is category, as did Hurricane Josephine of 1984. Implications of the result s far guiding in situ wind measurements to improve hurricane track forecast s are discussed.