AUGUST 2001 DAVIS AND BOSART 1859 Numerical Simulations of the Genesis of Hurricane Diana (1984). Part I: Control Simulation CHRISTOPHER A. DAVIS National Center for Atmospheric Research,* Boulder, Colorado LANCE F. B OSART Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York (Manuscript received 25 July 2000, in ®nal form 23 January 2001) ABSTRACT The complete transformation of a weak baroclinic disturbance into Hurricane Diana is reproduced by numerical simulations using the ®fth generation Pennsylvania State University±National Center for Atmospheric Research Mesoscale Model. Three distinct phases of the evolution are evident. First, baroclinic and barotropic development, strongly modi®ed by the effects of latent heating, occurs. During the latter part of this phase, the low-level circulation is strengthened through the axisymmetrization of remote potential vorticity anomalies that are gen- erated by condensational heating and then advected toward the incipient storm. The axisymmetrization process evinces properties of both nonlinear, discrete vortex merger and vortex Rossby wave dynamics. The transfor- mation from cold-core to warm-core vortex occurs in this development stage. In the second phase, lasting 10±12 h, little deepening occurs. Spiral bands of convection begin to form and the core of the storm moistens, eventually reaching 95% humidity averaged between the top of the boundary layer and 600 hPa at the radius of maximum wind. The third stage ensues, driven mainly by the positive feedback between ¯uxes of latent heat and the increase of the tangential wind. In this stage, the storm readily develops a clear eye. The transition to the hurricane stage occurs 12±24 h sooner in the model than in nature. The maximum intensity was also underestimated, with peak winds in the model being about 42 m s 21 (at 40 m above ground level) whereas sustained winds of nearly 60 m s 21 were observed. 1. Introduction cursors in many cases of tropical cyclone development, especially near and poleward of 208N latitude. The pro- a. Tropical cyclone genesis cess by which such an incipient mesoscale or synoptic- The general conditions favoring tropical cyclone for- scale disturbance becomes a coherent mesoscale cir- mation have been known for some time (e.g., Gray culation capable of self-ampli®cation has remained par- 1968; McBride and Zehr 1981). These consist of a series ticularly elusive and may not be a single mechanism of practically necessary, but by no means suf®cient, but, rather, any process that concentrates mesoscale vor- constraints on sea surface temperature (SST), environ- ticity. mental shear (weak), presence of ambient cyclonic vor- Observations to date have generally been inadequate ticity, and large-scale divergence aloft. Tropical cyclone to capture mesoscale aspects of the genesis stage of formation also requires a preexisting disturbance of suf- tropical cyclones except for brief glimpses during the ®cient amplitude such that air±sea interaction can occur process. Experiments such as the Tropical Experiment (Riehl 1948; Rotunno and Emanuel 1987). Recent ev- in Mexico (Bister and Emanuel 1997) and Tropical Cy- idence from studies by Hess et al. (1995), Lehmiller et clone Motion experiments TCM-92 (Ritchie and Hol- al. (1997), Molinari et al. (1998), and Bracken and Bos- land 1997) and TCM-93 (Harr et al. 1996a,b) over the art (2000) points to the importance of upper-level pre- western Paci®c Ocean are some recent examples. All of these studies suggest the importance of a preexisting large-scale disturbance that organizes convection and * The National Center for Atmospheric Research is sponsored by an importance of lower-tropospheric cyclonic potential the National Science Foundation. vorticity (PV) anomalies that form within the organized convection. One such vortex appears to make a trans- Corresponding author address: Christopher A. Davis, National formation to warm core and form the seed of the tropical Center for Atmospheric Research, P.O.Box 3000, Boulder, CO 80307. cyclone. However, there does not exist a dataset that E-mail: [email protected] temporally resolves this transformation. q 2001 American Meteorological Society Unauthenticated | Downloaded 09/26/21 04:27 AM UTC 1860 MONTHLY WEATHER REVIEW VOLUME 129 A fairly comprehensive review of tropical cyclone simulations is found in Liu et al. (1997). Simulations of the genesis phase, fully in three dimensions with domains large enough to capture both the evolving meso- and synoptic scales and the inner core dynamics, do not exist. Zhang and Bao (1996a) produced a mar- ginal tropical storm in a 90-h integration, but their res- olution (25-km grid spacing) was too coarse to capture the inner structure of the storm. Even Liu et al. did not simulate tropical storm genesis because their study of Tropical Cyclone Andrew began after the disturbance had reached tropical storm strength. All high-resolution (less than 20-km grid spacing) studies have had to em- ploy a bogusing scheme to initialize a vortex, and in simulations of observed cases, the initial vortex is usu- ally of tropical storm strength. Thus, the imposed initial disturbance is capable of self-ampli®cation, so the ques- tion of how this disturbance originates is not addressed. Several recent papers have dealt with the topic of the FIG. 1. Track and intensity of observed and simulated storms. formation of tropical depressions and tropical storms as Heavy solid line indicates the observed track, with the storm positions an amalgamation of diabatically produced potential vor- marked by L's. This line is the simulated track, with 3-hourly position ticity (PV) maxima in the lower and middle troposphere. marked by 1's. Heavy 1's denote time-matching observations. Small Observational evidence that a merger process is im- integers (1±9) refer to times listed at lower left. Inset ®gure depicts minimum SLP (hPa) as a function of time with observations indicated portant comes from Harr et al. (1996a,b) and Ritchie by ®lled circles. Shaded ®eld is SST (8C) obtained from manual and Holland (1997), who examined western Paci®c sys- analysis, interpolated to the 9-km domain. tems. Organized latent heating and the generation of multiple PV anomalies on the mesoscale appears to in- volve background synoptic-scale upward motion (Simp- rare class of tropical cyclones with origins as an extra- son et al. 1997), but the mechanisms producing wide- tropical, baroclinic cyclogenesis. Diana formed to the spread weak ascent are varied. Easterly waves (Reed east of Florida, and slightly poleward of a decaying 1979), the monsoon trough in the western Paci®c Ocean stationary front that had moved unusually far south for (e.g., Simpson et al. 1997), and extratropical troughs in early September. A large anticyclone dominated the east the upper troposphere (Riehl 1954; Molinari and Vollaro coast of the United States. To the south of this anti- 1989; DeMaria et al. 1993; Montgomery and Farrell cyclone and poleward of the surface front, strong east- 1993; Molinari et al. 1998) are all thought to provide erly ¯ow drove large latent heat ¯uxes over a mesoscale favorable environments for producing multiple cloud region, with observed values approaching 1000 W m22. clusters and associated PV anomalies, which can merge The incipient development as seen in water vapor and intensify into a nascent tropical cyclone. imagery was highly reminiscent of frontal cyclogenesis The view espoused by Ritchie and Holland is that the (Fig. 7 of BB). Tropical storm Diana formed on the merger process in the real atmosphere is a strong analog western edge of a baroclinic zone that was anomalously of two-dimensional barotropic dynamics, where the en- strong given its latitude and season. This incipient cy- ergy cascade is entirely upscale. Montgomery and En- clone itself was diagnosed by BB to grow in response agonio (1998) have viewed the vortex intensi®cation to mesoscale ascent and vortex stretching caused by a process in terms of Rossby waves propagating on the cold-core upper-tropospheric trough centered over Flor- PV gradient outside the radius of maximum wind. Vor- ida at 1200 UTC September. The upper-tropospheric ticity transport into the core is accomplished by wave trough had become increasingly cut off from the main ¯uxes. It is possible that these two views of axisym- westerlies farther north. The process of trough fracture metrization are complementary, but not necessarily so. could also be viewed as a large-scale anticyclonic wave- In the idealizations presented by Ritchie and Holland, breaking episode in the upper troposphere with attendant Rankine vortices are assumed and the vortices are large- ®lamentation of vorticity and PV (Thorncroft et al. amplitude, coherent structures. In Montgomery and En- 1993). agonio, disturbances are more wavelike, propagating on Figure 1 shows the track and intensity of Diana and a radially distributed vorticity gradient. It is not cur- the pre-Diana disturbance. Diana initially moved west- rently known which perspective more accurately re¯ects ward and featured a highly asymmetric distribution of nature. wind and precipitation (as judged from satellite). The storm was named around 1500 UTC 8 September b. Tropical Cyclone Diana (1984) (Lawrence and Clark 1985) even though it still resembled As discussed in detail by Bosart and Bartlo (1991, a baroclinic cyclone (Fig. 8c of BB). By 0000 UTC 9 hereafter BB), Hurricane Diana was one of a relatively September, the storm had deepened to moderate tropical Unauthenticated | Downloaded 09/26/21 04:27 AM UTC AUGUST 2001 DAVIS AND BOSART 1861 storm, had lost most of its baroclinic character, and began neously resolve the core of the storm, including the to exhibit a more axially symmetric pressure and wind eyewall development, and the synoptic scales, including ®eld at the surface. During this period of deepening, the preexisting upper-level trough and low-level baro- Diana drifted slowly northwestward to within 100 km of clinic zone.