ON THE OCCURRENCE OF DYNAMIC INSTABILITY IN INCIPIENT AND DEVELOPING HURRICANES

Abstract

Observations made by the specially instrumented aircraft, operated by the National Hurricane Research Project, establish the occurrence of dynamic instability, notably in the form of anomalous winds in the upper troposphere above hurricane Daisy, shortly after its inception. It is inferred that the dynamic instability released by these anomalous winds, which represent anticyclonic rotation in space, triggered Daisy's development. Since the observed dynamic instability occurred on a mesoscale, the above inference is not amenable to direct verification from synoptic maps. However, synoptic conditions favorable for the development of anomalous winds are discussed and it is found that these conditions prevailed in the upper troposphere directly above Daisy and Gracie a short time before they reached hurricane intensity. The role of negative absolute vorticity is shown to vary. In the presence of anomalous winds it is a stabilizing factor which is nonetheless important in channeling outflow at the top of the hurricane. With normal winds, negative absolute vorticity is a destabilizing agent and some evidence is presented that it may also be responsible for initiating the development of certain hurricanes.