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4B.6 Evolution of Low-Wavenumber Vorticity During Rapid Intensification: a Dual-Doppler Analysis

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4B.6 Evolution of Low-Wavenumber Vorticity During Rapid Intensification: a Dual-Doppler Analysis 4B.6 EVOLUTION OF LOW-WAVENUMBER VORTICITY DURING RAPID INTENSIFICATION: A DUAL-DOPPLER ANALYSIS Matthew D. Eastin∗1, Paul D. Reasor2, David S. Nolan3,Frank D. Marks Jr.4, John F. Gamache4 1 Department of Math and Computer Science, Central College, Pella, Iowa 2 Department of Meteorology, The Florida State University, Tallahassee, Florida 3 Rosentiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 4 NOAA/AOML Hurricane Research Division, Miami, Florida 1. INTRODUCTION The azimuthal distribution of deep convection in Table 1: Dual-Doppler composite times for the 10 a tropical cyclone inner core often consists of mul- passes through Hurricane Guillermo on 02 August tiple transient convective cells superimposed upon 1997 and the NOAA WP-3D aircraft (N42RF and a more slowly-evolving low-wavenumber (symmetric N43RF) flight tracks across the inner core. and asymmetric) structure. Numerous observations Composite Time N42RF track N43RF track and numerical simulations suggest such distributions Pass (UTC) (3.0 km) (5.8 km) are intimately linked to the environmental vertical 1 1845-1905 E to W N to S wind shear and internal dynamical processes (Rea- 2 1925-1944 S to N E to W sor et al. 2000; Black et al. 2002; Corbosiero et al. 3 1953-2010 NW to SE 2006; Kossin and Schubert 2001; Braun 2002). Un- 4 2033-2052 NE to SW derstanding the dynamics of the asymmetries is of 5 2106-2129 SE to NW SW to NE fundamental importance due their influence not only 6 2144-2159 SW to ENE NW to SE on storm structure, but also storm motion (Nolan 7 2215-2234 NNE to SSW et al. 2000) and intensity (Moller and Montgomery 8 2249-2307 SE to NW 2000). The vertical shear will induce a wavenumber- 9 2324-2341 SW to NE SE to NW one asymmetry with enhanced convergence and as- 10 2354-2412 NW to SE SW to NE cent downshear (Frank and Ritchie 1999, 2001). The higher-order asymmetries appear to be related to in- ternal dynamics. In particular, intensifying tropical cyclones often exhibit an annular tower (or ring) of 60 vertical vorticity with enhanced values in the eye- Wavenumber 0 wall and relatively depressed values in the eye and 55 Wavenumber 1 Wavenumber 2 outside the eyewall (Kossin and Eastin 2001). Schu- 50 Wavenumber 3 bert et al. (1999) showed that such vorticity distri- ) Wavenumber 4 -1 45 butions are barotropically unstable whereby coun- s 4 terpropagating vortex Rossby waves (VRWs) could 40 (10 become phase-locked, grow in amplitude, and even- ζ 35 tually break-down into coherent pools of vorticity, or "mesovortices". Depending on the initial struc- 30 ture of the vorticity ring, the mesovortices either 25 merge and relax to a monopole, or remain distinct and form a quasi-steady rotating lattice (Kossin and 20 Schubert 2001). In either case, the winds associated 15 with the mesovortices exchanged considerable mass Mean Magnitude of 10 between the eye and eyewall. Eastin et al. (2005) recently presented observational evidence suggest- 5 ing the mesovortices may help generate asymmet- 0 ric buoyant convection via the transfer of high en- 1 2 3 4 5 6 7 8 9 10 tropy air from the low-level eye into the relatively Pass Number low-entropy eyewall, and thus contribute to storm evolution. Unfortunately, previous attempts to doc- ument the structure and evolution of these Figure 1: Azimuthal mean magnitude of the dual{ Doppler-derived vertical vorticity averaged within ∗ the 10-25 km annulus between 1-3 km altitude of Corresponding Author Address: Matthew D. each composite wind field. Shown are the symmet- Eastin, Department of Math and Computer Science, ric and azimuthal wavenumbers 1-4 contributions. Central College, 812 University, Pella, IA 50219; email: [email protected] eye-eyewall mesovortices using Doppler radar were pass following Gamache (1998). The wind fields limited by insufficient data within the eye and large were decomposed into azimuthal mean and per- temporal gaps between composite wind fields. The turbation components following methods similar to objective of this study is to document the the Reasor et al. (2000). In contrast to previous three-dimensional structure and evolution of low- Doppler radar studies of intense tropical cyclones, wavenumber vorticity in a rapidly intensifying tropi- shallow convection and light precipitation within cal cyclone through the aid of a unique dual-Doppler Guillermo's eye has permitted unprecedented wind dataset with considerably fewer limitations. field documentation throughout a large fraction of the low-level eye. 2. DATA AND METHODS 3. RESULTS AND DISCUSSION Two NOAA WP-3D aircraft observed the inner core of Hurricane Guillermo between 1800 and 2400 Figure 1 shows the azimuthal mean magnitudes UTC on 2 August 1997. During this period, the hur- of the symmetric and azimuthal wavenumbers 1-4 ricane was moving westward over > 29◦C waters and components of vorticity within the 10-25 km annulus intensifying at an average rate of 2.4 mb hr−1 from between 1-3 km altitude for each pass, encompass- an initial central pressure of 959 mb. Dual-Doppler ing the mean eye-eyewall interface region inward of velocity data was collected for ten passes through the the radius of maximum wind (RMW). The symmet- inner core at 3.0 and 5.8 km altitude (Table 1). Dur- ric component of vorticity dominated the region at ing dual-aircraft passes, the upper aircraft employed all times and tended to increase with time. However, the fore-aft scanning technique (FAST) while the during two periods (Passes 2-3 and 6-7) the symmet- lower aircraft used a normal-plane scanning strategy. ric component decreased while higher wavenumber During single-aircraft passes the radar scanned in components increased in magnitude. FAST mode. Unique three-dimensional wind fields Figures 2 and 3 show the azimuthal mean (or were constructed for each symmetric) vorticity profiles at 3 km altitude for 60 60 Pass 1 FL Mean Pass 6 FL Mean 40 DD Mean 40 DD Mean 20 20 60 60 Pass 2 Pass 7 40 40 20 20 60 60 Pass 3 Pass 8 ) ) -1 40 -1 40 s s 4 4 20 20 (10 (10 ζ ζ 60 60 Pass 4 Pass 9 40 40 20 20 60 60 Pass 5 40 40 20 20 Pass 10 0 0 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 Radius (km) Radius (km) Figure 2: Azimuthal mean vorticity at 3 km altitude Figure 3: Azimuthal mean vorticity at 3 km altitude for Passes 1-5 computed from the flight-level data for Passes 6-10 computed from the flight-level data (black) and dual-Doppler composite winds (red). (black) and dual-Doppler composite winds (red). each pass, computed from both the Doppler wind hurricane throughout the observation period. Such fields as well as flight-level data. Despite the reso- shear resulted from 3-4 m s−1 southeasterly low- lution differences (0.5 and 2.0 km) and flight-level level flow and 4-5 m s−1 northeasterly upper-level means comprised of only two radial legs, the respec- flow. Note the similarity between the mean dual- tive profiles show good agreement at each time. Ini- Doppler-derived vector and the estimated shear from tially (Pass 1), the symmetric structure consisted the SHIPS predictor database (DeMaria and Ka- of a ring with maximum values in the eyewall (at 25 plan 1999). During each pass, low-level convergence km). During the next three passes, the eyewall max- maxima were often located in the southern eyewall imum decreased while the vorticity along the eye- (downshear) with upward vertical velocity (and eyewall boundary (10-20 km) generally increased. A smaller increase was also apparent outside the ini- tial maximum (near 30 km) by Pass 4. Such evolu- tion is consistent with a breakdown of the vorticity ring into mesovortical structures as discussed above. During Passes 4 and 5 the mean eyewall vertical ve- locity significantly increased as a result of several deep convective bursts (not shown). Concurrently, a pronounced ring of enhanced symmetric vorticity re-emerged in the eyewall, reaching a maximum dur- ing Pass 6. Over the course of the the next four passes (7-10) the eyewall maximum decreased while B C vorticity inside the eye and just outside the eyewall increased. Again, such evolution is consistent with a vorticity ring breakdown into mesovortical struc- tures. However, unlike the previous apparent break- A down, the profile relaxed to a monopole (Pass 10). Distance from Center (km) Kossin and Eastin (2001) noted that such transi- tions frequently occurred after maximum intensity, marking the end of intensification, but they did not find robust evidence of episodic transitions during in- a tensification. Clearly, the Guillermo profiles depict an evolving symmetric (and asymmetric) vorticity structure during rapid intensification. Estimates of the environmental vertical wind shear vector computed as the difference in mean wind (within 50 km from the center) between the 8-10 km and 1-3 km altitude layers are shown in Fig. 4 for each pass. A persistent 6-7 m s−1 north- northwesterly vertical shear was impinging on the 0 B C 1 Pass 1−10 2 Mean 3 SHIPS 4 A 5 Distance from Center (km) 6 7 8 9 Shear Magnitude (m/s) 10 b 11 11 10 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 Distance from Center (km) Shear Magnitude (m/s) Figure 5: (a) Total perturbation vorticity (black) and Figure 4: Estimates of the environmental vertical (b) azimuthal wavenumber 1 (green), wavenumber wind shear vector computed from each Doppler wind 2 (blue), and wavenumber 4 (red) components of field (black arrows) as the difference in mean wind vorticity at 2 km altitude for Pass 2.
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