Abstract
The interaction between a hurricane and its environment is studied by analysing the generation and influence of potential vorticity (PV) from the Geophysical Fluid Dynamics Laboratory hurricane model analysis system. Two sets of numerical experiments are performed: one with and the other without a bogused hurricane vortex in the initial time, for cases of Hurricanes Bob (1991), Gilbert (1988), and Andrew (1992).
The PV budget analysis of Bob shows that the condensational heating within the vortex redistributes the PV, causing a PV sink in the upper part of the vortex and a PV source in the lower part. This tendency is compensated for largely, but not entirely, by the upward transport of high-PV air from the lower levels to the upper levels. The net effect contributes to the increase of the negative upper-level PV anomaly during the vortex intensification period. This result indicates that the diabatic heating effect plays a crucial role in the evolution of the PV field in hurricanes. It also suggests the importance of accurate representation of the heating profile in hurricane models.
It is shown that the negative upper-level PV anomaly is spread out by the upper-level outflow and the large-scale background flow. The impact of the spread of the negative upper PV anomaly to the storm is quantitatively evaluated by computing the nonlinear balanced flow associated with the PV perturbation. Notable contribution to the steering of the storm from the upper-level PV anomaly is found. The result supports the theory advanced by Wu and Emanuel concerning the effect of the upper negative PV anomaly on hurricane motion. This study also indicates the need of enhanced observation and accurate analysis and prediction in the upper troposphere in order to improve hurricane track forecasting.
