tc lifecycle and intensity changes part ii: intensification
DESCRIPTION
TC Lifecycle and Intensity Changes Part II: Intensification. Hurricane Katrina (2005) August 24-29. Outline. Tropical Cyclone Intensification Large-Scale Factors Symmetric Route Asymmetric Route Maximum Potential Intensity (MPI) Eyewall Replacement Cycles - PowerPoint PPT PresentationTRANSCRIPT
Tropical M. D. Eastin
TC Lifecycle and Intensity ChangesPart II: Intensification
Hurricane Katrina (2005)August 24-29
Tropical M. D. Eastin
Outline
Tropical Cyclone Intensification
• Large-Scale Factors • Symmetric Route• Asymmetric Route• Maximum Potential Intensity (MPI)• Eyewall Replacement Cycles• Role of Trough Interactions• Role of Upper Ocean Features• Rapid Intensification
Tropical M. D. Eastin
TC Intensification
Intensity change can be a slow and steady process or it can occur rapidly over the course of several hours
Forcing exists on multiple scales
• Seasonal (SST, relative humidity)• Synoptic (wind shear)• Mesoscale (convective features, MCV, eyewall cycles)• Microscales (air-sea interface, water phase changes)
Complex interactions exist between the scales
Very difficult forecast problem!!!
Tropical M. D. Eastin
TC Intensification: Large Scale Factors
Conditions favorable for intensification:
• Favorable wind shear pattern• Moist mid-troposphere• Warm ocean with deep mixed layer• Enhanced outflow• Persistent deep convection near the cyclone center
Conditions favorable for weakening would be the opposite
Tropical M. D. Eastin
Symmetric Route to Intensification
Local Heat and Momentum Sources:
• In 1982, Lloyd Shapiro and Hugh Willoughby examined the response of “balanced” (slowly evolving), symmetric hurricanes to local sources of heat and momentum
• Idealized study (built upon many before)
• Symmetric vortex is in thermal wind balance• The eyewall is a uniform ring of convection• Local heat sources (mimic latent heat release in convection)• Local momentum sources (mimic vertical advection of momentum to upper levels by convection)
In hurricane-like vortices, the local sources induce secondary circulations that can slowly intensify the vortex ...How?
Hugh Willoughby
Lloyd Shapiro
No Picture Available
Tropical M. D. Eastin
Symmetric Route to Intensification
Local Heat Sources:
• Heating produces a local temperature anomaly (like a buoyant updraft) which disturbs the local pressure surfaces
• This effect on the local pressure surfaces induces an local secondary circulation
• In hurricanes, the inner circulation is more confined with radius than the outer
Streamfunction responseto a local heat source
(mathematical solution)
Streamfunction response to a local heat source in the mid-level eyewall
(numerical simulation)
H
L
AdiabaticWarming
AdiabaticWarming
Note the difference between the two circulations
Tropical M. D. Eastin
Symmetric Route to Intensification
Local Heat Sources:
• The sinking branches adiabatically warm the air (further pressure decreases)
• The radial confinement of the inner circulation limits the warming to a smaller area than that associated with the outer circulation
Change in pressure and tangential wind bylocal heat source in the mid-level eyewall
(numerical simulation)
Lowers pressure in the eye
Increases winds in the eyewall
Radius of the local heat source is denoted
Streamfunction response
Tropical M. D. Eastin
Streamfunction response to a localmomentum source in the upper-level eyewall
(numerical simulation)
Symmetric Route to Intensification
Local Momentum Sources:
• Increased tangential momentum results in a “super-gradient” state and an outward acceleration up the pressure gradient
• This acceleration produces an local secondary circulation to conserve mass
Streamfunction responseto a local momentum source
(mathematical solution)
H
L
GradientBalance
PGF
CentrifugalForce
SuperGradient
State
H
L
PGF
CentrifugalForce
Tropical M. D. Eastin
Symmetric Route to Intensification
Local Momentum Sources:
• The lower circulation’s inflow conserves angular momentum (increases the tangential wind)
• The upper circulation’s descent results in adiabatic warming confined in the eye (lowers pressure)
Lowers pressure in the eye
Increases winds in the eyewall
Radius of local momentum source is denoted
Change in pressure and tangential wind bylocal momentum source in the upper-level eyewall
(numerical simulation)
Streamfunction response
Tropical M. D. Eastin
Asymmetric Route to Intensification
Convective Bursts:
• In 1960, Joanne Malkus (Simpson) and Herbert Riehl first suggested that hurricane evolution was linked to a few, asymmetric, intense cumulonimbus clouds, which they called “hot towers”, that carried a large fraction of the high-θe inflow aloft in undiluted updrafts
• Observational study
• Eyewall convection was often asymmetric with many localized updraft cores
• Convection was often episodic with “bursts”
These “convective bursts” increase the latent heating aloft and the asymmetric secondary circulations that can intensify the vortex...How?
Joanne Simpson
Herbert Riehl
Tropical M. D. Eastin
Asymmetric Route to Intensification
Convective Burst in Hurricane Bonnie (1998) on 23 August
Tropical M. D. Eastin
Asymmetric Route to Intensification
Convective Bursts:
• Overshooting and diverging convection at upper levels drives asymmetric mesoscale descent (adiabatic warming) in the eye, which lowers the pressure, increasing the pressure gradient and tangential winds
• A recent survey of convective bursts:
• 80% of TCs have at least one “burst”• 70% of TCs intensify after a “burst”
Conceptual Model of Convective Burst
Tropical M. D. Eastin
Maximum Potential Intensity
Maximum Potential Intensity (MPI)
• Theoretical maximum intensity a TC could achieve if environmental conditions were infinitely perfect
Emanuel (1988)
• MPI is primarily a function of SST and the mean outflow temperature at the top of the eyewall• No eye subsidence
Holland (1998)
• MPI is primarily a function of environmental CAPE• Incorporates eye subsidence for strong hurricanes
MPI computed for Typical Conditions
800
820
840
860
880
900
920
940
960
26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0
SST (C)
Min
imu
m P
res
su
re (
mb
)
Observed
Emanuel
Holland
Note: Observed values should be higher since the dynamical environment will limit TC intensities
Tropical M. D. Eastin
Eyewall Replacement Cycles
Eyewall Replacement Cycles:
• Outer eyewall develops and begins to contract
• Inner eyewall begins to dissipate• Maximum winds decrease• Minimum central pressure increases
• Outer eyewall continues to contract• Maximum winds increase• Minimum central pressure decreases
Hurricane Gilbert (1988)
Radar at 2300 UTC13 September
Tangential Winds11-16 September
Tropical M. D. Eastin
Eyewall Replacement Cycles
Eyewall Replacement Cycles: Statistics
• More common in intense tropical cyclones• Process typically takes 36 hours
• Survey of multiple eyewall structures in TCs with maximum winds > 120 knots (Category 345) during 1997-2002
• 40% of Atlantic hurricanes• 60% of East Pacific hurricanes• 70% of West Pacific typhoons
• Significant factor in TC intensity changes
• Results in an outward expansion of the wind field (i.e., TC grows in size) and an “annular” (or symmetric) wind field
• An eyewall replacement cycle contributed the weakening of Katrina (2005) just prior to landfall near New Orleans
Tropical M. D. Eastin
Eyewall Replacement Cycles
Eyewall Replacement Cycles: Hurricane Ivan (2004)
Note the overallexpansion
of the wind fieldafter 6 EWRCs
Inner eyewallSecondary eyewall
Third eyewall
From Sitkowski et al. (2011)
Tropical M. D. Eastin
Role of Trough Interactions
Basic Idea:
• Upper tropospheric troughs can promote intensification by enhancing the upper-level divergence and outflow• Troughs can also promote weakening by enhancing the vertical shear experienced by the TC
• What are the differences between “good” and “bad” troughs (for intensification)?
Hanley et al. (2001):
• Examined 146 TCs which interacted with upper-level troughs• 68% of the TCs intensified• Composited the large-scale flow with respect to each TC center
Vorticity Cross-Section
Upper-level Trough
HurricaneDennis(1999)
Tropical M. D. Eastin
Role of Trough Interactions
Favorable Trough Interactions:
• Trough potential vorticity (PV) maximum comes within 400 km of TC center, but rarely closer
• Troughs are generally small in size
• Outflow is enhanced
• Mean vertical wind shear between 850 and 200 mb is less than 8 m/s
Composite 200 mb Flowand Potential Vorticity
Note: Asterick denotes TC center
Tropical M. D. Eastin
Role of Trough Interactions
Unfavorable Trough Interactions:
• Trough potential vorticity (PV) maximum comes within 100 km of TC center
• Troughs are generally larger in size
• Mean vertical wind shear between 850 and 200 mb is greater than 10 m/s
Composite 200 mb Flowand Potential Vorticity
Note: Asterick denotes TC center
Tropical M. D. Eastin
Role of Upper Ocean Features
Deep Warm Currents and Eddies:
• A shallow oceanic mixed layer can easily be eroded by TC induced upwelling of cold water, resulting in cold SSTs and and the potential weakening of the TC
• A deep oceanic mixed layer will experience less upwelling of cold water, resulting in higher SSTs, and a better chance for intensification
Deep warm water matters, not just SST
SST on 8-25-05
Depth of 26ºC on 8-25-05
Tropical M. D. Eastin
Role of Upper Ocean Features
Common Deep Warm Currents and Eddies:
Trajectories of NOAA buoys
from1978-2003
Gulf Stream
Loop Current
Warm Core
Eddies(Rings)
Tropical M. D. Eastin
Rapid Intensification (RI)
Definition and Statistics:
• Increase in maximum wind speed of 15.4 m/s (30 knots) over a 24 hour period
• A survey of Atlantic basin TCs (1989-2000)
• All category 4 and 5 hurricanes underwent a period of RI during their life• ~60% of all hurricanes undergo a period of RI• ~30% of all tropical storms undergo RI
When is Rapid Intensification more likely?
• Storm is far from it’s MPI (weak system)• Storm over high SST and deep warm oceanic mixed layer• Higher than normal mid-tropospheric humidity• Low vertical wind shear
Tropical M. D. Eastin
Rapid Intensification (RI)
Hurricane Opal (1995)
• Weak hurricane stalled in southern Gulf of Mexico
• Moved rapidly NE during the night of 4 October
• Rapidly intensified from 965 to 916 mb in 14 hours
• Coastal residents not warned appropriately (unexpected intensification)
Tropical M. D. Eastin
Rapid Intensification (RI)
Hurricane Opal (1995)
Tropical M. D. Eastin
Rapid Intensification (RI)Forecasting: 37-GHz Imagery
• Kieper and Jiang (2012) evaluated precipitation patterns prior to and during RI for 84 Atlantic TCs
Rapid intensification often occurred 6-12 hrs after the first appearance of a “ring pattern” in the 37-GHz passive microwave (SSMI) imagery (75% of all RI cases in 2003-2007)
Ring of shallow precipitationaround a small “eye”
Tropical M. D. Eastin
TC Lifecycle and Intensity ChangesPart II: Intensification
Summary
• Large-Scale Factors
• Symmetric Intensification (assumptions, physical processes, cases)• Intensification via Hot Towers (assumptions, physical processes)
• MPI (basic idea)• Eyewall Replacement Cycles (process, impacts)• Upper-level Trough Interactions (favorable/unfavorable, impacts)• Upper Ocean Features (examples, physical processes, impacts)• Rapid Intensification (definition, favorable situations, forecasting)
Tropical M. D. Eastin
ReferencesBosart, L. A., C. S. Velden, W. E. Bracken, J. Molinari, and P. G. Black, 2000: Environmental influences on the rapid
intensification of Hurricane Opal (1995) over the Gulf of Mexico. Mon. Wea. Rev., 128, 322-352
Emanuel, K. A., 1988: The maximum intensity of hurricanes. J. Atmos. Sci., 45, 1143-1155.
Hanley, D. E., J. Molinari, and D. Keyser, 2001: A composite study of of the interactions between tropical cyclones andupper-tropospheric troughs. Mon. Wea. Rev., 129, 2570-2584.
Heymsfield, G. M., J. B. Halverson, J. Simpson, L. Tian, and T. P. Bui, 2001: ER-2 Doppler radar investigations of the eyewall of Hurricane Bonnie during the Convection and Moisture Experiment-3. J. Appl. Met., 40, 1310-1330.
Holland, G. J., 1997: The maximum potential intensity of tropical cyclones. J. Atmos. Sci., 54, 2519-2541.
Kaplan, J., and M. DeMaria, 2003: Large-scale characteristics of rapidly intensifying tropical cyclones in the north Atlanticbasin. Wea. Forecasting, 18, 1093-1108.
Kieper, M., and H. Jiang, 2012: Predicting tropical cyclone rapid intensification using the 37-GHz ring pattern identified from
passive microwave measurements, Geophysical Research Letters, 39, L13804.
Kossin, J. P., and M. D. Eastin, 2001: Two distinct regimes in the kinematic and thermodynamic structure of the hurricaneeye and eyewall. J. Atmos. Sci., 58, 1079-1090.
Kossin, J. P., and M. Sitkowski, 2012: Predicting hurricane intensity and structure changes associated with eyewallreplacement cycles, Wea. Forecasting, 27, 484-488.
Knaff, J. A., M. DeMaria, and J. P. Kossin, 2003: Annular hurricanes. Wea. Forecasting, 18, 204–223.
Malkus, J., and H. Riehl, 1960: On the dynamics and energy transformations in steady-state hurricanes. Tellus, 12, 1–20.
Moeller, D. J., and M. T. Montgomery, 1999: Vortex Rossby Waves and hurricane intensification in a barotropic model.J. Atmos. Sci., 56, 1674-1687.
Tropical M. D. Eastin
ReferencesMontgomery, M. T., and R. J. Kallenbach, 1997: A theory for vortex Rossby waves and its application to spiral bands and
intensity changes in hurricanes. Quart. J. Roy. Meteor. Soc., 123, 435–465.
Shapiro, L. J., and H. E. Willoughby, 1982: The response of balanced hurricanes to local sources of heat and momentum.J. Atmos. Sci., 39, 378–394.
Sitkowski, M. J. P. Kossin, and C. M. Rozoff, 2011: Intensity nad structure changes during eyewall replacement cycles.Mon. Wea. Rev., 139, 3829-3847.
Sitkowski, M. J. P. Kossin, and C. M. Rozoff, 2011: Intensity nad structure changes during eyewall replacement cycles.Mon. Wea. Rev., 139, 3829-3847.
Willoughby, H. E., and M. L. Black, 1992: The concentric eyewall cycle of Hurricane Gilbert. Mon. Wea. Rev., 120, 947-957