Effects Of Vertical Wind Shear On The Structures And Intensity Of Hurricane Bonnie (1998)

A5-day cloud-resolving simulation of Hurricane Bonnie (1998) with the finest grid length of4km is used to examine the effects of vertical wind shear (VWS) on hurricane structures and intensity changes. Results show the feature of wavenumber-1asymmetries in the vertical motion, simulated radar reflectivity, horizontal wind speed, vorticity and radial speed fields strengthens when VWS increases. In low VWS intensity situation, though the beta effect still works, Bonnie’s structure maintains axisymmetries, which means VWS effect is dominant in real tropical cyclone structure changes.It is found that the upper-level strong ventilation on the upshear side(Z=6-12km) tends to destroy the warm core in the eye while the VWS-induced intrusion of lower equivalent potential temperature from higher levels descends with cyclonic rotation around eye region, and finally into the boundary layer core region,which plays an important role in weakening the storm. In addition, the aidabatic heating from descending air creates warm area in the neighbourhood of middle level eyeywall, which strengthens the stability and suppresses convection. In view of little variations of the mean sea-level temperature during the integration period, VWS may be considered as one of the major factors in determining the structures and evolution of Bonnie.The relation between Bonnie’s central pressure and equivalent potential temperature distribution in boundury layer may make latter possible to be the indicator of strom intensity change. Thus, the results above indicate that the thermodynamical impact of VWS could be as significant as its dynamical impact on Bonnie’s intensity and intensity changes. Results suggest that higher-level ventilation is more detrimental to hurricane intensity compared to lower-level ventilation.