Dr. Gabriel Williams
Office: JC Long 223
Weather Analysis Links
- Research Interests-I have research interests specifically within the field of tropical cyclone dynamics and in geophysical fluid dynamics more generally. The problems that I'm working on currently are summarized below.
1. Structure and Evolution of Hurricane Boundary Layer
The boundary layer of a mature hurricane has been long recognized as an important feature of the storm. In particular, it controls the radial distribution of heat, moisture, vertical motion, and absolute angular momentum that ascends into the eyewall clouds. In addition, turbulent processes within the boundary layer transfer momentum to the ocean, generating damaging storm surge and waves, and also transfer energy from the oceanic reservoir to the TC heat engine, generating and maintaining the storm. My research focuses on the dynamics of the structural features within hurricane boundary layer (such as low-level jets, horizontal convective rolls, and shocks) and examines how these structural features lead to intensity and structural changes for mature hurricanes. In my research, emphasis is given to the dynamics of the boundary layer associated with the tropical cyclone inner core and tropical cyclone outer rainband. My research also examines how environmental forcing affects the evolution of the boundary layer.
2. The Dynamics of Secondary Eyewall Formation and Eyewall Replacement Cycles
Secondary eyewall formation (SEF) is widely recognized as an important research problem in the dynamics of mature tropical cyclones, but as of yet there is not a consensus on the phenomenon's fundamental physics. The conceptual and empirical linkage between secondary eyewalls to hurricane intensity change and storm growth has fostered renewed interest in SEF, with substantial efforts currently underway in developing statistical forecasting tools for the operational community. However, as of today, such forecasting instruments tend to rely more on empirical relations than in the understanding of the physical processes that lead to SEF. Only recently has a clear dynamical link been made between the overarching mechanisms of tropical cyclone intensification and the physics of SEF. This dynamical link is associated with the interaction of the tropical cyclone boundary and the free atmosphere. My research focuses on the dynamics and the thermodynamics of secondary eyewall formaion. In particular, my research examines how thermodynamic processes within the boundary layer can initiate SEF and how the thermal structure of the tropical cyclone boundary layer evolves during SEF and eyewall replacement cycles.
Figure 3: An animation of the Morphed Integrated Microwave Imagery at CIMSS (MIMIC) product revealed that Hurricane Danielle (which had intensified into a Category 4 storm) was undergoing an Eyewall Replacement Cycle (ERC) during the 27 August - 28 August 2010 period. See the CIMSS Satellite Blog for more details.
3. Dynamical Instabilities in Geophysical Vortices
Unsteady, asymmetric processes near and within the cores of geophysical vortices is a topic of increasing meteorological and geophysical interest. The growth of disturbances associated with barotropic and baroclinic instability of the symmetric hurricane vortex has been argued as a cause of rapid structural variability in a mature tropical cyclone and for phenomena such as polygonal eyewalls, the formation of mesocyclones. Moreover, there is growing evidence that asymmetric dynamics play an important role in both the track and intensity changes associated with tropical cyclones. My research examines additional dynamical instability mechanisms that cause rapid structural variability within the core of geophysical vortices, such as tropical cylone vortices. More specificaly, my research examines how growth of these asymmetries initiates irreversible mixing within the core of geophysical vortices. With applications to tropical cyclones, my research investigates how this irreversible mixing occurs within the inner core of a mature TC, within the outer region of a mature TC, and for a tropical cyclone with primary and secondary eyewalls.
Figure 6: A numerical simulation of the inner core vorticity mixing for an annular vorticity ring up to 48 hours. Barotropic instability produces counterpropagating vortex Rossby waves that redistribute vorticity from the eyewall to the eye. See Schubert et al. (1999) for more details.
4. Physics of Vortex Merger and Vortex Resiliency
During its development, an atmospheric vortex may experience episodes of external vertical shear. A vertically tilted vortex in the atmosphere either succumbs to external vertical shear by irreversibly shearing apart or by resisting external forcings to align, a process called vortex resiliency. Recent research has shown that atmospheric vortices under weak vertical shear remain vertically upright through a wave mean-flow interaction of the mean vortex with the shear-induced vortex Rossby waves. However, most of these studies examine weakly rotating vortices under weak unidirectional shear. My research extends this by examining the dynamics of vortex resiliency for rapidly rotating vortices under moderate directional shear. These results have relevance to the problem of tropical cyclogenesis and the observations of vortex resiliency for mature hurricanes. My research also examines the physics of vortex merger for rapidly-rotating vortices, which have applications for the dynamics of hurricane-trough interactions. My research also examines how environmental shear affects inner core mixing for a mature hurricane.
If you're interested in any of these research topics or might be interested in joining me in research, contact me and I'll be glad to talk to you about my ongoing research.