An experimental examination of hurricane mesovortices: Flow apparatus and particle image velocimetry measurements

Radar observations of intense hurricanes suggest the existence of intense "meso-vortices" within the hurricane eyewall region. These vortices are believed to be associated with the polygonal shapes of hurricane eyes observed with precipitation-based radar and visible satellite images. Surprisingly little is understood, however, about the dynamics of such vortices in hurricanes.

We suggest that the mechanism of their generation and maintenance is similar to that of the formation of a system of vortices above an orifice in the bottom of a vessel with a stead supply of circulating liquid along the vessel's periphery. The experimental set up for producing "hurricane-like" flows has been manufactured and used at the Fluid Dynamics Laboratory, Department of Mathematics, Hong Kong University of Science and Technology. Among the governing parameters, the most important are found to be the aspect ratio (fluid depth divided by orifice diameter) and flow rate. Both are chosen to give a dynamically similar flow to the hurricane eyewall region.

Measurements of fluid velocity are obtained using a Particle Image Velocimetry (PIV) system. Results indicate that a reasonably stable flow regime with 3, 4, and 5 vortices can exist for the same external conditions. Transitions from one regime to another can also occur. Near the radius of maximum wind, velocity pulsations are observed to get as high as 1.4 times the azimuthal mean trangential wind.

These measurements are believed to be useful indicatiors of the wind variability in intense hurricanes over the open ocean. Work is planned to consider the modification of these results for landfalling storms.