Transcript Attributing Tropical Cyclogenesis to Convective Wave Modes

Equatorial Waves and Tropical
Cyclogenesis
Carl J. Schreck, III
University at Albany
Equatorial Waves
• Wheeler & Kiladis (1999)
and others have shown
the importance of
equatorial wave in
tropical convective
variability
– The Madden–Julian
Oscillation (MJO)
– Kelvin waves
– Equatorial Rossby (ER)
waves
– Mixed Rossby-gravity
(MRG) waves
– Tropical Depression (TDtype) disturbances
Spectrum of TRMM multisatellite
precipitation analysis Eq–15°N, May–Nov,
1998–2007, divided by a red background
following Roundy & Frank (2004)
Influences of Equatorial Waves on
Tropical Cyclone (TC) Genesis
• Equatorial waves can modulate the background
conditions for cyclogenesis (e.g., Liebmann et al. 1994;
Bessafi & Wheeler 2006; Frank & Roundy 2006; Camargo et al.
2009)
– Convection
– Low-level vorticity
– Low-level convergence
– Vertical wind shear
– Mid-level relative humidity
• Low-level confluence associated with the MJO can also
amplify higher frequency modes (e.g., Sobel and Maloney
2000)
• What fraction of tropical cyclone formations may be
attributed to each wave type?
Time–Longitude Composite of Western
Pacific Tropical Cyclogenesis
• The westward moving
developing TC is the
dominant feature
• The genesis location
anomaly is 89 mm/day
• Weaker (< 16 mm/day)
eastward and westward
envelopes of precipitation
could indicate equatorial
waves
Composite unfiltered precipitation for 145 TC formations
during the warm season (May–Nov) in the western Pacific (0°–20°N; 120°E–180°)
Time–Longitude Composite of Western
Pacific Tropical Cyclogenesis
MJO
Kelvin
ER
MRG
TD
Contours indicate filtered anomalies of
the shaded composite unfiltered field
• The westward moving
developing TC is the
dominant feature
• The genesis location
anomaly is 89 mm/day
• Weaker (< 16 mm/day)
eastward and westward
envelopes of precipitation
could indicate equatorial
waves
Artificial Precipitation Maximum
MJO
Kelvin
ER
MRG
TD
• A stationary precipitation
anomaly of the same
magnitude as the
composite TC can project
onto many different wave
modes
• How do we determine
whether anomalies are
associated with TCs or
equatorial waves?
Contours indicate filtered anomalies of the
shaded Gaussian precipitation field
Artificial Precipitation Maximum
MJO
Kelvin
ER
MRG
TD
• A stationary precipitation
anomaly of the same
magnitude as the
composite TC can project
onto many different wave
modes
• How do we determine
whether anomalies are
associated with TCs or
equatorial waves?
Contours indicate filtered anomalies of the
shaded Gaussian precipitation field
Time–Longitude Composite of Western
Pacific Tropical Cyclogenesis
MJO
Kelvin
ER
MRG
TD
• A stationary precipitation
anomaly of the same
magnitude as the
composite TC can project
onto many different wave
modes
• How do we determine
whether anomalies are
associated with TCs or
equatorial waves?
Contours indicate filtered anomalies of
the shaded composite unfiltered field
Removing TC-related Anomalies
• Calculate the
anomalies by
removing the first four
harmonics of the
annual cycle
• NCDC’s global best
tracks are used for
TC locations
– Only fixes with
maximum winds
≥ 13 m/s are used
21 Aug 2000
Range rings every 500 km
Removing TC-related Anomalies
• A Gaussian centered on
the storm is used to
determine which
anomalies are TC-related
– The anomaly at the TC
center is assumed to be
completely TC-related
– Anomalies at large radii are
assumed to be entirely
associated with the
environment
– Anomalies at a radius of
500 km are considered to
be half from TC and half
from environment
21 Aug 2000
Range rings every 500 km
Removing TC-related Anomalies
• This removal process greatly
reduces the intense
precipitation in the core of the
TCs
• Precipitation is slightly
increased farther away from
the storm where compensating
subsidence may suppress
rainfall
• Some potentially TC-related
features remain, but the results
are not sensitive to expanding
the radius for the Gaussian
21 Aug 2000
Range rings every 500 km
TC-related Spectrum
5 m/s
Spectrum of removed TC anomalies,
Eq–15N, May–Nov, 1998–2007
• The removed TCs
produce power in a broad
region associated with
westward propagation at
roughly 5 m/s
• Another maximum in TCrelated power lies in the
MJO band
• The TCs produce power
in most of the equatorial
wave bands, but it is
much smaller than the
total power
TC-related Spectrum
Total Spectrum, Eq–15N, May–Nov,
1998–2007
Percent of total spectrum that is
associated with removed TC signal
Method for Attribution
1.
2.
3.
•
•
Remove the TC-related
anomalies
Filter the remaining rainfall
rates for a given wave type
Test the filtered anomaly at
the 1° box containing the
genesis location against
some threshold
At Lingling’s genesis
location, the MRG-band
anomaly is 3.97 mm/day
How large should the
anomaly be to attribute a
storm to that wave type?
Map of Typhoon Lingling’s genesis.
Shading indicates remnant unfiltered
rainfall rates following removal of all
TCs. MRG-band anomalies are
contoured.
Method for Attribution
1.
2.
3.
•
•
Remove the TC-related
anomalies
Filter the remaining rainfall
rates for a given wave type
Test the filtered anomaly at
the 1° box containing the
genesis location against
some threshold
At Lingling’s genesis
location, the MRG-band
anomaly is 3.97 mm/day
How large should the
anomaly be to attribute a
storm to that wave type?
Map of Typhoon Lingling’s genesis.
Shading indicates remnant unfiltered
rainfall rates following removal of all
TCs. MRG-band anomalies are
contoured.
Method for Attribution
1.
2.
3.
•
•
Remove the TC-related
anomalies
Filter the remaining rainfall
rates for a given wave type
Test the filtered anomaly at
the 1° box containing the
genesis location against
some threshold
At Lingling’s genesis
location, the MRG-band
anomaly is 3.97 mm/day
How large should the
anomaly be to attribute a
storm to that wave type?
Map of Typhoon Lingling’s genesis.
Shading indicates remnant unfiltered
rainfall rates following removal of all
TCs. MRG-band anomalies are
contoured.
Selecting the Attribution Threshold
• More than one wave type may
exceed the threshold for a
given tropical cyclone
• For each wave type, the
majority of TCs develop in
association with convective
anomalies (consistent with Frank
& Roundy 2006)
The percent of 145 Western Pacific TCs
May–Nov, 1998–2007 that form where the
filtered anomaly exceeds a given threshold
• The number of storms
attributed to each wave type
decreases as threshold
increases
• The relative importance of the
wave types is generally
insensitive to the threshold
Summary
• TCs can contaminate the filtered
anomalies for many equatorial wave types
• The influence of TCs on wave
climatologies is relatively small
• TC contamination can be mitigated by
removing the TC-related anomalies using
a Gaussian function centered on the best
track position before filtering
Summary
• Even following the TC removal, each wave
type produces favorable convective
anomalies at most genesis locations
(consistent with Frank & Roundy 2006)
• The number of storms attributed to each
wave type decreases as the attribution
threshold increases
• For a wide range of thresholds, TD-type
disturbances are attributable for the most
TC developments
Sensitivity of attributions to radius
used to remove TCs
• This figure uses a 3
mm/day threshold
• The attributions are
relatively insensitive
to the removal radius
beyond 500 km
Composite WestPac TC before and
after removal
Rings every 500 km
Mean Rainfall Rate
Original
TC-related
Original
TC-related
Rainfall Variance
Genesis locations for storms attributed to each wave
type using a 3 mm/day threshold overlaid on the
filtered variance for that wave
TD
May–November 1998–2007
145 total TCs
Kelvin
51%
23%
MRG
MJO
26%
13%
ER
None
29%
19%