Dr. Gabriel Williams
Assistant Professor
Office: JC Long 223
Phone: 843.953.0278
Email: williamsgj@cofc.edu

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Gabriel J. Williams

- 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.

Observation of boundary layer shock in Hurricane Hugo (1989). Observation of roll vortices in Typhoon Fengshen (2002).
Figure 1: NOAA WP-3D (N42RF) aircraft data from an inbound leg in the southwest quadrant (red, 434 m average height) and an outbound leg in the northeast quadrant (blue, 2682 m average height) of Hurricane Hugo on 15 September 1989. (top) The solid curves show the tangential wind component, while the dotted curves show the radial wind component. (bottom) The vertical component of the velocity. A boundary layer shock develops near the eyewall of Hurricane Hugo. See Williams et al.(2013) for more details. Figure 2:RADARSAT-1 synthetic aperture radar (ScanSAR Wide B) image of Typhoon Fengshen showing evidence of finescale roll circulations across much of the image (e.g., black arrow). The center of the typhoon is just to the southwest of the image at 28.3 N, 140.7 E, with the signature of eyewall convection visible in the lower left of the image (white arrow). See Morrison et al. (2005) for more details.

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.

Observation of ERC in Hurricane Danielle (2010).

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.

Observation of mesovortices in Typhoon Nari (2001). Observation of polygonal eyewalls in Hurricane Isabel (2003) .
Figure 4: A MODIS image showing a swirling pattern of eye clouds for Typhoon Nari, indicative of inner core mixing between the eye and eyewall. See Kossin et al. (2002) for more details. Figure 5: Defense Meteorological Satellite Program (DMSP) image of Hurricane Isabel at 1315 UTC 12 Sep 2003. The starfish pattern is caused by the presence of six mesovortices in the eye (one at the eye center and five surrounding it). See Kossin and Schubert (2004) for more details.

Asymmetric Mixing and Eye Breakdown from 0-8 hr Asymmetric Mixing and Eye Breakdown from 10-20 hr Asymmetric Mixing and Eye Breakdown from 22-48 hr

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.

Observation of mesovortices in Typhoon Nari (2001). Observation of polygonal eyewalls in Hurricane Isabel (2003) .
Figure 7: A schematic of the TC alignment mechanism when the TC vortex is tilted by vertical shear. As the TC vortex is tilted by the vertical shear, VRW damping counters the differential advection of the TC by the vertical shear. See Reasor et al.(2004) for more details. Figure 8: Cross-cut experimental dye visualizations of two laminar co-rotating vortices before, during, and after merging. See Meunier et al. (2005) for more details.

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.

updated: 13 May 2013