Transcript Slide 1

Convective Oscillations in a
Strongly Sheared Tropical Storm
Jaclyn Frank and John Molinari
The University at Albany, SUNY
Introduction
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Tropical cyclones usually do not form or intensify
in vertical shear greater than ~12.5 ms-1.
Tropical Storm Edouard formed off the coast of
Florida in 2002 and reached a peak intensity of
55 kt despite vertical shear in excess of 13 ms-1.
Although shear increased with time to well over
20 ms-1, Edouard maintained tropical storm
intensity until landfall.
Convective Oscillations
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Forecasters have long been aware of the pulsing
nature of convection in tropical cyclones.
“Always try to analyze more than one image leading
up to the analysis time…This is particularly applicable
to shear patters that often go through a cyclic regime
of convection blow-up near the low-level center
followed by increasing separation of the overcast from
the low-level center. This can lead to rapidly varying
DT numbers over several hours.”
― A. Burton, Australian Bureau of Meteorology
“Notes on the Application of the Dvorak Technique”
Convective Oscillations in Edouard
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Edouard underwent a series of pulses in which
deep convection formed near the storm center,
then shifted more than 100 km downshear
within a few hours.
Bursts of convection near the storm center
continued in spite of strong vertical shear,
probably helping the storm maintain intensity.
One such event will be examined here.
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0715 UTC 3 Sept.
IR image
1500 m recon
winds (white
barbs, 0613 UTC
center pass)
Positive ( ) and
negative ( )
lightning flashes
(30 min. centered
on 0615 UTC)
Shear direction
(yellow arrow;
850-200 hPa,
13-15 ms-1)
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1115 UTC 3 Sept.
IR image
250 m recon
winds (1154 UTC
center pass)
30 min. lightning
centered on 1115
UTC
850-200 hPa shear
13-15 ms-1
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1515 UTC 3 Sept.
IR image
325 m recon
winds (1546 UTC
center pass)
30 min. lightning
centered on 1115
UTC
850-200 hPa shear
13-15 ms-1
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1915 UTC 3 Sept.
IR image
350 m recon
winds (1942 UTC
center pass)
30 min. lightning
centered on 1115
UTC
850-200 hPa shear
13-15 ms-1
A plot of flash count
versus distance from
storm center reveals
that lightning seemed to
move inward with time
and increase in
frequency from 05-12
UTC 3 September.
A similar plot from 1317 UTC 3 September
shows the inner core
convective maximum
waning and a new burst
of lightning reforming
more than 100 km from
the storm center.
SW-NE crosssection of 250350 m stormrelative recon
tangential
winds
11:04:40 - 12:33:50
15:17:30 – 15:59:00
23:19:40 – 00:28:10
Boundary layer storm-relative tangential winds peak when
convection is near the center, then weaken as it moves away
once again. The wind profile becomes more flat and symmetric
with time.
Summary of Convective Oscillation
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In the convective burst shown here, lightning
seemed to propagate inward toward the storm
center.
The apparent inward propagation and
subsequent downshear reforming made the
convection seem to oscillate, with a period of
less than one day.
Edouard intensified briefly when lightning was
near the storm center, then weakened again
rapidly.
Possible Mechanisms for Oscillation
of Convection
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Vertical shear effects
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Downdrafts induced by mid-level dry air
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Boundary layer recovery
Vertical Shear Effects
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Asymmetric
convection
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Vertical tilt
1535
UTC
3 Sept.
MODIS
image
0715 UTC 3 September IR, 1500 m
recon winds (green barbs, 0613 UTC
center pass), and dropsonde surface
wind (orange barb, 0616 UTC).
Vertical Shear Effects
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Dropsonde winds and visible satellite imagery clearly
reveal the convective asymmetry and downshear tilt with
height of Edouard.
To maintain balance conditions in a tilted vortex, a
vertical circulation develops (e.g. Jones 1995, DeMaria
1996).
This circulation favors upward motion downshear and
downward motion upshear of the storm center.
Convection only occurs in the downshear direction of the
center of Edouard throughout the oscillation.
Vertical shear can account for the re-development of
convection downshear, but not the inward propagation.
Dry Air / Downdrafts
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Radiosondes over Florida and dropsondes over Edouard
reveal very dry air above 500 hPa.
Dry air in the middle troposphere can fuel convective
downdrafts.
Downdrafts can bring cool, dry air to the boundary layer,
impeding development of convection near the storm
center for a time.
Conversely, downdrafts can create a gust front, which
can trigger new convection, and may have fueled the
apparent inward propagation of lightning.
Boundary Layer Recovery
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Once convection forms near the storm center, the
atmosphere stabilizes due to vertical mixing and the
effects of cold downdrafts.
Convection may be impeded near the center for a time,
but still able to form downshear of the center.
Since the SSTs are warm, surface fluxes of heat and
moisture allow the boundary layer to recover relatively
quickly.
As the atmosphere destabilizes, convection can again
occur near the center.
Hypothesis
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Ambient or shear-induced dry air fuels convective
downdrafts.
Downdraft cooling, followed by boundary layer recovery
due to surface fluxes over warm water could cause the
oscillation.
The cause of the apparent inward propagation of
convection remains unclear, but may be forced by gust
fronts.
The reason convection recurs downshear after collapsing
at the core may be related to the shear-induced
circulation.
Periodic deep convection near the storm center probably
helped Edouard resist the strong shear to some extent
and maintain tropical storm intensity.