APRIL 2012 D O L L I N G A N D B A R N E S 1177 Warm-Core Formation in Tropical Storm Humberto (2001) KLAUS DOLLING AND GARY M. BARNES University of Hawaii at Manoa, Honolulu, Hawaii (Manuscript received 27 July 2011, in final form 18 October 2011) ABSTRACT At 0600 UTC 22 September 2001, Humberto was a tropical depression with a minimum central pressure of 1010 hPa. Twelve hours later, when the first global positioning system dropwindsondes (GPS sondes) were jettisoned, Humberto’s minimum central pressure was 1000 hPa and it had attained tropical storm strength. Thirty GPS sondes, radar from the WP-3D, and in situ aircraft measurements are utilized to observe ther- modynamic structures in Humberto and their relationship to stratiform and convective elements during the early stage of the formation of an eye. The analysis of Tropical Storm Humberto offers a new view of the pre-wind-induced surface heat exchange (pre-WISHE) stage of tropical cyclone evolution. Humberto contained a mesoscale convective vortex (MCV) similar to observations of other developing tropical systems. The MCV advects the exhaust from deep con- vection in the form of an anvil cyclonically over the low-level circulation center. On the trailing edge of the anvil an area of mesoscale descent induces dry adiabatic warming in the lower troposphere. The nascent warm core at low levels causes the initial drop in pressure at the surface and acts to cap the boundary layer (BL). As BL air flows into the nascent eye, the energy content increases until the energy is released from under the cap on the down shear side of the warm core in the form of vigorous cumulonimbi, which become the nascent eyewall. This series of events show one possible path in which a mesoscale convective system may evolve into a warm-cored structure and intensify into a hurricane. 1. Introduction and Riehl 1960; Ooyama 1964). Some have proposed that the intensification process should be split, with a pre- Wind-induced surface heat exchange (WISHE) is one WISHE phase, in which some other mechanism spins up of the leading theories explaining the intensification of a vortex to a critical point at which the WISHE theory can a strong tropical depression to a full-fledged hurricane then take over (Montgomery and Farrell 1993; Bister and (Emanuel 1986; Rotunno and Emanuel 1987). In this Emanuel 1997; Ritchie and Holland 1997; Montgomery theory the boundary layer (BL) air acquires heat and and Enagonio 1998; Davis and Bosart 2001; Hendricks water vapor from the sea, which in turn is transported et al. 2004; Molinari et al. 2004). upward in the eyewall resulting in a maximum temper- Theories about the pre-WISHE phase have recently ature anomaly in the upper troposphere. This warming been framed as either being top down or bottom up. Both hydrostatically reduces the surface pressure in the core of these approaches assume that the inner core of the cy- of the tropical cyclone (TC), and the enhanced pressure clone evolves from a mesoscale convective system (MCS). gradient between the core and the environment pro- Some MCSs develop a mesoscale convective vortex duces the destructive winds associated with a hurricane. (MCV) in the stratiform region, which trails the leading The acceptance of the WISHE theory for mature TCs is edge of convection (Gamache and Houze 1982; Leary and widespread among the scientific community, essentially Rappaport 1987; Brandes 1990; Chen and Frank 1993). formalizing the importance of sea to air energy transfer Others have hypothesized that if an MCS moves into an recognized earlier as vital for tropical cyclone develop- area favorable for genesis to occur, the stratiform rain ment (Byers 1944; Riehl 1948; Kleinschmidt 1951; Malkus region might play a role in tropical cyclogenesis (Bosart and Sanders 1981; Velasco and Fritsch 1987; Menard and Fritsch 1989; Frank and Chen 1991; Chen and Frank 1993; Corresponding author address: K. Dolling, Department of Mete- orology, University of Hawaii at Manoa, 2525 Correa Rd., Honolulu, Simpson et al. 1997; Rodgers and Fritsch 2001). HI 96822. Top-down theories propose that the MCV migrates to E-mail: [email protected] the surface at which point WISHE takes over. Simpson DOI: 10.1175/MWR-D-11-00183.1 Ó 2012 American Meteorological Society Unauthenticated | Downloaded 09/27/21 12:53 PM UTC 1178 MONTHLY WEATHER REVIEW VOLUME 140 et al. (1997) emphasize the importance of MCV Enagonio and Montgomery 2001; Hendricks et al. 2004; mergers during the formation of TC Oliver during the Montgomery et al. 2006). Recently, VHTs have been Tropical Ocean and Global Atmosphere Coupled Ocean– hypothesized to be part of the marsupial pouch paradigm Atmosphere Response Experiment (TOGA COARE). (Wang et al. 2010a,b). In this scenario, an area of quasi- The mergers increase the vertical extent of the resulting closed Lagrangian circulation (the cat’s eye) is moistened vortex so a surface connection can be established. The by convection and mostly protected from dry air intru- vorticity-rich MCV environment reduces the Rossby ra- sion. The parent wave is maintained and amplified by dius of deformation, which allows for the more efficient convection within the cat’s eye, defined as the intersec- warming of the troposphere. Harr et al. (1996) used tion of the wave critical latitude and the trough axis. Omega dropwindsondes (ODWs) to investigate the for- Stossmeister and Barnes (1992) documented the de- mation of Typhoon Ofelia (1993) that formed from one velopment of a new second vortex that became the persistent MCS. Although Ofelia’s formation is viewed as circulation center for Isabel (1985). This new vortex a top down, Harr et al. (1996) found warm, dry midlevel formed beneath the downwind anvil of intense convec- air near the circulation center and argues that subsidence tion in a rainband. They hypothesized that subsidence induced on the concave side of the main convective band warming beneath the anvil, similar to the mesolows might have played a role in warming the troposphere and found at the trailing edge of MCSs, could lower the in the formation of an eye. pressure and serve as the initial perturbation for a TC. In the Tropical Experiment in Mexico (TEXMEX), Heymsfield et al. (2006) investigated Tropical Storm Bister and Emanuel (1997) suggested that after an MCS Chantal in a highly sheared environment. Although has formed with a cold core in the lower atmosphere and Chantal failed to intensify into a hurricane, detailed a cyclonic circulation in the midtroposphere, the tropo- analysis of the storm provides clues to warm-core for- sphere becomes nearly saturated in the core of the vortex. mation. The low-level circulation center (LLCC) is ob- This prevents the evaporation of rain and inhibits served 80 km west-southwest of the deep convection in downdrafts that bring cool dry air to the BL. Enhanced a rain-free region. Two broad regions of warming are surface fluxes associated with increased surface winds found, one of these was argued to be due to subsidence near the core will increase the BL moist static energy. from the interaction of the storm with high wind shear, After convection occurs, the temperature of the vortex as suggested earlier by Ritchie and Elsberry (2001). core will increase, and the WISHE mechanism can then This area had a maximum temperature perturbation at act as a positive feedback to the warm-core cyclone. 500 hPa and was located partially over the low-level Bottom-up theories suggest that a low-level vorticity- circulation center. The other area of warming was found rich environment in concert with deep convection can ini- to have a maximum at the 700-hPa level. Heymsfield et al. tiate tropical cyclogenesis. Montgomery and Kallenbach (2006) hypothesized that this area of warming was due (1997), Molinari et al. (2004), and Reasor et al. (2005) have to similar mechanisms as observed by Stossmeister and hypothesized that the axisymmetrization of vorticity Barnes (1992). centers is important in spinning up a TC. Here vorticity A rare set of observations collected in Tropical Cy- is increased in localized areas by vortical hot towers clone Humberto (2001) are interpreted to gain insights (cumulonimbi with high vorticity) embedded in a well- into how a hurricane may form. The observations pro- organized, but weak low-level cyclonic circulation. vide evidence that WISHE theory, though vital for in- Axisymmetrization and the stretching of these vorticity tensification, may not explain the formation of a tropical centers intensify the initial surface vortex to a critical cyclone where deep clouds become organized and ro- point at which time WISHE becomes dominant. tational flow develops. The flights into Tropical Storm To examine the formation of TC Diana (1984), Humberto on 22 September provide observations on the Hendricks et al. (2004) used a model with a 3-km hori- pre-WISHE phase of tropical cyclone development. zontal grid that is near–cloud resolving. The simulation GPS sondes, radar from the WP-3D, and in situ aircraft shows the presence of vortical hot towers (VHTs), Cbs measurements are utilized to observe thermodynamic with high vertical vorticity. They develop from the and kinematic structures in Humberto and their rela- buoyancy-induced stretching of the local absolute vertical tionship to stratiform and convective elements. In this vorticity. At this grid spacing, VHTs are the preferred study, we examine the following questions: mode of convection. VHTs produce diabatically gener- ated low-level PV anomalies that axisymmetrize and 1) Does the warm core form in deep convection or does merge, which is a step in the intensification of the newly it form in the stratiform area of precipitation? formed vortex (Montgomery and Lu 1997; Montgomery 2) Where does the warm core form as a function of and Enagonio 1998; Montgomery and Franklin 1998; height and do the observations verify WISHE theory? Unauthenticated | Downloaded 09/27/21 12:53 PM UTC APRIL 2012 D O L L I N G A N D B A R N E S 1179 3) Is there an MCV present and what role does it play in the formation of the tropical cyclone? 2.