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Mesoscale Observations of the Genesis of Hurricane Dolly (1996)

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Mesoscale Observations of the Genesis of Hurricane Dolly (1996) SEPTEMBER 2005 R E A S O R E T A L . 3151 Mesoscale Observations of the Genesis of Hurricane Dolly (1996) PAUL D. REASOR AND MICHAEL T. MONTGOMERY Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado LANCE F. BOSART Department of Earth and Atmospheric Sciences, The University at Albany, State University of New York, Albany, New York (Manuscript received 17 August 2004, in final form 23 February 2005) ABSTRACT Recent numerical studies of tropical cyclone genesis suggest a new paradigm for how the surface vortex is established based on a highly nonaxisymmetric mechanism involving the interaction of low-level cyclonic circulations generated by deep cumulonimbus convection. A reexamination of mesoscale observations during the genesis of Hurricane Guillermo (1991) confirms the presence of multiple cyclonic circulations. More recently, airborne Doppler radar wind observations during the genesis of Atlantic Hurricane Dolly (1996) also reveal multiple lower-to-middle-tropospheric mesoscale cyclonic circulations during sequential 15–20-min compositing periods. A particularly well-organized, but initially weak (mean tangential wind of7msϪ1), low-level cyclonic vortex embedded within the pre-Dolly tropical disturbance is observed coincident with deep, vertically penetrating cumulonimbus convection. The earliest observations of this vortex show the peak circulation near 2-km height with a mean diameter of 30–40 km. The circulation undergoes a slight intensification over a 2-h period, with the maximum tangential winds ultimately peaking below 1-km height. Approximately 18 h after these observations Dolly is classified as a hurricane by the National Hurricane Center. A synthesis of observations during the early development of Dolly supports a stochastic view of tropical cyclone genesis in which multiple lower-to-middle-tropospheric mesoscale cyclonic circulations are involved in building the surface cyclonic circulation. It is suggested that, in particular, the interaction of low-level circulations generated by a series of deep cumulonimbus convective events, like the one documented here, within an environment of elevated cyclonic vorticity was instrumental to the formation of the Dolly surface vortex. 1. Introduction Montgomery and Enagonio 1998; Hendricks et al. 2004; Montgomery et al. 2005, manuscript submitted to J. Tropical cyclone (TC) genesis requires the transfor- Atmos. Sci., hereafter M05). Here we examine the me- mation of a tropical disturbance, generally character- soscale and convective-scale aspects of the problem uti- ized by a mesoscale region of convection, and in some lizing a unique airborne in situ and Doppler radar cases a midlevel cyclonic flow, into a warm-core TC dataset, described in detail by Bracken (1999, hereafter vortex with maximum winds near the surface. Globally, B99) that documents the three-dimensional wind struc- the majority of tropical disturbances do not develop ture of TC Dolly (1996) during the genesis stage. into TCs. The relative scarcity of TC genesis events may Based on recent observations and numerical studies in part be a consequence of the complex multiscale of the mesoscale and convective-scale aspects of TC nature of the phenomenon, involving a cooperation of genesis, a growing consensus exists for separating the the synoptic scale (e.g., Bracken and Bosart 2000), me- early development of the TC into two stages. The latter soscale (e.g., Ritchie and Holland 1997, hereafter stage of development occurs when air–sea interaction RH97), and convective scale (Simpson et al. 1997; processes begin to dominate the growth of a finite- amplitude surface circulation in a manner consistent Corresponding author address: Dr. Paul D. Reasor, Dept. of with the wind-induced surface heat exchange (WISHE) Meteorology, The Florida State University, Tallahassee, FL theory of Rotunno and Emanuel (1987). The onset of 32306. WISHE is characterized by increases in surface fluxes E-mail: [email protected] © 2005 American Meteorological Society JAS3540 3152 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 62 with wind speed, and is generally accompanied by in- convection discussed by Riehl and Malkus (1958) in creases in the surface equivalent potential temperature that the tilting and stretching of elevated preexisting ␪ ( e) above ambient values. Molinari et al. (2004) term vorticity by the intense convection yields a strong rota- the initial genesis stage before significant increases of tional component of the flow. M05 clarified the dynam- ␪ surface e as pre-WISHE. During the pre-WISHE pe- ics of VHTs and their role in the formation of the TC riod mesoscale and convective-scale vortical interac- surface circulation through a series of idealized simula- tions take place to build the seedling surface circula- tions of convection initiated within a preexisting MCV. tion. It is the pre-WISHE stage that we explore here. In the present study we examine data from National We believe that the establishment of the surface vor- Oceanic and Atmospheric Administration (NOAA) re- tex during the transformation of a tropical disturbance search flights into Hurricane Dolly (1996) during the is central to the problem of TC genesis. Using a syn- genesis phase. The Dolly case compliments prior ob- thesis of flight-level aircraft measurements and com- servational studies in the sense that the flight-level and posited airborne Doppler radar winds, Bister and satellite data confirm the important role of midlevel Emanuel (1997, hereafter BE97) proposed that the sur- MCVs during the development of the TC from an in- face vortex is established in part through vertical ad- cipient tropical disturbance. The Dolly case is unique vection of vorticity associated with midlevel mesoscale because of the detailed look at the wind field on the convective vortices (MCVs) by downdrafts in the strati- cloud scale to mesoscale provided by the airborne form rain region. RH97 used flight-level measurements, Doppler radar during the period when the surface vor- omega dropwindsondes, and satellite imagery to dem- tex began to form. We believe this to be the first TC onstrate the interaction between lower-tropospheric genesis dataset to clearly document the wind evolution synoptic-scale circulation and MCVs during the genesis of a low-level cyclonic circulation on the VHT scale process. Simpson et al. (1997) also emphasized the role (10–20 km) in apparent association with deep cumulon- of MCVs using a dataset similar to RH97. They argued imbus convection. that the stochastic interaction of midlevel MCVs in an The outline of the paper is as follows. Section 2 re- environment of elevated ambient cyclonic vorticity views the use of Doppler radar observations during the (e.g., a monsoon trough) builds the circulation down to Tropical Experiment in Mexico (TEXMEX) in 1991 the surface. (BE97; Raymond et al. 1998), which aside from Dolly Montgomery and Enagonio (1998) argued that the (1996) is the only known source of high temporal reso- surface vortex need not originate at midlevels. Moti- lution three-dimensional wind measurements of TC vated by Zehr’s (1992) observations of convective puls- genesis to date. The aircraft flight-level data, Doppler ing during the early genesis stage, they focused on the radar data, and satellite measurements used in the dynamical role of convectively generated potential vor- present study of Dolly are described in section 3. In ticity anomalies within MCV environments. The lim- section 4, the genesis of Hurricane Dolly is presented ited spatial and temporal resolution of the observations through a synthesis of the data. A discussion of the available at the time made numerical simulation the Dolly results in the context of recent cloud-resolving only viable option for studying the smaller-scale pro- numerical simulations is presented in section 5. cesses. Using a three-dimensional quasigeostrophic model, they demonstrated how low-level vorticity gen- 2. TEXMEX genesis field experiment erated by convective bursts within a preexisting midlevel MCV can spin up the surface circulation be- The first organized field campaign designed specifi- neath the MCV through horizontal axisymmetrization. cally to examine the mesoscale structure of tropical dis- Reasor and Montgomery (2001) clarified the vertical turbances in the genesis phase was TEXMEX in 1991. alignment dynamics of the preexisting vortex and con- TEXMEX focused on the eastern Pacific, climatologi- vectively generated vorticity anomaly. More recently, cally the most active region of TC development on Davis and Bosart (2001) found in a numerical case earth (based on the density of genesis cases per unit study of Hurricane Diana (1984) using parameterized area per unit time). The experiment ran from July to convection that the surface vortex can arise through August and captured six tropical disturbances in vari- multiple mergers of low-level cyclonic vorticity. Hen- ous stages of development, four of which ultimately dricks et al. (2004) used cloud-resolving numerical became TCs. One of the central goals of TEXMEX, as simulations to demonstrate the importance of “vorti- discussed by BE97, was to test the hypothesis that the ␪ cal” hot towers (VHTs) in the generation of low-level elevation of middle-tropospheric e (i.e., moistening) vorticity during Diana’s genesis phase. The VHTs differ above a near-surface vorticity maximum is a sufficient from the deep, vertically penetrating cumulonimbus condition for TC genesis to occur. SEPTEMBER 2005 R E A
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