Doughnut Hurricanes

1
Doughnut Hurricanes

• Presented by
• John Knaff
• CIRA
• January 29, 2001

Research in Cooperation with Jim Kossin, Mark
DeMaria and Vince Larson
2

• IR imagery is the visual means by which we
  determine whether a storm is a doughnut
  hurricane.
• A hurricane is identified as a doughnut if
• the hurricane persists for at least three hours
  in an axisymmetric state
• the hurricane has a normal-to-large-sized
  circular eye surrounded by a single band of deep
  convection containing the inner core region and
• the hurricane has little or no rainband structure
  as determined from IR imagery.

3
(No Transcript)
4
What environmental conditions are associated with
formation and existence of doughnut hurricanes?
5
Summary of Environmental Conditions
6
Characteristic Environment

• Weak east southeasterly vertical wind shear
• SSTs ranging from 25.5 to 28.5 C, decreasing or
  remaining constant.
• Located under an upper-level ridge with no
  evidence of trough interaction.

7
How are doughnuts formed?
8
From Seasonal Reports and Preliminary Reports

• Almost all doughnuts formed after peak intensity,
  except for the Atlantic systems which moved into
  warmer water and westerly wind shear later in
  there evolution.
• All storms had an eyewall rearrangement just
  before the doughnut phase, in which a smaller
  asymmetric eye was replaced by a larger circular
  eye.

9
(No Transcript)
10
Howard
11
Beatriz
12
Darby
13
Dora
14
What is the structure of doughnut hurricanes?
15
Hurricane Luis (1995) 2200 UTC 3 September -
0400 UTC 4 September
16
Hurricane Dora (1999) 0200 UTC 15 August - 0900
UTC 15 August
17
Vorticity maximum (updraft) near the radius of
maximum wind
18
Vorticity maximum (updraft) near the radius of
maximum wind
19
Vorticity maximum (updraft) inside the radius of
maximum wind
20
Vorticity maximum (updraft) inside the radius of
maximum wind
21
Can the study of doughnut hurricanes help us
better understand and predict the intensity of
non-axisymmetric tropical cyclones?
22
Implications of Doughnut Hurricanes

• Potential Maximum Intensity vs. Maximum Potential
  Intensity
• Spiral Bands and Inner Core dynamics
• Intensity Forecasting

23
Observations of Doughnuts and MPI Theory

• Observations of MPI is shown to flatten off in
  very warm SSTs
• Eyewall cycles only occur over very warm (28.5
  C) water
• Doughnuts stabilize at 85 MPI wrt SST

• Doughnuts are observed Stable wrt Barotropic
  instabilities after a dramatic transformation
  likely associated with eyewall mixing
• Eyewall mixing is necessary for intensification
  (Emanuel 1997)

24
MPI vs PMI speculation

• There exists a thermodynamic regime where
  tropical cyclone maximum intensity is controlled
  solely by the thermodynamics or MPI this where
  doughnut hurricanes are observed to occur
• There exists a dynamic regime where tropical
  cyclone maximum intensity (observed) is
  controlled by internal dynamics this is where
  eyewall cycles are observed to occur

25
Spiral Bands?

• Doughnut hurricanes have a lack of significant
  spiral bands and are stable to barotropic
  instability in the inner core region.
• These same storms during the pre-doughnut period
  show significant banding.
• Doughnuts are still convectively active.

26
Implication wrt to spiral bands

• Significant spiral bands are likely the result of
  PV wave (vortex Rossby wave) activity (Guinn and
  Schubert (1993)
• That spiral band activity may represent a proxy
  for eyewall to eye inward mixing by asymmetric
  eye contraction. (see Schubert et al. 1999 KE and
  angular momentum conservation)

27
Importance of southeasterly vertical wind shear

• Suggests that asymmetries are related to vertical
  wind shear.
• Southeasterly shear offsets the effects of
  differential vorticity advection by the
  baroclinic vortex (Beta effect).

28
Beta Gyre induced relative flow
Taken From Bender (1997)
29
Beta (cont.)
Taken From Bender (1997)
30
Intensity Characteristics of Doughnuts

• Long-lived intense tropical cyclones
• Do not fill rapidly after maximum intensity
• Intensity is very close to 85 MPI wrt SST.
• Have a specific environment (decreasing SST, weak
  southeasterly vertical wind shear, and no trough
  interaction)

31
(No Transcript)
32
(No Transcript)
33
Nature produces nearly symmetric hurricanes that
have been shown to have wind and vorticity fields
that are barotropically stable. These doughnut
hurricanes form in environments characterized by
weak easterly or southeasterly vertical wind
shear, and where SSTs are less than 28.5oC.
They form after a vorticity mixing event
(eyewall replacement cycle, or eyewall
rearrangement) and as they move into cooler or
constant SST conditions. Once doughnut
hurricanes have formed, they can maintain their
doughnut shape for days if the environmental
conditions include easterly wind shear and
decreasing or constant SSTs greater than 25.5 oC.
Doughnut hurricanes are very intense roughly
85 of their MPI with respect to SST.
34
Doughnut hurricanes offer a unique opportunity to
study a simple version of a hurricane, and just
like simple modeling studies, give valuable
insights into how nature operates. The
observation of doughnut hurricanes has several
implications that are germane to basic research
and forecasting.