The Genesis of Hurricane Guillermo: TEXMEX Analyses and a Modeling Study

Abstract

The transformation of a mesoscale convective system into Hurricane Guillermo was captured by aircraft and Doppler wind data during the Tropical Experiment in Mexico. The early phase of the system evolves in a way very similar to previously documented mesoscale convective systems, with a midlevel mesocyclone developing in the stratiform precipitation region. More unusually, the cyclone extends to low altitudes: A weak cyclone is discernable even in the 300-m altitude wind field. After another day of evolution, a small, surface-based warmcore cyclone is observed to develop within the relatively cold air associated with the mesocyclone aloft. This mesocyclone develops into a hurricane over the subsequent day. A nonhydrostatic, axisymmetric numerical model is used to explore the evolution of the initially cold-core, midlevel vortex into a tropical cyclone. A mesoscale, midlevel ''showerhead'' is switched on in an initially quiescent, tropical atmosphere overlying a warm ocean surface. Evaporation of the falling rain cools the lower troposphere and leads to the spinup of a midlevel vortex, while divergent outflow develops near the surface. After some time, the midlevel vortex expands downward toward the boundary layer, and later a warm-core, surface-flux-driven cyclone develops within it. Experiments with the model show that both the cyclone, with its associated cold anomaly, and the relatively humid air associated with the evaporatively cooled mesoscale cyclone are important for the subsequent development of the warm-core system. The simulations also suggest that, for the warm-core development to occur, the stratiform rain must last long enough to drive the midlevel vortex down to the boundary layer. The authors present a simple argument for why this must be so and suggest that this process would be significantly impeded by the presence of background vertical wind shear.

Thermodynamic control of tropical cyclogenesis in environments of radiative‐convective equilibrium with shear

“…Such a diabatic heating profile leads to the formation of a mesoscale convective vortex (MCV) near the melting level roughly 36 hours into each simulation. At, or just prior to 2 days, a rapid collapse in low-level vorticity is seen in all cases due to downdraught-driven lowlevel divergence beneath the mid-level vortex (Bister and Emanuel, 1997, their Figure 13(a)). …”

Tropical‐cyclone intensification and predictability in three dimensions

“…Following Bergeron (1950), recent work has hypothesized that a tropical depression vortex will begin to intensify only after the column-integrated relative humidity has become nearly saturated on the vortex scale (Rotunno and Emanuel, 1987;Emanuel, 1989;Bister and Emanuel, 1997;Raymond et al, 1998). The basis for this idea originates in observations showing that regions of deep cumulus convection have adjacent downdraughts that advect mid-tropospheric air possessing a minimum of moist entropy into the boundary layer (e.g.…”

Objective Estimation of the 24-h Probability of Tropical Cyclone Formation

“…Dry air at middle levels can have a negative impact on convective activity by reducing the updraft buoyancy via entrainment and decreasing the precipitation efficiency within developing systems (Ruprecht and Gray 1974;Gray 1975;Bister and Emanuel 1997). As described by Moody et al (1999), the geostationary satellite water vapor imagery in cloud-free regions is sensitive to the relative humidity in the mid-to upper troposphere.…”

“…Palmé n (1948) discussed the link between TC formation and sea surface temperature (SST) and concluded that TCs typically form in regions where the SST exceeds 268C. It has been shown that dry air at midlevels, via its entrainment into convective updraft cores and subsequent reduction in both buoyancy and precipitation efficiency (Ruprecht and Gray 1974;Gray 1975;Bister and Emanuel 1997), can inhibit sustained deep convection and have a negative effect on TC formation. Furthermore, the explicit role of tropical convection in TC formation has received considerable attention.…”