15B.1 INTERNAL DYNAMIC CONTROL OF HURRICANE INTENSITY CHANGE: THE DUAL NATURE OF POTENTIAL VORTICITY MIXING James P. Kossin 1* , Wayne H. Schubert 2 , Christopher M. Rozoff 2 , and Pedro J. Mulero 1 1 University of Wisconsin-Madison, Madison, WI 2 Colorado State University, Fort Collins, CO 1. INTRODUCTION Hurricane intensity change is known to be gov- erned by a number of factors. The climatology of in- tensity change was most recently demonstrated by Emanuel (2000) and shows that an average storm intensifies at a rate of about 12 m s -1 day -1 for about 5 days, and then begins to weaken at a slower rate of about 8 m s -1 day -1 . We also know that environmental conditions play a key role — for ex- ample, if a storm moves over colder water or land, or if the ambient environmental vertical wind shear in- creases, weakening typically follows. Alternatively, an environment that is not conducive for intensifi- cation can become more favorable over time, and strengthening would typically occur. Ideally then, we would be able to explain the variance of hur- ricane intensity change in terms of the variance of the synoptic-scale storm environment. This is not the case however, and it is fairly typical for storms to strengthen or weaken, sometimes rapidly, without any commensurate changes in the external storm en- vironment. Although the specific processes involved remain an open question, this behavior is widely be- lieved to result from internal vortex-scale processes that can have a profound effect on how storm inten- sity evolves, and this means that our ability to model and ultimately predict hurricane intensity change is dependent on our ability to contemporaneously model a very broad range of spatial scales. From a more pragmatic viewpoint, it’s reveal- ing to note that our operational ability to accurately forecast hurricane motion (track) has improved dra- matically in the past 20 years, and that the reason for this lies in our improving ability to capture evolv- ing synoptic-scale fields with our present numerical guidance. With the exception of occasional small- amplitude trochoidal oscillations, which are on the order of tens of km and are caused by transient po- tential vorticity asymmetries near the storm center, the track is controlled almost entirely by the envi- ronmental steering flow that the storm vortex is em- * Corresponding author address: James P. Kossin, UW-Madison, Madison, WI 53706; kossin@ssec.wisc.edu bedded in. Contrary to track forecasting, our ability to forecast hurricane intensity change has shown al- most no progress in the past 20 years and again it is widely believed that this is due to our present inability to model small-scale internal processes in hurricanes. Here we seek to uncover and elucidate these small-scale processes in the hurricane-inner core. It is presently unknown what the exact inter- nal processes are, but it is believed that the re- arrangement of the inner-core potential vorticity (PV) structure plays an important role. To gain further understanding of these internal governing mechanisms, we are considering the role of two- dimensional barotropic processes in the hurricane inner-core when non-conservative forcing (diabatic heating and friction) is applied. 2. RESULTS Using a 2D barotropic numerical model with spatially and temporally varying PV forcing, we’ve uncovered an internal mechanism that interrupts the intensification process resulting from diabatic forc- ing in the hurricane eyewall. This internal control mechanism is due to PV mixing in the region of the eye and eyewall, and can manifest itself in two antithetical forms — 1) as a transient “intensifica- tion brake” during symmetric diabatically-forced in- tensification, or 2) as an enhancer of intensification through efficient transport of PV from the eyewall, where it is generated, to the eye. We find that when non-conservative forcing is included in the physics, PV mixing occurs episod- ically in the form of well-defined events and these events act as a suppressant to intensification until a critical point in the lifetime of the vortex. After this time, the PV mixing events allow the vortex to intensify well beyond the potential intensity that could be realized through diabatic heating in the eyewall in the absence of PV mixing. The ampli- tude of the disruption of intensification is found to depend on a number of factors, such as eyewall size and diabatic heating rates, and frictional dissipation