Extreme helicity and intense convective towers in Hurricane Bonnie (1998)

We have calculated cell-relative helicity from tropospheric-deep dropsondes released by NASA in the CAMEX experiments. The presentation will focus on the results from Hurricane Bonnie (1998). We have found that:

(i) Helicity varied strongly with the ambient wind shear, with large values downshear and small values upshear. CAPE varied in the same manner.

(ii) Intense localized convective cells existed only in the vicinity of large helicity.

(iii) Helicity maxima over 3, 6, and 12-km layers represent the largest ever reported in the literature for each of the three layers. This indicates the likelihood of deep, long-lasting cells.

(iv) Large environmental helicity occurred in the form of strongly curved, semi-circular hodographs. The latter maximize the ability of a cell to gain helicity from its environment.

(v) Bluestein and Jain (1985) argued that in the presence of large environmental helicity, the most likely convective structures were isolated supercells and back-building squall lines. Both behaviors were seen in Hurricane Bonnie. The back-building behavior resulted in a half-open open eye wall almost completely closing off. There appear to be parallels to middle latitude severe weather in the core convection in Bonnie.

We believe that the helicity distribution in tropical cyclones contains considerable information as to where intense convection will occur. Helicity concepts also give some insight into the nature of the resistance of tropical cyclones to vertical wind shear. The implications of these findings will be addressed in the talk.