ER-2 Doppler radar investigations of the eyewall of Hurricane Bonnie during the convection and moisture experiment-3

A persistent, mesoscale region of intense eyewall convection contained with in Hurricane Bonnie on 23 August 1998 is examined from multiple observation s synthesized from the National Aeronautics and Space Administration ER-2 a nd DC-8 aircraft. The intense convection occurred late in the day as Bonnie was attaining its minimum central pressure and during a stage when the inn er core featured a markedly asymmetric structure. The internal structure of this convective burst and its relationship to the warm core are presented using a synthesis of high-resolution satellite, aircraft radar, and in situ data. An exceptionally vigorous eyewall tower within the burst and penetra ting to nearly 18 km is described. A second intense eyewall tower, adjacent to the eye, is shown to be associated with a mesoscale subsiding current o f air, with vertical velocities on the order of several meters per second t hat descends at least 9 km and extends horizontally nearly 25 km into the e ye interior. The subsidence is a much deeper and broader-scale feature than the convectively induced, symmetric overturning that commonly occurs on th e upper-level flanks of convective towers in other tropical environments. T he air supplying the deep current probably originates both at tropopause he ight and also from air detrained out of the adjacent updraft at midlevels. Strong downdrafts within the eye could not be associated with every hot tow er. Whether this result was due to undersampling by aircraft or whether dee p eye downdrafts are indeed sporadic, it is plausible that up to 3 degreesC of midlevel eye warming observed in Bonnie may arise from one or more of t hese convectively induced episodes rather than as a result of a gradual sin king motion applied uniformly throughout the eye.