The mesoscale aspects of the genesis of Hurricane Danny (1997) are examined, starting from an MCS over the northern Gulf of Mexico, ending at the onset of hurricane intensity. The storm remained within range of the National Lightning Detection Network throughout the period.

Moderate vertical wind shear (5-10 m/s over the troposphere) occurred throughout MCS, depression, and storm stages. Convective cloudiness and lightning were highly asymmetric, with maxima downshear of the vortex center, and only low cloud swirls upshear. On the second day at depression strength, a strong downshear cell developed (cell strength was measured by ground flash density). The circulation appeared to redevelop within this downshear cell and quickly intensified to tropical storm intensity. Another even more intense cell developed downshear from the tropical storm, and the hurricane core appeared to develop within this latter cell. Prior to hurricane formation, three distinct vortices were visible: the initial tropical depression, represented mostly by low clouds in otherwise clear air; the tropical storm that formed downshear of the depression, visible in both clouds and lightning; and the intense downshear cell within which the hurricane formed, visible primarily in the lightning field (characterized by 1000 flashes of lightning per hour, a very large number over water in the subtropics). There is evidence within the lightning distribution of filaments of potential vorticity connecting the vortices.

Values of equivalent potential temperature did not develop a large radial gradient until a single strong vortex formed from the multiple vortices noted above. The development of such a gradient is viewed as a visualization of the WISHE mechanism of Emanuel (1989; see also the study of Raymond et al. 1998). It is suggested that this mechanism was effective only because two other "pre-WISHE" processes occurred: midlevel moistening, and the interaction of vortices generated by downshear convection. The vortex interactions looked remarkably similar to those described in idealized numerical models by Michael Montgomery and his collaborators (Montgomery and Enagonio 1998, and Fig 12 of Enagonio and Montgomery 2001). Simpson et al. (1997) also described a tropical cyclone genesis arising from mesoscale vortex interactions.

The details of the evolution will be presented in the talk, and the relevance of current theories of tropical cyclogenesis will be examined.