Dr. Gabriel Williams
Office: RHSC 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 tools that I use to analyze tropical cyclones can be found on my tropical cyclone research page. A more in-depth discussion of the research 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. The boundary layer is the lowest 1 - 2 km of the atmosphere, the region most directly influenced by the exchange of momentum, heat and water vapour at the earth's surface. Therefore, the hurricane boundary layer (HBL) 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 structural features within the HBL (such as low-level jets and horizontal convective rolls). In particular, I use computer modeling, aircraft reconnissance data, and thermodynamic sounding data to examine how these structural features lead to intensity and structural changes for mature hurricanes. My research also examines how environmental forcing (such as the presence of land, upper-level troughs, and continental air masses) affects the evolution of the boundary layer.
2. The Dynamics of Secondary Eyewall Formation and Eyewall Replacement Cycles
Intense hurricanes at times undergo eyewall replacement cycles (ERCs), in which an outer eyewall and wind maximum form at several times the radius the radius of the primary eyewall. The outer eyewall usually then contracts and intensifies, while the inner eyewall weakens, disappears, and is replaced by the outer eyewall. These ERCs have significant impact. The formation and the contraction of the outer eyewall often coincides with a weakening, or a pause in the intensification, of the storm. The formation of the outer eyewall [known as secondary eyewall formation (SEF)] is associated with an increase in the area covered by the strong winds, which leads to more severe storm surges near land.
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.
SEF is widely recognized as an important research problem in the dynamics of mature hurricanes, but as of yet there is not a consensus on the phenomenon's fundamental physics. Recently, a clear dynamical link has been made between the overarching mechanisms of hurricane intensification and the physics of SEF. This dynamical link is associated with the interaction of the hurricane boundary layer (HBL) and the free atmosphere.
My research examines how thermodynamic processes within the hurricane boundary layer (HBL) can initiate SEF and how the thermal structure of the tropical cyclone boundary layer evolves during SEF and eyewall replacement cycles. Another scientific question that my research seeks to answer is why do ERCs commonly form near land?
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 small-scale disturbances within the core 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 and 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.
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.
My research examines additional dynamical instability mechanisms that cause rapid structural variability within the core of geophysical vortices, such as tropical cylone vortices. More specifically, my research examines how the growth of these asymmetries initiates irreversible mixing within the core of geophysical vortices. Using computer modeling along with satellite and radar imagery, my research also examines how convective processes affect the irreversible mixing that 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.
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 self-axisymmetrization process. 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 observations of vortex resiliency for mature hurricanes and severe convective storms. 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.