The Thermodynamic Evolution of Recurving Tropical Cyclones

The Thermodynamic Evolution of
Recurving Tropical Cyclones
Clark Evans
ASP Research Review
17 November 2010
With credit to: Bob Hart (FSU) and Heather Archambault (Univ. at Albany, SUNY)
A Couple of Definitions…
1. “Recurving Tropical Cyclones”
(aka “extratropically transitioning tropical cyclones”)
Basic Definition: A cyclone that moves from the tropical and sub-tropical
latitudes to the mid-latitudes.
Extended Definition: A cyclone that transitions from a tropical cyclone into an
extratropical (or frontal) cyclone (Jones et al. 2003).
(Hurricane Floyd of 1999; Images courtesy Bob Hart, FSU)
A Couple of Definitions…
“The Thermodynamic
Evolution of Recurving
Tropical Cyclones”
Symmetric Component: The cooling
of the tropical cyclone’s inner
core during the extratropical
transition (or ET) process.
Right: Area-averaged (inside 500 km radius from the
surface center) potential temperature (K; contoured)
and temporally-integrated potential temperature
change (K; shaded) of Atlantic Tropical Cyclone Bonnie
between 1200 UTC 28 August 1998 and 0000 UTC 31
August 1998 from U.S. Navy operational forecast
model (NOGAPS) analyses.
Pressure (hPa)
Time (UTC)
A Couple of Definitions…
“The Thermodynamic
Evolution of Recurving
Tropical Cyclones”
Asymmetric Component: The
transition from a symmetric,
warm-core to an asymmetric,
cold-core thermal structure
(for most ET cases).
End of ET
Start of ET
Right: Cyclone phase space (Hart 2003) evolution of
Atlantic Tropical Cyclone Bonnie between 1200 UTC 28
August 1998 and 0000 UTC 31 August 1998 as
diagnosed by analyses obtained from the U.S. Navy’s
operational forecast model (NOGAPS).
Why Do We Care?
Impacts a number of structural evolutions of the
transitioning cyclone, including the wind field structure
(e.g., Evans and Hart 2008) and vertical distributions of
static stability and potential vorticity.
The sensible weather expected from post-tropical cold-core
and warm-seclusion cyclones can be quite different! Better
understanding of this evolution can lead to better forecasts.
Thermodynamic processes influence the downstream largescale weather pattern, leading to forecast skill degradation
and potential high-impact weather events well-removed
from the cyclone.
Example: Wind Field Evolution
Cooling preferentially
near the center leads to
the outward movement
of the near-surface
radius of maximum
winds (RMW; Evans and
Hart 2008).
Understanding how this
cooling occurs may lead
to a better ability to
predict the expansion of
the RMW (Fogarty et al.
K day-1
Above: Vertically-integrated, azimuthally-averaged temperature
tendency (shaded; K day-1) between 0-500 km radius from the local
of minimum sea level pressure as obtained from a 4 km simulation
of the ET of TC Bonnie (1998). The solid black line represents the
simulated radius of maximum 925 hPa tangential wind.
Track of Typhoon Malakas
Day in September 2010
Example: Downstream Impacts
Downstream Development
Record Western U.S. Heat
Heavy Eastern U.S. Rains
Slide courtesy Heather Archambault,
University at Albany (SUNY).
Above: 40°–60°N 250-hPa meridional wind anomaly (shaded, σ)
and magnitude of departure from climatology (solid, every 15
m s−1, zero line omitted). Data are from 0.5° Global Forecast
System (GFS) and 2.5° NCEP/NCAR Reanalysis analyses.
Here, we conduct a case study
of a representative ET event,
Atlantic Tropical Cyclone
Bonnie of 1998.
Similar to ~70% of recent Atlantic
ET events (Hart et al. 2006)
Cold-core transition with decay
after ET; minimal land interaction;
no merger with another cyclone
A triply-nested (36/12/4 km)
simulation is conducted using
the Penn State/NCAR
Mesoscale Model v5 (MM5).
Physics chosen as appropriate for
TC and ET modeling studies
Track and intensity of Bonnie are
reasonably well-simulated
(Satellite image obtained from NOAA/OSEI)
Primary diagnostic tool: thermodynamic budgets
Reasoning? We can understand how temperature is changing if
we can quantify every possible means by which it changes!
Horizontal Vertical Adiabatic
advection advection processes
Dynamical Terms
Parameterized (Diabatic) Terms
(Radiation image obtained from, boundary layer image from
Results: Symmetric Evolution
Cooling structure inside 500 km radius:
vertical (left), radial (top right), temporal (bottom right)
Fields in panels at right are vertically-integrated.
Fields in panels at left and lower right are area-averaged.
Field in upper right panel is azimuthally-averaged.
K day-1
Results: Symmetric Evolution
Cooling structure inside 500 km radius: components
Left: Area-averaged,
temporally-integrated (to
0000 UTC 31 August 1998)
temperature change (K) from
selected thermodynamic
budget components.
Results: Symmetric Evolution
Comparisons to hypotheses from previous studies
Vag < 0
Upper Level Jet
(Sinclair 1993, their Figure 10)
Vag > 0
(McTaggart-Cowan et al. 2003, their Figure 14)
Sinclair (1993):
McTaggart-Cowan et al. (2003):
Evans and Hart (2008):
Our findings: in the lower troposphere, primarily horizontal advection with some vertical redistribution
horizontal advection as other terms largely offset
horizontal advection in the right upper jet entrance region
adiabatic cooling driven by inner core quasigeostrophic rising motion
Results: Asymmetric Evolution
Cyclone phase space evolution: lower and upper troposphere
Lower Troposphere
Thermal Wind vs. Thickness
Lower Troposphere
Thermal Wind vs. Upper
Troposphere Thermal Wind
Results: Asymmetric Evolution
In the lower troposphere, the magnitude of the thickness asymmetry
between the left-of-track and right-of-track hemisphere grows steadily
through the model simulation.
The lower tropospheric warm-core structure of the cyclone is
maintained well into the ET process before decaying starting near 0000
30 August 1998.
The upper tropospheric warm-core structure of the cyclone steadily
decays and has transitioned into a cold-core structure shortly after 0000
UTC 30 August 1998. It continues to become increasingly cold-core in
nature through the remainder of the model simulation.
What influences the thickness asymmetry evolution?
What causes the warm-to-cold-core structural evolution?
Results: Asymmetric Evolution
Thickness evolution: before ET and late in the ET process
r = 500 km
r = 500 km
1200 UTC 28 August 1998
1400 UTC 30 August 1998
~ 4 K across vortex
~ 10 K across vortex
Further analysis is needed to determine why these fields evolve as they do. Use of quasi-LaGrangian
trajectories in conjunction with thermodynamic budget output may provide insight. The role of vertical wind
shear in creating these asymmetries (e.g., Jones 1995,2000) must also be considered in this context.
Results: Asymmetric Evolution
Structural evolution: a hypothesis
Steady erosion of upperlevel warm-core
The warm-to-cold-core structural evolution is hypothesized to be driven by vertical wind shear, eroding the
vortex from the top-down (e.g., Frank and Ritchie 2001), and the atmospheric response to restore thermal
wind balance as an upstream trough is superimposed upon the shallow remnant vortex and its environment.
The symmetric evolution appears to be driven largely by horizontal
advective processes. Other processes play minor or redistributive roles.
Our findings most closely resemble the hypothesis of McTaggart-Cowan et
al. (2003), particularly considering that the greatest cooling is found north
of the vortex. Further analyses are needed, however, to quantify this.
The increasingly cold-core structure in the upper troposphere appears to
coincide well with the decay of the vortex aloft, suggesting that the warmto-cold-core structural evolution may be driven by vertical wind shear and
thermal wind balance restoration with the vortex.
We’ve only started scratching the surface with respect to tying this
evolution into the many factors that it potentially influences!
Future Work
Refine findings related to the asymmetric structural evolution using our
simulation output and theory derived from previous works
Extend our methods to further cold-core and warm-seclusion ET cases
and tie findings to forecasting the wind field evolution
Quantify uncertainty in the thermodynamic evolution to the selection of
physical parameterizations, numerical model, and initial conditions
Quantify the adiabatic (dynamical) and diabatic (parameterized)
contributions to the large-scale flow reconfigurations and downstream
heat transport associated with ET events
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