Tropical cyclone formation in a sheared environment: A case study

The development of Hurricane Danny (1997) from depression to hurricane was examined using cloud-to-ground lightning data, reconnaissance aircraft data, and satellite imagery. Vertical wind shear between 850 and 200 hPa of 5-11 m s(-1) produced persistent downshear convective outbreaks that became progressively more intense and closer to the center during the development. Early in the period the storm intensified steadily in the presence of this downshear convection. During the last and most intense outbreak, a second vortex appeared to develop within the convection. Evidence is presented that the new downshear vortex became the dominant vortex and absorbed the original. Based on these events, it is hypothesized that the presence of moderate vertical wind shear accelerated the early development process. Equivalent potential temperature fields within 500 m of the surface were examined. Only well after the period of vortex interaction did the characteristic mature tropical cyclone radial profile of equivalent potential temperature appear. This came about by the virtual elimination of both low theta(e) values in the core and high theta(e) values outside the core that had been present at previous hours. The growth of Hurricane Danny is viewed in terms of the wind-induced surface heat exchange (WISHE) theory. During the tropical depression and early tropical storm (pre-WISHE'') periods, few if any of the assumptions of WISHE were met: vertical wind shear exceeded 5 m s(-1), considerable azimuthal asymmetry was present, transient highly buoyant convection occurred, and low values of theta(e) in the storm core suggested the presence of convective downdrafts. It is proposed that 1) vortex interactions and subsequent axisymmetrization produced a single dominant vortex at the surface, and 2) vertical mixing of moist entropy by strong convection moved the sounding toward moist neutrality. By this reasoning, the disturbance then met the key tenets of the known finite-amplitude WISHE instability, and the storm intensified to hurricane strength.