Baroclinically induced tropical cyclogenesis

The authors investigate the transition of numerous subtropical cyclones into late season tropical storms and hurricanes during the 2000 and 2001 Atlantic tropical cyclone seasons. In all transitioning cases (10), the 900-200-hPa wind shear was initially near or in excess of the upper limit of vertical shear deemed suitable for tropical cyclogenesis. In many of these cases, the vertical shear decreased markedly near or prior to the time of tropical cyclone formation. In cases that did not become named tropical cyclones, either the tropospheric vertical shear remained in excess of 15-20 m s^-1 or the underlying sea surface temperature (SST) dropped below about 26°C prior to or during the weakening of the shear. Cases in which the shear remained large featured multiple, shortwave upper-tropospheric troughts interacting with the developing lower-tropospheric disturbance such that classical occlusion did not occur. Through detailed analysis and simulation of the development of Hurricane Michael in 2000, it was found that the rapid reduction of vertical shear occurred within the precursor baroclinic development. This shear reduction is explained by the diabatic redistribution of potential vorticity, involving both the nonconservative redistribution of potential vorticity (PV) along the three-dimensional vorticity vector and divergent, diabatically induced outflow in the upper troposphere. While some shear reduction occurred in an adiabatic simulation, the diabatic processes were found crucial to reducing the shear on a short (12 h) timescale. Such a rapid reduction in shear may be particularly important for the formation of late season hurricanes because of the greater poleward steering influence of disturbances in the westerlies that tends to move such storms over cool water.