10-15% of eye trajectories escape into the eyewall

1. Evolution of Mass Transport between the Eye and Eyewall in Hurricane Guillermo (1997) Matthew D. Eastin1 and Paul D. Reasor2 1Department of Geography and Earth Sciences, University of North Carolina at Charlotte 2Hurricane Research Division, NOAA/AOML Motivation and Objectives Several theoretical, numerical, and observational studies have suggested that coherent mesovortices along the eye-eyewall interface can play a significant role in modulating the azimuthal distribution of eyewall convection as well as overall hurricane intensity (see discussions in Braun & Wu 2007; Bryan & Rotunno 2009; Reasor et al. 2009). Moreover, the export of high entropy air from the low-level eye into the relatively low entropy eyewall can generate/enhance transient, buoyant, convective updrafts. Some studies argue that such entropy injections can produce “superintense” storms in terms of their steady-state maximum potential intensity (Persing & Montgomery 2003; Cram et al. 2007; Bell & Montgomery 2008). In contrast, others argue such injections play a minimal role in determining maximum intensity (Bryan & Rotunno 2009). However, the observational evidence supporting such asymmetric mass transports has been circumstantial; often relying on comparisons between simulations and satellite/radar imagery (Kossin & Schubert 2001) or on thermodynamic comparisons of the low-level eye and the midlevel eyewall (Kossin & Eastin 2001; Eastin et al. 2005; Barnes and Fuentes 2010). The objective of this study is to provide the first observationally-based documentation of asymmetric mass transport across the eye-eyewall interface using a unique set of 20 dual-Doppler wind analyses from Hurricane Guillermo (1997). In particular, we attempt to elucidate the extent, evolution, and impact of mesovortex-induced transport between the low-level eye and eyewall during an initial period of rapid intensification (discussed here) and a second steady-state period near maximum intensity. Eye – Eyewall Boundary Multiple criteria for the eye-eyewall boundary were tested (including various radar reflectivity thresholds, the radius of maximum winds, and the radius of maximum symmetric vertical vorticity) but the criteria that best matched the inner edge of the eyewall cloud [as defined by cloud water content (CWC) observed at flight level (FL) by in situ sensors] was the inner edge of the azimuthal mean updraft exceeding 1 m/s as shown below. Trajectories that crossed this boundary were deemed “escape trajectories”, and they ascended within the eyewall updrafts. Trajectories Seeded in the Low-Level Eye during Rapid Intensification Summary of Common Trajectory Behavior Key to Individual Pass Figures (below) Total percentage of trajectories that escaped the low-level eye as a function of time (or pass) • 10-15% of eye trajectories escape into the eyewall • with more frequent escapes during passes with • stronger mesovortices along the eyewall edge • Escape trajectories originate within 8 km of the • eye-eyewall boundary • A strong spatial correlation exists between escape • location and mesovortex location (escapes often • occur just upwind of strong mesovortices) • A strong spatial correlation exists between the • eyewall updrafts and the escape trajectories • (eye air often rises through strongest updrafts) Top Panels: Seed locations (2 km) in eye (gray circles) Seed locations of escapes (red circles) Escape locations (blue circles) Eyewall locations (6 km) of escapes (green +) Mesovortex locations (purple asterisks) Bottom Panels: Low-level asymmetric vorticity (shading) [0.5, 1.0, 1.5, 2.0, 2.5, 3.0 ×10-3 s-1] Low-level asymmetric winds (vectors) Mid-level vertical motion (blue contours) [updrafts at 3, 4, 5, 6, 7, 8 m/s] Symmetric vorticity maximum (pink ring) Range rings at 20 and 30 km (black) Percentage of trajectories that escaped the low-level eye as a function of radius from the inner eyewall edge Total Escape Percentage (%) Escape Percentage (%) Inner edge of the azimuthal mean w > 1.0 m/s Pass Number Radius from Inner Eyewall Edge(km) Pass 02 – 1933 UTC Pass 03 – 2001 UTC Pass 04 – 2042 UTC Pass 05 – 2117 UTC Altitude (km) Pass 01 – 1855 UTC Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Radius (km) Distance from center (km) Sample Escape Trajectories Show below are 16 sample escape trajectories during Pass 01. All seed locations at z = 2.0 km within the eye are denoted by gray circles. Of those trajectories which escape the eye below 4 km, their seed location (red circles), escape location (blue circle), and eyewall location at 6 km altitude (green “+”) are shown. The locations of coherent low-level mesovortices are denoted by purple asterisks. A similar display format is used in the summary figures for each pass (shown to your right). Notice how the escape locations are spatially linked to the mesovortices and how each trajectory rotates around the eyewall during their ascent Methods and Definitions The Doppler radar analyses and wind field decomposition for each of the 10 passes on 02 August 1997 are processed following the methods outlined in Reasor et al. (2009). Trajectories are seeded at 2.0 km altitude throughout the eye during each pass and are calculated forward for 3-h using a 10-s time step and a second-order Runga-Kutta advection method. The eye diameter was defined by the inner edge of the azimuthal mean vertical motion exceeding 1.0 m/s (see adjacent box). Since the average time elapsed between passes was ~35 min, the wind analyses were not linked to provide structural evolution. Rather, the wind field of each pass is treated as a unique case. Structural evolution is broadly accounted for by assuming all convective features (i.e., updrafts and mesovortices) translate at 75% of the local azimuthal mean horizontal wind, and trajectory advection is adjusted accordingly. Low-level mesovortices are defined as coherent with asymmetric vertical vorticity features exceeding 1.0 ×10-3 s-1 in the 1-3 km layer. Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Pass 06 – 2154 UTC Pass 07 – 2225 UTC Pass 08 – 2258 UTC Pass 09 – 2333 UTC Pass 10 – 2404 UTC Distance from center (km) Distance from center (km) Altitude (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Radius (km) References and Additional Reading Barnes, G. M., and P. Feuntes, 2010: Excess eye energy and the rapid intensification of Hurricane Lili (2002). Monthly Weather Review, 138, 1446-1458. Bell, M. M., and M. T. Montgomery, 2008: Observed structure, evolution, and potential intensity of Category-5 Hurricane Isabel (2003) from 12 to 14 September. Monthly Weather Review, 136, 2023-2046. Braun, S. A., and L. Wu, 2007: A numerical study of Hurricane Erin (2001). Part II: Shear and organization of eyewall vertical motion. Monthly Weather Review, 135, 1179-1194. Bryan, G. H., and R. Rotunno, 2009: The influence of near-surface, high-entropy air in hurricane eyes on maximum hurricane intensity. Journal of the Atmospheric Sciences, 66, 148-158. Cram, T. A., J. Persing, M. T. Montgomery, and S. A. Braun, 2007: A lagrangian trajectory view on transport and mixing processes between the eye, eyewall, and environment using a high-resolution simulation of Hurricane Bonnie (1998). Journal of the Atmospheric Sciences, 64, 1835-1856. Eastin, M. D., W. M. Gray, and P. G. Black, 2005: Buoyancy of convective vertical motions in the inner core of intense hurricanes. Part II: Case studies. Monthly Weather Review, 133, 209 – 227. Kossin, J. P., and M. D. Eastin, 2001: Two distinct regimesin the kinematic and thermodynamic structure of the hurricane eye and eyewall. Journal of the Atmospheric Sciences, 58, 1079-1090. Kossin, J. P., and W. H. Schubert, 2001: Mesovortices, polygonal flow patterns, and rapid pressure falls in hurricane-like vortices. Journal of the Atmospheric Sciences, 58, 2196-2209. Reasor, P. D., M. D. Eastin, and J. F. Gamache, 2009: Rapidly intensifying Hurricane Guillermo (1997). Part I: Low-wavenumber structure and evolution. Monthly Weather Review, 137, 603 – 631. Persing, J, and M. T. Montgomery, 2003: Hurricane superintensity. Journal of the Atmospheric Sciences, 60, 2349-2371. Future Work Determine the relative origin of air rising through midlevel eyewall updrafts through backward trajectory analysis Determine the fractions of low-level and midlevel environmental inflow that rises through the eyewall or enters the eye. Examine “residence times” in the low-level eye within the context of a simple entropy budget. Complete all of the above analyses for the second steady-state period near maximum intensity. Explore any sensitivity to the seed altitude. Explore any sensitivity to the assumed translation speed. Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km) Distance from center (km)