Title: Formation of the Hurricane Eye
1Formation of the Hurricane Eye
- Jonathan Vigh
- Colorado State University
- April 24, 2006
Hurricane Emily as viewed from the ISS at 1852
GMT, July 16, 2005.
Image courtesy of the Image Science Analysis
Laboratory, NASA Johnson Space Center.
(ISS011-E-10509)
2Hurricane Wilma, as seen from the International
Space Station 822 AM CDT, 19 October 2005. Wilma
was near peak intensity at this time with a
minimum sea level pressure of 882 hPa and maximum
sustained surface winds of 160 kt. The lower eye
diameter was 2 n mi, a new record for smallest
observed eye. NASA Photo ISS012-E-5241.
3High altitude aerial photograph of Super Typhoon
Ida taken from a U-2 spy plane 25 September
1958. Photo courtesy of Frank Marks
(NOAA/AOML/HRD).
4Philosophical Approach
- Eye formation apparently involves two structural
trends - convection begins to form into an annulus
- central subsidence begins over the storm center
- Goal describe transition from a one-cell vortex
-gt two-cell vortex
From Willoughby (1995) and Palmen and Newton
(1969)
5Formation of Hurricane Michelles (2001) primary
eye, courtesy of Brian McNoldy link
6Photo looking straight up in the eye. Photo by
Scott A. Dommin (Hurricane Hunters)
7Secondary Circulation induced in a balanced vortex
Heat source
Momentum source
From Willoughby (1995)
Heating at RMW
Tangential wind tendency
Momentum at RMW
Heating at 2xRMW
From Shapiro and Willoughby (1982)
Vortex intensity
8Importance of the Boundary Layer
- Eliassen (1971) and Eliassen and Lystad (1977)
- Boundary layer type critically affects the radial
distribution of upward mass flux out of BL top - laminar BL produces radially-constant upward mass
flux near the vortex center - turbulent BL produces a maximum of upward motion
out some radial distance from center
919 Sep 2005 1336 UTC 1136Z recon 997 mb
64 kt flt lvl, N
1019 Sep 2005 1919 UTC 1931Z recon 995 mb
62 kt flt lvl, NE
1119 Sep 2005 2312 UTC 2302Z recon 993 mb
67 kt flt lvl, SW (850 mb)
1219 Sep 2005 2349 UTC 2302Z recon 993 mb
67 kt flt lvl, SW (850 mb)
1320 Sep 2005 0052 UTC 0213Z Recon 992 mb
51 kt flt lvl, SE (700 mb) good banding,
30 eye feature, open SSE-SSW
1420 Sep 2005 0828 UTC 0811Z Recon 988 mb
1520 Sep 2005 1220 UTC 1203Z Recon 985 mb
69 kt flt lvl, E 50 n mi elliptical eye, open
to S
1620 Sep 2005 1323 UTC 1330Z Recon 982 mb
78 kt flt lvl, NW
1720 Sep 2005 1500 UTC 1517Z Recon 980 mb
77 kt flt lvl, NW 40 n mi elliptical eye,
open NW
1820 Sep 2005 1835 UTC 1730Z Recon 976 mb
85 kt flt lvl, W 40 n mi ragged, elliptical
eye
1921 Sep 2005 0154 UTC 0204Z Recon 965 mb
103 kt flt lvl, NE 28 n mi circular eye,
closed wall
2021 Sep 2005 0909 UTC
2121 Sep 2005 1424 UTC 1517Z Recon 934 mb
137 kt flt lvl, E 25 n mi circular eye, closed
wall
2221 Sep 2005 1918 UTC 1936Z Recon 914 mb
(Ext.) 161 kt flt lvl, NE 20 n mi circular
eye, closed wall
23Hurricane Rita, as seen by MODIS at 1920 GMT on
21 September 2005. Best track maximum sustained
winds of 145 kt, minimum central pressure of 914
hPa. Image credit SSEC/Univ. Wisconsin.
24Hurricane Bonnie Hot Tower
- Local strong subsidence can produce a
prototypical warm core (Simpson et al. 1998) - An encircling rainband may also induce
subsidence (Willoughby 1979,1990,1995) - Stochastic in nature
- But once started, positive feedback process may
lock in
From Heymsfield et al. (2001), Fig. 12
25Other possible factors
- Vorticity distribution theories
- Chaotic PV mixing at vortex boundaries leads to
transport barriers in the polar stratospheric
vortex (Mizuta and Yoden 2001 Shuckburgh and
Haynes 2003 Nakamura 2004). - Inertial stability plays a key role in
determining the nature of the vortex scale
subsidence (Schubert and Hack 1982). - Air-sea interaction
- Formation may happen at an air-sea interaction
threshold where sea spray and drag effects
radically change transfer coefficients of heat
and momentum - Waves
- Vortex Rossby Waves transport angular momentum
out of core and in towards the center - Inertia-gravity waves may be absorbed near the
core, contributing to warm core
26Questions
The eyes of Hurricane Wilma at two different
times during the storms lifecycle. Image
courtesy of Scott Bachmeier (CIMSS/SSEC/Univ.
Wisconsin).
27Future Work
- Observational
- Formulate a useful definition for detecting eye
formation - Determine the intensity/size characteristics
- Trace kinematic and thermal structural evolution
- Characterize convective morphology and ecology
- Determine the role of environment in formation
- Diagnose cases of failed eyes in nondeveloping
systems, contrast to successful eye developments - Modeling with WRF and HWRF
- Verify historical theories (symmetric subsidence,
role of BL friction) - Diagnose local descent from hot towers
- Determine threshold of eye formation
- Examine air-sea interaction
- Theory?
28Observational Goals
- Formulate a useful definition for detecting when
an eye has formed. - Determine the intensity/size characteristics of
the initial eye at the time it is first detected
and when it reaches a mature state. - Trace the evolution of the complete wind profile
during eye formation. - Determine the observable internal and external
factors which control initial eye size. - Characterize the convective morphology
(rainbands, convective arcs, convective rings,
isolated cells) and ecology during eye formation
using microwave imagery and space-, land-, and
aircraft-based radar. - Determine the role of the environment during eye
formation by examining the thermodynamic and
angular momentum distribution at the inflow
source radius. - Diagnose cases of failed eyes in nondeveloping
systems and contrast these to successful eye
cases in developing systems. - Strategy
- Ideally, highly observed case studies are
available from coordinated aircraft missions - Synthesize comprehensive case studies from the
diverse data available - Do broader studies of the data to answer some of
the above questions (e.g. intensity threshold at
time of eye formation) - Gain understanding to motivate modeling work and
theory. - Reanalysis/data assimilation approach.
29Modeling Methodology
- Conduct both idealized and real storm studies of
eye formation using Advanced Research WRF (ARW)
and/or HWRF. - Models are new and relatively untried, so initial
work will be to find optimal research
configuration (domain size, lateral boundary
conditions, underlying boundary conditions,
physics options, nesting configuration, and
gridpoint spacing). - Challenges for idealized experiments how to
initialize the model and the correct method of
expressing convective morphology (axisymmetric or
asymmetric, parameterized vs. explicit, thermal
vs. wind asymmetries). - Experiments to determine relationship between eye
formation and initial vortex intensity and size.
Investigate the parameter space involving initial
vortex structure (intensity, wind profile shape,
swirl ratio), physics (precipitation loading and
cloud microphysics effects), convection
(distribution and mass fluxes of updrafts and
downdrafts), and surface properties (transfer
coefficients for drag and heat). - Real storm experiments determine role of
genesis mode in eye formation (a storms
pedigree) and the role of the environment.
Determine what mechanisms prevented eye formation
in past storms. - Evaluation of HWRFs structure and intensity
forecasts during eye formation.
30Model Goals and Diagnostics
- To determine useful measures of the model
vortex's intensity, aspect ratio, swirl
parameter, inflow angle, and inflow source
radius. - To examine the evolution of the momentum, E, and
potential vorticity (PV) fields during eye
formation. - To diagnose the strength of the induced axial
subsidence and determine the contributive
mechanisms (i.e. by comparing the subsidence
predicted from balanced dynamical theory to the
subsidence observed in the model). - To compute Lagrangian parcel back-trajectories
from the eyewall and eye during eye formation.
This should help determine the source
characteristics for these two types of air and
shed light on the mass recycling rates in the
incipient eye (following Cram et al., 2006).
Budgets of angular momentum, thermodynamic
energy, moisture, kinetic energy, and PV of the
eye and eyewall air can then be computed along
the parcel trajectories. - To test the sensitivity of eye formation to
surface fluxes. These fluxes could be held fixed,
or allowed to vary through a coupling of the
air-sea interface.
31Avenues of Inquiry
- What is the most useful way to define the eye?
Are there different types or classes of eyes
(i.e. rapidly rotating vs. weakly rotating)? - What are salient mechanisms and dynamics that
drive a single-cell vortex structure to a
two-cell vortex structure? - What role does central subsidence play in eye
formation? - What forces the subsidence? Can subsidence
trigger eye formation? - Convection must obviously play an important role
in eye formation, but what role? What role does
the convective strength play? The distribution
and concentration of convective elements and
their radial distance from the storm center?
Their morphology and geometrical arrangement into
rings, spiral bands, or clusters? - It is also believed that friction plays a
critical role in eye formation. What is that
role? As the storm intensifies, are feedbacks
between the sea state and the resulting
frictional drag exerted on the atmosphere
important for eye formation? What are the
microphysical effects of increasing sea spray on
the storm's cloud and precipitation microphysics? - What role does intensification play during eye
formation? Is eye formation an instability
process triggered at an intensity threshold? If
so, what is the nature of the trigger and the
actual intensity threshold for eye formation?
What is the least intense tropical cyclone to
sport a bona de eye? What is the most intense
storm to not possess a clearly defined eye?
32- Is eye formation a bifurcation phenomenon, with
multiple states of equilibria? If so, what
mechanisms are responsible for pushing a storm
back and forth between equilibria states? Can a
phase space diagram be constructed for eye
formation? What are the relevant parameters of
this phase space? - Are there multiple dynamical pathways to eye
formation, or do all intensification routes lead
to one common eye formation pathway, perhaps
dictated purely by geometry and friction? - If there are multiple modes of eye formation,
which one is optimal for the greatest
intensification rate? How much of the storm's
actual realized maximum intensity depends on the
storm's initial structure and the route it
followed to get there? - What are the relationships between overall storm
size and initial eye size? Eye size and
intensification rate? Eye size and a storm's
ultimate realized intensity? What role does the
environment play in these relationships? - How much of constraint does initial structure
place on the final mature structure? - What role do asymmetries play in eye formation?
At what threshold do asymmetric mixing processes
become important as the storm strengthens? - What is the exact role of gravity waves during
eye formation? Of vortex Rossby waves? - What determines the eye shape and eyewall slope
in real storms? How is eye shape affected by
intensity or rate of intensification? Movement?
Shear? - Why do some storms rapidly intensify as they form
eyes, yet others do not? Are there commonalities
in the developing eyes of storms which
subsequently undergo rapid intensification in the
hurricane stage? - What role does eye and eyewall buoyancy play
during eye formation?
33Willoughby (1995)
Willoughby (1995)
34Hurricane Inez (1966)
From Hawkins and Imbembo (1976)
35Hawkins and Imbembo (1976)
36Hawkins and Imbembo (1976)
37lt- Response to heat source centered at the
radius of maximum winds in a baroclinic vortex
Response to a momentum source -gt
Shapiro and Willoughby (1982)
38- Shapiro and Willoughby (1982) find that the
vortex is sensitive to the location of heat and
momentum sources, with the largest vortex
response occurring when sources are located at or
just inside the RMW - Character of vortex response to symmetric forcing
depends critically only on the vortex intensity
(not aspect ratio) - At vmax gt 35 m s-1, the character of the response
changes dramatically due to increased inertial
stability in the core and recirculation inside
the RMW forms an eye - Theory describes vortex response to given
forcings at various vortex intensities, but does
not explain initial vortex structure evolution - Does not include effects such as eddy momentum or
cumulus momentum fluxes, boundary layer effects
Shapiro and Willoughby (1982)
39Schubert and Hack (1982)