TC Lifecycle and Intensity Changes Part II: Intensification

Tropical M. D. Eastin

TC Lifecycle and Intensity ChangesPart 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• Role of Trough Interactions• Role of Upper Ocean Features• Rapid Intensification


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!!!


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


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



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



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


Lowers pressure in the eye

Increases winds in the eyewall

Radius of the local heat source is denoted

Streamfunction response


Streamfunction response to a localmomentum source in the upper-level eyewall



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



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


Streamfunction response


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



Convective Burst in Hurricane Bonnie (1998) on 23 August



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


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


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



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



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)


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)



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



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


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


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)


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



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)



Hurricane Opal (1995)


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”


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)


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.


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

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TC Lifecycle and Intensity Changes Part II: Intensification