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

Abstract

SUMMARYThe 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.

The Tropical Transition in the Western North Pacific: The Case of Tropical Cyclone Peipah (2007)

“…For the 7 days prior to the formation of Peipah, SSTs and vertical wind shear provided favorable environments for tropical transition (Davis & Bosart, , , ; Sadler, , , ). In Figure a, pre‐Peipah remained over regions where SST exceeded 26 °C.…”

Increased sensitivity of tropical cyclogenesis to wind shear in higher SST environments

“…In a sheared environment, the mid‐level, mesoscale convective vortex (MCV) is advected downstream from the surface vortex, and the two must remain dynamically coupled for development to proceed. The low‐level vortex can slowly intensify by building upwards [e.g., Davis and Bosart , 2006; Tory et al , 2007]; or a new low‐level vortex can form underneath the mid‐level vortex as a result of a large burst of convection, the so called “down‐shear reformation” [ Molinari et al , 2006].…”

Vertical structure of tropical cyclones at onset of the rapid intensification in the HWRF model

“…In high‐resolution experiments with the Weather Research and Forecasting model, Davis and Bosart [, ] and Nolan [] provided more concrete evidence about the necessity of an upper level warm core and a midlevel vortex for the RI onset. By tracking the evolution of moist convection within the inner core region of an idealized storm, Nolan [] demonstrated that convective bursts could moisten the low‐to‐middle level troposphere gradually for a period of several days.…”

“…The intensification can be further enhanced if the initial vortex has a larger vertical extent to upper levels. This is consistent with the analysis obtained from the real‐time experiments by Davis and Bosart [], which also revealed that both a midlevel vortex and a near‐saturated inner core from the surface to 500 hPa were required for the genesis of Hurricane Diana (1984).…”