The formation of hurricane Humberto (2001): The importance of extra-tropical precursors

The authors examine the formation of hurricane Humberto (2001) in the Atlantic basin. Precursor disturbances originating in the extratropics appear critical for inducing tropical cyclogenesis. One precursor near the tropopause arose during the development of hurricane Erin on 7 September and could be tracked continuously for 12 days until 1200 UTC 19 September. A fully explicit numerical simulation with the PSU/NCAR mesoscale model produces a generally realistic evolution of the developing storm during a 4-day period beginning 0000 UTC 19 September. Diagnostics of the simulations reveal that moderate vertical shear was crucial for organizing convection, and this shear was enhanced on the periphery of an upper-level vortex, within a lower- to mid-tropospheric baroclinic zone. Downdraughts and evaporative cooling produced a rearward propagating bore that organized new convection in a notably moistened environment with small CAPE. Vorticity generation maximized near 1 km above mean sea level and the vortex built upward with time. This evolution contrasts markedly with 'downward development' conceptual models of tropical cyclogenesis and points to the crucial need for observations in the lower troposphere before depression formation. Convective-scale dynamics were governed by intense convective cores which produced most of the diabatic heating and lower-tropospheric vertical vorticity. They were constrained by vertical shear to populate the downshear-left quadrant of the vortex, and therefore their collective behaviour was organized despite being quasirandom within the favoured region. Ultimately, the shear that helped organize the convection in the developing depression hampered Humberto's subsequent intensification.