Transcript Document

Thermodynamic Structure of
Tropical Cyclones From
Aircraft Reconnaissance
Kay Shelton
MS Thesis Presentation
Thesis Outline
• Composite
– Created composite radial and vertical profiles of
qe categorised by storm intensity
• Bret (1999)
– Case study of the qe evolution in a TD–H4 storm
• Claudette (2003)
– Detailed case study of storm under strong shear
– System very short-lived as a hurricane
Thesis Outline
• Composite
– Created composite radial and vertical profiles of
qe categorised by storm intensity
• Bret (1999)
– Case study of the qe evolution in a TD–H4 storm
• Claudette (2003)
– Detailed case study of storm under strong shear
– System very short-lived as a hurricane
• How did Claudette intensify to a hurricane under
strong shear?
• Why was the hurricane phase so short-lived?
TC Genesis and Intensification
• Mid-level moistening
– Emanuel (1995), Bister and Emanuel (1997)
– Moistening in the mid-troposphere helps to
eliminate cold downdrafts reaching the surface
• Vertical Wind Shear
– Vertical shear is a negative factor
– Downshear vortex reformation, eg. Hurricane
Danny (1997) and Hurricane Gabrielle (2001)
[Molinari et al 2004,2005]
WISHE Hypothesis
• Proposed by Emanuel (1986)
• Assumes a pre-existing axisymmetric (surface
concentrated) disturbance in symmetric neutrality
• Feedback mechanism between maximum surface
winds and surface fluxes
• Smith (2003) – modelled the boundary layer for an
axisymmetric hurricane
– Cold downdrafts not included
– Calculated radial profiles of qe and latent and sensible
heat fluxes
– Results support WISHE hypothesis
• Neither of the assumptions for WISHE are usually
met in early stages of TC development
Pre-WISHE Hypothesis
• Proposed by Molinari et al (2004)
• Can have wind shear acting to create strong
asymmetries, strong buoyant convection and cold
downdrafts occurring
• But, via
– i) vortex merger processes, vorticity can be
axisymmetrised
– ii) convective mixing, profile becomes more nearly
neutral and cold downdrafts are eliminated
• System can approach a state where WISHE is valid
and a hurricane can form
Background - Shear
• Shear tilts vortex
• Enhanced convection and maximum upward motion
expected downshear and downshear-left
• Dynamically induced downdrafts upshear cover a
large area
• Frank and Ritchie
(2001)
• Corbosiero and Molinari
(2002)
• Jones (1995) & Reasor
et al (2004)
From Eastin et al (2005) 
Claudette (2003) – Case Study
• USAF reconnaissance data
– Flight level observations
– Dropsondes
• IR, visible and microwave satellite imagery
• Gridded ECMWF (1.125ox1.125o) analyses
• NHC Best Track (modified using
reconnaissance centre fixes)
Track

SSTs in central/western
Caribbean: 26-28oC
Case Study period
H1 (12 UTC/10th July)

Pressure and
 Windspeed
Very rapid pressure
change at H1 time
850-200hPa Shear
H1

Storm reaches
hurricane intensity
when shear is close to
its maximum and still
increasing.
A large portion of the
storm’s life occurs with
 shear greater than
12.5ms-1, which is
considered a cut-off
value, above which
storm’s can no longer
sustain themselves
(Zehr, 1992)
IR Animation
Flight 6 - Composite Observations
400km
• 850hPa
• ~ 3 hours of
observations
• Centred on 0715
UTC on 10th July
– 5 hours prior to H1
time
Closed circulation is very
small scale and located
under deep convection.
Larger-scale wave
structure evident.
Fl. 6 Pass 1 – Centre Cross-section
qe
Windspeed
T, Td
50m
D-value
SW
 
S E
NE
SW
 
S E
NE
Fl. 7 Pass 1 – Centre Cross-section
qe
Windspeed
T, Td
145m
9oC
D-value
 
WSW SW SSW S-N E SE
WSW SW
 
SSW S-N E SE
Surface Observation
• At time of H1
• No eye seen in IR
image
• Dropsonde (cyan)
– 1203 UTC
• 850hPa winds
(white)
– 1131-1208 UTC
• 700hPa winds
(black)
– 1209-1300 UTC
• No vortex tilt from
surface to 850hPa
Flight 7 - Composite Observations
400km
• 700hPa
• ~ 4.5 hours of
observations
• Centred on 1415
UTC on 10th July
– 2 hours after
Claudette named H1
Small-scale vortex
embedded in larger-scale
wave
Embedded circulation
under deep convection
Fl. 7 Pass 2 – Centre Cross-section
qe
Windspeed
T, Td
100m
7oC
D-value
SW
NE
SW
NE
1330UTC/10th – SSM/I
85GHz HW
Storm centre
Good for weak TCs
Sensitive to warm precip.
85GHz PCT
Isolates convection
1456UTC/10th – TRMM
85GHz H
Sensitive to ice phase
Shows upper levels (5-9km)
85GHz – Colour Composite
Cloud free/dry – grey
Low level clouds – blue/green
Deep convection - red
Intensification Summary
• Repeated convective pulses over centre
• Initially a very small vortex forms embedded within
a larger-scale wave
• Vortex remains aligned from surface to 850hPa
• Vortex deepens and eyewall structure begins to
develop
• High qe and winds clearly define eyewall structure
• Dry air seen persistently upshear at 700hPa and
moist air remains downshear
• Hurricane strength reached at time of nearmaximum and increasing shear
– In which case, why did the convective pulses occur over
the centre?
Fl. 7 Pass 3 – Centre Cross-section
qe
Windspeed
T, Td
7oC
7oC
100m
D-value
NW
SE
NW
SE
Fl. 7 Pass 4 – Centre Cross-section
qe
Windspeed
T, Td
12oC
D-value
SW

S SSE N NNE N
SW

S SSE N NNE N
Weakening Summary
• Dry air upshear and also to the left and right of the
shear vector persistent at 700hPa
• Strong small-scale circulation exists until 1530UTC
• No evidence of this strong circulation or a D-value
depression in Pass 4
• The dry air in three quadrants of the storm could
possibly have allowed cold downdrafts to occur,
acting to weaken the storm
• Also the advection of the mid-level dry air around
the storm by its own circulation could have brought
about the weakening
Comparison with Theories
1. Mid-level moistening or rather, the lack of:
–
Why wasn’t this a factor in the spin-up phase of
Claudette and yet, so instrumental in the weakening
Shear
DRY
Shear
Circulation
spins up
MOIST
Dry air not advected
by weak circulation
DRY
MOIST
Strong circulation advects
dry air downshear
2. Vertical wind shear:
–
–
–
This creates the dry/moist anomalies and drives the
downshear convection
But, it is seemingly not a negative factor for this case
Why did the storm form if shear was so high?
Comparison with Theories
3. WISHE and Pre-WISHE:
– Vortex interactions and convection create a symmetric,
neutral system
– But, multiple vorticies are not found in Claudette
– The qe profile is not WISHE-like, and yet a hurricane
develops
4. How did this hurricane form when all the
conventional theories suggest it should not have?
Hypothesis
• A: 700hPa circulation located upshear of surface
circulation. Shear advects 700hPa circulation over
surface circulation
• B: Circulations are vertically aligned, convection
breaks out  rapid intensification
• C: Shear advects 700hPa circulation further
downshear, circulations decouple  system weakens
Z
Shear
700hPa
Surface
A
B
C
Conclusions
• Although Claudette did manage to become a
hurricane, it was never going survive for long
• How many more of these embedded, minihurricanes occur?
• The flight level and dropsonde data available from
the USAF from their missions into developing TCs
has never been so abundant. If this type of system
is fairly common, NHC forecasters will have to
address the issue of being able to predict such
small, short-lived hurricanes.
Acknowledgements
• John Molinari
• Dave Vollaro
• Chris Thorncroft and the rest of the Faculty
• Kevin Tyle and Dave Knight
• Celeste, Diana, Sharon, Lynn and Sally
• Kristen Corbosiero, Anantha Aiyyer and Eyad Atallah
• Grad students past and present
• Gareth Berry