Mesoscale Structures in Tropical Cyclones
John Knaff
NOAA/CIRA
Colorado State University
Outline
• Definitions
• Eyewall region
– Symmetric Structure
– Replacement Cycles
– Asymmetric Aspects
• Rainbands
– Structure
– Location
• Convective Asymmetries
• Diurnal Convective Oscillations
RAMMT/CIRA
Definitions
Core - The inner 100-200 km of a tropical cyclone where
flow is dominated by the swirling motion of the tropical
cyclone where local Rossby numbers are always greater
than unity, inertial stability is very strong, and
convection dominates.
Rainbands - Spiral-shaped patterns of cloud or
precipitation. Such bands are typically 5 to 50 km wide
and spiral in toward the center over a radial distance of
100 to 300 km.
RAMMT/CIRA
Definitions (Cont.)
Convective Rings - Rain bands that completely encircle the
cyclone center, although not at a constant radius.
Eyewall - is the innermost convective ring that surrounds
the almost circular, clear eye region.
Principle Band or Stationary Band- A quasi-stationary
rainband which is located at the outer edge of the core
region and whose location is determined by the relative
flow around the tropical cyclone.
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Tropical Cyclone Core Region
Scale Perspective
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Structure Near the Core (the primary circulation)
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Eyewall Symmetric Structure
WMO 1992
From the Sawyer-Eliassen equation: Which states that the secondary
circulation is determined by the forcing (heat and momentum fluxes), the
inertial frequency, static stability, and baroclinicity of the vortex for cases
when the equation is elliptic or when (4AC-B2 > 0), where A is the Brunt-
Vaisala frequency squared, B is the baroclinicity, and C is the inertial
frequency.
Ad2y/dy2 + Bd2y/dyp + Cd2y/dp2 = Forcing
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Examples of Eyewall Cross Sections
M.Black 1999
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Eyewall Cycles and Replacement
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Causes
• The cause of the formation of the initial convective ring
that becomes the outer eyewall is yet unclear.
• Once formed the outer eyewall move inward because the
inertial stability is dictating the shape of the secondary
circulation causing the greatest adiabatic
sinking/warming and hydrostatic change in heights at or
just inside the radius of maximum wind. This
hydrostatic change cause wind speed increase inward the
radius of maximum wind.
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Instabilities Associated With Wind Maxima
• The necessary conditions
for combined
baroclinic/barotropic
Instability often occurs
with wind maxima in
tropical cyclones dq/dy
changes sign, where q is
potential vorticity and y
is in this case radius.
Kossin et al. 1999
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What questions can be answered using barotropic
models
• What happens to the inner eyewall?
• What causes of elliptical eyewalls?
• Why some secondary wind maxima appear
stable?
Note: These calculations cannot not account for
diabatic effects and vertical structure.
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Inner Eyewall Dissipation
Kossin et al. 1999
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Elliptical and Polygonal Eyewalls
Kossin et al. 1999 Schubert et al. 1999
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Stable Secondary Wind Maxima
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Eyewall Mesovorticies
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Associated Track Variations
Black et al. 1999
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Rainbands
General Structure: Propagating Bands
Powell 1989
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Rainband Kinematics
Powell 1989
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Similarities to Tropical Squall Lines
• Convection is orientated perpendicular to low-level crossband
vertical shear vector.
• Stratiform rain extends mostly down shear above 4km.
• Maximum low-level convergence and barrier effects are at the
mesoscale updraft position with pressure minima produced by
hydrostatic and dynamic effects.
• Warm moist, low-level inflow can be supplied from the relatively
undisturbed environment.
• Surface cold pools containing theta-E decreases of 10 -20 K from
the environment can force the mesoscale updraft.
Powell 1990
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Differences from Tropical Squall Lines
• Updraft of the rainband are on the upshear side
of the axis. Cells of convection propagate along
band producing outflow then can generate new
cells along band.
• Propagation can be inward, outward and along
band or bands can remain stationary.
Powell 1990
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The Effect of Rainbands on the Boundary Layer
• Downdrafts cool and dry the boundary layer
outside the core region.
• This defies conventional wisdom that air spirals
into the core isothermally with an air sea contrast
on about 1 degree Celsius.
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Observed Boundary Layer Cooling
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Boundary Layer Drying
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Fluxes
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Stationary Bands
Storm is moving toward the west with
westerly vertical shear. Low level flow
is through the storm from east to west.
The Stationary Band Complex (or
principle band) develops on the edge of
the core region where the inertial
stability of the core forces the
environmental flow around the center of
the storm, causing convergence and
vertical motion along the boundary of
the core.
RAMMT/CIRA
Convective Asymmetries
• Translation of a vortex results in asymmetric
frictional drag, winds and convergence
• Vertical Wind Shear or relative flow moving
across the vortex core produces asymmetric
advection of vorticity, likely resulting in
asymmetries in divergence.
– Beta Gyre result in a low level relative flow
– Environmentally induced relative flow
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Relative Wind
Convergence Divergence
Z
Divergence Convergence
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Translation induced asymmetries
Taken from Shapiro ( 1983)
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Beta Gyre induced relative flow
Taken
From
Bender (1997)
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Beta (cont.)
Taken From Bender (1997)
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Environmentally induced relative flow
Easterly basic current
Vertical with 3 m/s westerly
Velocity shear.
950 mb
(Frank and Ritchie 1997)
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Procedure
• Individual images are placed in a storm relative,
rotated, centered format
– Rotated with respect to the direction of
motion (upward)
– Radar projection (equal area)
– Centered
• Images are then averaged over a six hour period
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Hurricane Bonnie 23 August 12Z
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Hurricane Georges 20 Sept 12Z
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Hurricane Danielle 1 Sept 00 Z
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Typhoon Todd 1 Sept 6Z
Typhoon Todd Sept 17, 1998
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Diurnal Oscillations of Deep Convection
in Tropical Cyclones
Observations
• A maximum in deep convection is observed in
the early morning (local).
• The amplitude varies with convective
organization.
• Oscillations are not related to the semi-diurnal
pressure tide.
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Tropical Cyclone Examples
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Theories
• Direct radiation-convective interaction: During the day,solar
warming of the convectively produced cloud shields acts to stabilize
the atmosphere through reducing the lapse rates. At night the lapse
rate become more favorable due to radiative cooling to space.
(Randel et al. 1991).
• Cloud-cloud-free radiation difference: Emphases the dynamic
consequences of the differential radiative heating over the
convective region and over the surrounding less cloud or clear
regions. The net radiative cooling in the upper levels is less over
cloudy regions during the day and greater at night compared to the
surrounding environment. This situation is reversed at the lower
levels. This situation produces variations in the daily convergence
fields and thus in convective activity (Gray and Jacobson, 1977)
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