Numerical Simulation of
Hurricane Alex (2004)
R. McTaggart-Cowan, L.F. Bosart
and C. Davis
Outline
●
Hurricane Alex (2004) background
–
Easterly wave (EW) development
–
Midlatitude interactions
●
Operational NWP guidance evaluation
●
Simulations of Hurricane Alex genesis
●
–
Development of tropical vortex
–
Spurious hurricane generation in the Gulf of Mexico
Summary and discussion
Storm Summary (Background)
●
●
Formation from a
tropical wave and cold
upper low on 31 July
Hurricane status on 3
August and sustained
Cat 1 force winds on the
Outer Bank
Strengthens to Cat 3
storm at 38ºN 5 August
NHC Best Track
●
SSM/I IR
channel
satellite
image for
~1200 UTC
5 August
(courtesy of
FNMOC)
Easterly Wave (Background)
Layer-averaged
925-850 hPa winds
and relative vorticity
from the 1º GFS
analysis. Winds are
shown in knots with
short, long and
pennant barbs
indicating 5, 10 and
50kt winds,
respectively.
Vorticity is plotted at
intervals of 2x10-5 s1 above 1x10-5 s-1.
E
0000 UTC 27 July
Easterly Wave (Background)
E
Layer-averaged
925-850 hPa winds
and relative vorticity
from the 1º GFS
analysis. Winds are
shown in knots with
short, long and
pennant barbs
indicating 5, 10 and
50kt winds,
respectively.
Vorticity is plotted at
intervals of 2x10-5 s1 above 1x10-5 s-1.
1200 UTC 27 July
Easterly Wave (Background)
E
Layer-averaged
925-850 hPa winds
and relative vorticity
from the 1º GFS
analysis. Winds are
shown in knots with
short, long and
pennant barbs
indicating 5, 10 and
50kt winds,
respectively.
Vorticity is plotted at
intervals of 2x10-5 s1 above 1x10-5 s-1.
0000 UTC 28 July
Easterly Wave (Background)
A
E
Layer-averaged
925-850 hPa winds
and relative vorticity
from the 1º GFS
analysis. Winds are
shown in knots with
short, long and
pennant barbs
indicating 5, 10 and
50kt winds,
respectively.
Vorticity is plotted at
intervals of 2x10-5 s1 above 1x10-5 s-1.
1200 UTC 28 July
Easterly Wave (Background)
A
E
Layer-averaged
925-850 hPa winds
and relative vorticity
from the 1º GFS
analysis. Winds are
shown in knots with
short, long and
pennant barbs
indicating 5, 10 and
50kt winds,
respectively.
Vorticity is plotted at
intervals of 2x10-5 s1 above 1x10-5 s-1.
0000 UTC 29 July
Easterly Wave (Background)
A
E
Layer-averaged
925-850 hPa winds
and relative vorticity
from the 1º GFS
analysis. Winds are
shown in knots with
short, long and
pennant barbs
indicating 5, 10 and
50kt winds,
respectively.
Vorticity is plotted at
intervals of 2x10-5 s1 above 1x10-5 s-1.
1200 UTC 29 July
Easterly Wave (Background)
A
E
Layer-averaged
925-850 hPa winds
and relative vorticity
from the 1º GFS
analysis. Winds are
shown in knots with
short, long and
pennant barbs
indicating 5, 10 and
50kt winds,
respectively.
Vorticity is plotted at
intervals of 2x10-5 s1 above 1x10-5 s-1.
0000 UTC 30 July
Easterly Wave (Background)
A
Layer-averaged
925-850 hPa winds
and relative vorticity
from the 1º GFS
analysis. Winds are
shown in knots with
short, long and
pennant barbs
indicating 5, 10 and
50kt winds,
respectively.
Vorticity is plotted at
intervals of 2x10-5 s1 above 1x10-5 s-1.
1200 UTC 30 July
Easterly Wave (Background)
A
Layer-averaged
925-850 hPa winds
and relative vorticity
from the 1º GFS
analysis. Winds are
shown in knots with
short, long and
pennant barbs
indicating 5, 10 and
50kt winds,
respectively.
Vorticity is plotted at
intervals of 2x10-5 s1 above 1x10-5 s-1.
0000 UTC 31 July
Trough Interaction (Background)
T
C
E
Dynamic tropopause (2 PVU) pressure (hPa) and deep layer shear. Shear is plotted in knots,
with short, long and pennant barbs for 5, 10 and 50kt, respectively. Shear <20kt is plotted in blue.
Trough Interaction (Background)
T
CA
E
Dynamic tropopause (2 PVU) pressure (hPa) and deep layer shear. Shear is plotted in knots,
with short, long and pennant barbs for 5, 10 and 50kt, respectively. Shear <20kt is plotted in blue.
Trough Interaction (Background)
T
CA
E
Dynamic tropopause (2 PVU) pressure (hPa) and deep layer shear. Shear is plotted in knots,
with short, long and pennant barbs for 5, 10 and 50kt, respectively. Shear <20kt is plotted in blue.
Trough Interaction (Background)
T
C
A
E
Dynamic tropopause (2 PVU) pressure (hPa) and deep layer shear. Shear is plotted in knots,
with short, long and pennant barbs for 5, 10 and 50kt, respectively. Shear <20kt is plotted in blue.
Trough Interaction (Background)
A
E
Dynamic tropopause (2 PVU) pressure (hPa) and deep layer shear. Shear is plotted in knots,
with short, long and pennant barbs for 5, 10 and 50kt, respectively. Shear <20kt is plotted in blue.
Trough Interaction (Background)
A
Dynamic tropopause (2 PVU) pressure (hPa) and deep layer shear. Shear is plotted in knots,
with short, long and pennant barbs for 5, 10 and 50kt, respectively. Shear <20kt is plotted in blue.
Trough Interaction (Background)
A
Dynamic tropopause (2 PVU) pressure (hPa) and deep layer shear. Shear is plotted in knots,
with short, long and pennant barbs for 5, 10 and 50kt, respectively. Shear <20kt is plotted in blue.
Trough Interaction (Background)
T
C
A
E
Dynamic tropopause (2
PVU) pressure (hPa) and
deep layer shear. Shear is
plotted in knots, with short,
long and pennant barbs for
5, 10 and 50kt, respectively.
Shear <20kt is plotted in
blue.
Trough Interaction (Background)
T
A
C
Dynamic tropopause (2
PVU) pressure (hPa) and
deep layer shear. Shear is
plotted in knots, with short,
long and pennant barbs for
5, 10 and 50kt, respectively.
Shear <20kt is plotted in
blue.
Trough Interaction (Background)
T
A
C
Dynamic tropopause (2
PVU) pressure (hPa) and
deep layer shear. Shear is
plotted in knots, with short,
long and pennant barbs for
5, 10 and 50kt, respectively.
Shear <20kt is plotted in
blue.
Diagnosis (Background)
●
●
●
●
Easterly wave enhances convection as it moves
towards the eastern Caribbean.
PV tail beneath a fold-over ridge over the
central North Atlantic begins to interact with a
digging trough over eastern North America.
As the wavelength between the troughs
collapses, the deep-layer shear is reduced.
The shear vorticity maximum of the EW is
stretched by convective warming to form Alex.
Operational Guidance (NWP)
●
●
NHC forecasts:
–
near-average track forecasting skill
–
well below-average intensity errors (200% of mean)
Numerical guidance:
–
compare 0-48h guidance from 0000 UTC 29 July
2005 initializations of GFS, Eta and CMC-R models
–
evaluate genesis skill based on 925-850 hPa layeraveraged relative vorticity since surface features
are weak until long lead times
–
DT pressure is used to evaluate large-scale flow
X
E
T
X
E
Eta Model 00h
C
CMC Regional Model 00h
C
GFS Model 00h
NCEP Global Analysis
T
T
C
X
E
00 UTC 29 July
T
C
X
E
A
EX
C
A
X
Eta Model 24h
T
CMC Regional Model 24h
T
GFS Model 24h
NCEP Global Analysis
C
T
C
A
XE
00 UTC 30 July
T
C
A
X
X
Eta Model 48h
T A
CMC Regional Model 48h
A
GFS Model 48h
NCEP Global Analysis
T
T
A
X
00 UTC 31 July
T
A
Operational Guidance (NWP)
●
●
●
GFS: good 3D structures and track, but vorticity
centre is significantly weaker than observed.
Eta: re-arrangement of DT perturbations leads
to a baroclinic development and strong frontal
features at 48-60h lead times.
CMC-R: rapid erosion of depressed DT as the
model incorrectly intensifies the curvature EW
component and strengthens it to Cat1 hurricane
status by 48h.
Model Description (Simulations)
●
MC2 and GEM models initialized with NCEP
Analysis (1º resolution)
●
Grid: 40km spacing (300x220 L30 grid)
●
Dynamics: Semi-Lagrangian advection
●
Physics:
–
Modified Kain-Fritsch convection
–
Moist kinetic energy closure for boundary layer
–
5 category bulk microphysics
18 UTC 29 July
X
GEM Simulation 18h
T
NCEP Global Analysis
C
●
C
A
X
MC2 Simulation 18h
T
●
T
C
A
X
Trough interaction strengthens over the next 12h
as separation decreases.
Convective DT ridging is
already separating the
cutoff from the PV tail.
00 UTC 30 July
X
GEM Simulation 24h
T
NCEP Global Analysis
C
●
A
MC2 Simulation 24h
T
C
●
T
C
A
X
Convective development
continues to re-arrange
the larger scale flow.
Southern cutoff disappears in MC2.
06 UTC 30 July
GEM Simulation 30h
X
C
NCEP Global Analysis
T
●
T
A
MC2 Simulation 30h
C
●
A
DT features in GEM lost
due to active convection.
Southern cutoff completely lost in MC2, eliminating
the shear-reduction
mechanism.
Shear Reduction (Simulations)
●
The shear
reduction
triggered by
trough
interaction is
weak in both
the GEM and
MC2
simulations.
Time series of shear averaged over a 4x4 degree box centered on the EW vorticity centre.
Shear is computed as the vector difference between the DT and 850-925hPa mean winds.
Evaluation (Simulations)
●
●
Both simulation and mesoscale operational
models show varying degrees of rapid
breakdown of the coherent DT field at 18-30h.
Large-scale shear reduction by trough combination is suppressed by flow re-arrangements.
Are the models failing to handle the strongly
nonlinear pattern (predictability problem), or are
mesoscale physical processes adversely
affecting the larger-scale flow?
Sensitivity Tests (Simulations)
●
●
●
Diagnostics indicate that shear reduction during
genesis is driven by the large scale trough
interaction well-described by dry dynamics.
The dynamical cores of the GEM and MC2
models are isolated and run as sensitivity tests.
Dry-dynamical predictability is good despite the
strongly nonlinear nature of the initial state and
early evolution of the system.
X
NCEP Global Analysis
C
T
GEM Core Simulation 30h
06 UTC 30 July
●
T
X
MC2 Core Simulation 30h
C
●
C
T
X
Troughs interaction is
delayed by ~6h.
Evolutions of PV tail and
digging trough structures
are well represented by
dry dynamics alone.
06 UTC 30 July
GEM Simulation 30h
X
C
NCEP Global Analysis
T
●
T
MC2 Simulation 30h
C
●
Troughs interaction is
delayed by ~6h.
Evolutions of PV tail and
digging trough structures
are well represented by
dry dynamics alone.
X
NCEP Global Analysis
C
T
GEM Core Simulation 30h
06 UTC 30 July
●
T
X
MC2 Core Simulation 30h
C
C
T
X
Growth of the incipient
vortex is not expected in
these tests since it is
largely convectivelydriven and inconsistent
with the dry dynamical
formulation.
Sensitivity Tests (Simulations)
●
Dry dynamics
are responsible
for the development and maintenance of low
shear during
trough interaction.
Time series of shear averaged over a 4x4 degree box centered on the EW vorticity centre.
Shear is computed as the vector difference between the DT and 850-925hPa mean winds.
Error Evaluation (Simulations)
●
●
The re-arrangement of the coherent initial DT
features is consistent with incorrect convective
triggering:
–
rapid destruction of DT depressions occurs on a 612h (convective) timescale
–
tropopause lifting is accompanied by an increase in
lower-level relative vorticity
Comparison of the model precip fields and
observed OLR shows errors in convection.
GOES-East OLR and GFS Analysis DT
Pressure valid ~0600 29 July 2005
●
MC2 Simulated Rain Rates and DT Pressure
after 6h of integration
Modelled convection under the cutoff PV tail
fragments the feature and reduces the effectiveness
of trough interaction/shear reduction processes.
GOES-East OLR and GFS Analysis DT
Pressure valid ~1200 29 July 2005
●
MC2 Simulated Rain Rates and DT Pressure
after 12h of integration
Modelled convection under the cutoff PV tail
fragments the feature and reduces the effectiveness
of trough interaction/shear reduction processes.
GOES-East OLR and GFS Analysis DT
Pressure valid ~1800 29 July 2005
●
MC2 Simulated Rain Rates and DT Pressure
after 18h of integration
Modelled convection under the cutoff PV tail
fragments the feature and reduces the effectiveness
of trough interaction/shear reduction processes.
GOES-East OLR and GFS Analysis DT
Pressure valid ~0000 30 July 2005
●
MC2 Simulated Rain Rates and DT Pressure
after 24h of integration
Modelled convection under the cutoff PV tail
fragments the feature and reduces the effectiveness
of trough interaction/shear reduction processes.
GOES-East OLR and GFS Analysis DT
Pressure valid ~0600 30 July 2005
●
MC2 Simulated Rain Rates and DT Pressure
after 30h of integration
Modelled convection under the cutoff PV tail
fragments the feature and reduces the effectiveness
of trough interaction/shear reduction processes.
GOES-East OLR and GFS Analysis DT
Pressure valid ~1200 30 July 2005
●
MC2 Simulated Rain Rates and DT Pressure
after 36h of integration
Modelled convection under the cutoff PV tail
fragments the feature and reduces the effectiveness
of trough interaction/shear reduction processes.
GOES-East OLR and GFS Analysis DT
Pressure valid ~1800 30 July 2005
●
MC2 Simulated Rain Rates and DT Pressure
after 42h of integration
Modelled convection under the cutoff PV tail
fragments the feature and reduces the effectiveness
of trough interaction/shear reduction processes.
Summary
●
●
●
The importance of trough interaction for shear
reduction over Alex’s incipient vortex is
investigated in operational NWP and simulations.
Higher resolution regional models poorly
represented the upper-level flow.
Rapid increases in tropopause heights due to
convection beneath the troughs led to fragmentation and reduced the effectiveness of shear
reduction compared to dry dynamical studies.
Conclusions
Upscale effects of the misrepresentation of mesoand convective scale features during Alex’s
genesis lead to the erroneous destruction of DT
features that in a dry dynamical setting act to
rapidly reduce the shear over the developing
system.
More Generally:
In a highly nonlinear flow, errors in mesoscale perturbations can
be amplified rapidly by the larger scales. All forcings in this case
must be small in amplitude and applied with caution – coarse
resolution models may outperform their mesoscale counterparts
by smoothly evolving the upper-level flow.