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Inflow Layer Energetics of Hurricane Bonnie (1998) Near Landfall

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Inflow Layer Energetics of Hurricane Bonnie (1998) Near Landfall 1600 MONTHLY WEATHER REVIEW VOLUME 131 In¯ow Layer Energetics of Hurricane Bonnie (1998) near Landfall DEREK R. WROE AND GARY M. BARNES University of Hawaii at Manoa, Honolulu, Hawaii (Manuscript received 10 December 2001, in ®nal form 3 January 2003) ABSTRACT On 26 August 1998, a NOAA WP-3D aircraft executed a curved track that mimics an in¯ow trajectory to the eyewall of Hurricane Bonnie. Global positioning system (GPS) sondes and airborne expendable bathyther- mographs jettisoned along the trajectory provide the observations to conduct an energy budget for the 1600-m- deep in¯ow to the eyewall. Surface ¯uxes are estimated via the bulk aerodynamic equations and the ¯ux at the top of the in¯ow is solved as a residual. From 170- to 125-km radial distance from the circulation center the mean ue of the in¯ow remains constant despite combined sensible and latent surface ¯uxes in excess of 500 W m 22. Convective cells remove energy from the in¯ow boundary layer at a rate similar to the inputs from the sea. From 125 to 100 km, in the annulus adjacent to the eyewall, mean ue increases 4.5 K in response to higher surface ¯uxes and little loss through the in¯ow top. Energy balance may be achieved by either entrainment of higher ue through the top of the in¯ow layer, or by inclusion of just half the estimated heat from viscous dissipation. The authors infer that the secondary circulation of the eyewall inhibits convective cells from forming in this region and thus facilitates the rapid increase of energy in the in¯ow. The results support hypotheses that hurricane intensity appears to be strongly modulated by energy exchange in a meso-b region adjacent to and under the eyewall. 1. Introduction vides an unprecedented, detailed portrayal of the in¯ow layer to the eyewall of Hurricane Bonnie (1998). Where and how does the in¯ow boundary layer to an eyewall acquire the requisite additional energy to create and sustain a tropical cyclone? Impediments to an an- a. Importance of in¯ow energy content swer include the challenging measurement conditions associated with a hurricane. What few observations we Riehl (1954), Malkus and Riehl (1960), Palmen and do have come from either providentially sited buoys Newton (1969), and Emanuel (1986) deduce that a trop- (e.g., Cione et al. 2000) or carefully executed aircraft ical cyclone (TC) cannot be sustained by simply using experiments (e.g., Jorgensen 1984; Black and Holland the ambient convective available potential energy 1995). Since Hurricane Hugo (1989), low-level pene- (CAPE). Additional energy, extracted from the sea, is trations of the eyewall of high-category hurricanes by needed to drive the TC heat engine (e.g., Riehl 1951, manned reconnaissance is not considered prudent. This 1954; Emanuel 1986). This energy enters into the TC has left the meteorological community with an impaired via the in¯ow boundary layer. The importance of this view of the thermodynamics affecting hurricane inten- additional energy is highlighted in the successful inten- sity. si®cation and maintenance of simulated TCs initiated in The global positioning system (GPS) dropwindsonde an environment devoid of CAPE (Rotunno and Emanuel (sonde), developed by the National Center for Atmo- 1987). Observations verify that CAPE is reduced as air spheric Research, the National Oceanic and Atmospher- ¯ows to the eyewall (Bogner et al. 2000); this is partly ic Administration (NOAA), and the German Aerospace due to a cooling of the in¯ow temperature (Korolev et Research Group, has become available for hurricane re- al. 1990; Cione et al. 2000; Barnes and Bogner 2001), search (Hock and Franklin 1999). With a 2-Hz sampling and partly due to warming aloft (Jordan and Jordan rate, the GPS sonde delivers 7-m vertical resolution of 1954; Sheets 1969; Frank 1977). kinematic and state variables from shortly after the The in¯ow layer acquires the additional energy epi- sonde is jettisoned to the sea surface. A novel ¯ight sodically. One may infer this from the height±radius pattern has been used to deploy these sondes and pro- cross sections of equivalent potential temperature (ue) (Hawkins and Imbembo 1976; Jorgensen 1984). These cross sections show radial swaths with little change in Corresponding author address: G. M. Barnes, Dept. of Meteorol- ogy, University of Hawaii at Manoa, 2525 Correa Rd., Honolulu, HI ue, and other swaths, usually adjacent to the eyewall, 96822. where ue increases rapidly (Fig. 1). Simulations of an E-mail: [email protected] axisymmetric TC by Rotunno and Emanuel (1987) also q 2003 American Meteorological Society Unauthenticated | Downloaded 09/30/21 04:02 AM UTC AUGUST 2003 WROE AND BARNES 1601 FIG. 1. Radial cross section of ue in Hurricane Inez (1966). [From Hawkins and Imbembo (1976).] show little gain in ue in certain radial bands despite high is often not uniform spatially, contrary to what was surface ¯uxes. Entrainment of dry air from above, or assumed in early energy budgets (Hawkins and Rubsam convective exchanges, drain the in¯ow layer of any na- 1968; Hawkins and Imbembo 1976). Cool wakes, iden- scent surplus of energy, countering the surface ¯uxes. ti®ed with airborne expendable bathythermographs Barnes et al. (1983) and Powell (1990a,b) indicate that (AXBTs) may result in little or no sensible energy being convectively active rainbands can tap the energy-rich transferred to the in¯ow layer (Black and Holland 1995), in¯ow and replace it with low-ue downdraft air. Betts while warm eddies may enhance the ¯uxes (Shay et al. and Simpson (1987) concur that losses from convective 2000). transports can overwhelm gains from surface ¯uxes. Bister and Emanuel (1998) discuss the possibility that They argue that to obtain an increase in the ue of the viscous dissipation of turbulent kinetic energy in the in¯ow and develop a more intense hurricane, the evap- surface layer could be an additional heat source for the oration in the subcloud layer and upward ¯uxes at cloud TC. Maximum dissipative heating would tend to occur base need to be minimized. The ®ndings support the in the high wind regime near and under the eyewall, argument that energy ¯ux divergence of the in¯ow layer where rain and spray would be available to evaporate, changes sign over small spatial or correspondingly short perhaps resulting in a latent energy input into the TC. temporal scales. The factors contributing to the episodic A critical point is that the stress expended in wave pro- increases in the ue of the in¯ow layer are only partially duction would not be available for dissipative heating. documented. Energy contributions to the in¯ow may come from Much of the uncertainty deals with the air±sea inter- the layer above. Simulations by Anthes and Chang face. Sensible and latent heat ¯uxes through the ocean (1978) and a budget analysis by Frank (1984) show that surface increase markedly near the eyewall in response the entrainment of high potential temperature (u) can to the escalating wind speeds (Riehl and Malkus 1961; contribute to the maintenance of an isothermal subcloud Hawkins and Rubsam 1968). Spray, which becomes layer. Barnes and Powell (1995) describe conditions ra- ubiquitous in high winds, complicates the situation dially outward of a strong rainband where higher ue (Fairall et al. 1994; Andreas and DeCosmo 1999; Wang overlays the in¯ow layer; shear-induced entrainment led et al. 2001; Andreas and Emanuel 2001). Most re- to increases in the energy content. The results support searchers expect a radical change in the magnitude and the hypothesis that rainbands can help as well as hinder distribution of the ¯uxes once the atmospheric surface the stoking-up process needed to drive the TC heat en- layer is ®lled with spray droplets. gine. The sea surface temperature (SST) ®eld under the TC The prior observational work (Malkus and Riehl Unauthenticated | Downloaded 09/30/21 04:02 AM UTC 1602 MONTHLY WEATHER REVIEW VOLUME 131 1960; Riehl and Malkus 1961; Miller 1962; Hawkins and Rubsam 1968; Hawkins and Imbembo 1976; Frank 1984; Powell 1990b) established methods to estimate an energy budget, but were often limited by one or more constraints that included single-layer sampling, incom- plete knowledge of the SST ®eld, no radar to assess convective activity, and limited vertical resolution. The preponderance of the evidence supports the argument that energy increases rapidly over a short distance ad- jacent to the eyewall. A wide range of surface ¯uxes have been offered, and ¯uxes at the top of the in¯ow, in the form of convective overturning or entrainment from the adjacent layer, appear to play a vital role in the energy content of the in¯ow, and ultimately hurri- cane intensity. b. Goals GPS sondes deployed in Hurricane Bonnie are used to 1) identify the thermodynamic and kinematic struc- tures of the in¯ow layer, 2) determine changes in the energy content of a column of air along an in¯ow tra- jectory, and 3) estimate the energy ¯uxes necessary to achieve balance. Our focus is on the entire low-level FIG. 2. Portions of the aircraft ¯ight pattern (dotted lines) executed in¯ow layer, not just the mixed layer. Based on the wind to obtain the three sampled trajectories. GPS drops are shown (*), ®elds adjacent to and in the eyewall (Jorgensen 1984; and the resultant SST from AXBTs is displayed in 8C. The HRD track of the circulation center of Bonnie is the dashed line. Marks et al. 1992) we assume that the entire in¯ow is processed by the eyewall. Turbulence in the eyewall updraft will homogenize the ue of this in¯ow.
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