Equatorial Rossby Waves and Twin Tropical Cyclogenesis Carl J. Schreck, III John Molinari Department...
-
date post
21-Dec-2015 -
Category
Documents
-
view
218 -
download
1
Transcript of Equatorial Rossby Waves and Twin Tropical Cyclogenesis Carl J. Schreck, III John Molinari Department...
Equatorial Rossby Waves and Twin Tropical Cyclogenesis
Carl J. Schreck, IIIJohn Molinari
Department of Earth and Atmospheric SciencesUniversity at Albany
State University of New York
Twin Tropical Cyclones
Hawaii
Paka
Pam
Pre-Ivan Pre-Joan
Lusi
Convectively Coupled ER waves
• Shading indicates divergence, hatching indicates convergence
• Create Symmetric regions of convergence and cyclonic vorticity
• Propagate westward
Composite ER wave from Molinari et al. (2006)
• Composited about maximum vorticity at time of tropical cyclogenesis
• Cyclonic vorticity contoured every 0.5 × 10-5 s-1
• OLR anomalies shaded every 10 W m-2
• Convectively coupled ER waves provide favorable regions for tropical cyclogenesis– See also Bessafi &
Wheeler (2006) and Frank & Roundy (2006)
– None of them mention twins
Convectively Forced ER Waves (Gill 1980)
• Cyclonic regions develop on both sides of the equator in response to near-equatorial heating
• Heckley & Gill (1984) found that this steady state solution could be reached within 3 days of the sudden “switch-on” of heating
Vertical Motion
Height
Liebmann et al. (1994): MJO
• Composite based on peak 35-95 day band-pass OLR anomalies at 12°N and 12°S rotated to 0° longitude– 35-95 day 850-hPa relative
vorticity (contours)– 35-95 day OLR (shaded)– Tropical cyclogenesis (*)
• About half the tropical cyclones form within 45° to the east of the heating
• No mention of twin tropical cyclogenesis, but both composites show cyclogenesis in both hemispheres
Northern Hemisphere Composite
Southern Hemisphere Composite
Keen (1982): Mid-latitude interactions
Lander (1990): Convectively Forced ER waves
1 2
3 4
Twin Tropical Cyclone Definitions
Keen (1982) Lander (1990) Harrison & Giese (1991)
N.H. storm is within 9° to the east or 17° to the west of the S.H. storm
Along the same longitude
Both form between 160°E and 160°W
Storms form within 22° latitude of each other
Both form at about 5° latitude
Both from between 20°S and 20°N
Form within 9 days of each other
Form nearly simultaneously
Named within 8 days of each other (most were
within 5 days)
22 sets happen from
1971 to 1979
Twins with Typhoon intensity happen “Once
every two or three years”
5 sets happen from
1955 to 1979
3.25 days
1.4°
24 h
2.8°
1.5°
42 h
8.6°
6.75 days
• Red-filled symbols indicate N.H. storms
• Blue-filled symbols indicate S.H. storms
• All equatorward of 10.5° latitude
• Four Sets of twin tropical cyclones form in during a two-month period from 4 October to 2 December– All occur in the
central Pacific– Consistent with
the intense El Nino of 1997-1998
Fall 1997 Pacific Tropical Cyclogenesis
Data & Methods
• ECMWF operational analyses– 1.125° grid– 12-hour temporal resolution
• CLAUS Brightness Temperature data– 0.5° grid– 3-hour temporal resolution
• Combined JTWC and NHC global best track data– Tropical cyclogenesis is considered to occur when a
storm first appears in the best track
Data & Methods
• Unfiltered Data
• Time-filtered data– 15-40 day band-pass– 40-day low-pass
• Space-time filtered data (Wheeler & Kiladis 1999)– ER-band– MJO-band
Wheeler & Kiladis (1999) Space-time filters
• Shading indicates OLR power above a red background
• Thin lines are shallow water dispersion curves
• Heavy lines outline the space-time filters
Unfiltered map on 26 September
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 27 September
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 28 September
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 29 September
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 30 September
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 1 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 2 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 3 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 4 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 5 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 6 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 7 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 8 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 9 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 10 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered map on 11 October
• 850-hPa winds
• 850-hPa heights (contours every 10 m)
• Brightness temperature – 265-290 K is shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Tropical cyclone locations
Unfiltered• Twin tropical
cyclogenesis– Red-filled symbols
indicate N.H. storms – Blue-filled symbols
indicate S.H. storms• 850-hPa u
– Contours every 3 m s-1
– Westerlies in red– Easterlies in blue– Averaged 4.5°S-4.5°N
• Brightness temp.– 265-290 K shaded
with cyan– Less than 265 K
shaded with warm colors in 25 K intervals
– Averaged 10°S-10°N
40-day low-pass filtered
• Twin tropical cyclogenesis– Red-filled symbols
indicate N.H. storms – Blue-filled symbols
indicate S.H. storms• 850-hPa u
– Contours every 2 m s-1
– Westerlies in red– Easterlies in blue– Averaged 4.5°S-4.5°N
• Brightness temp.– 270-280 K shaded
with cyan– Less than 270 K
shaded with warm colors in 10 K intervals
– Averaged 10°S-10°N
MJO-band space-time filtered
• Twin tropical cyclogenesis– Red-filled symbols
indicate N.H. storms – Blue-filled symbols
indicate S.H. storms• 850-hPa u
– Contours every 1 m s-1
– Westerlies in red– Easterlies in blue– Averaged 4.5°S-4.5°N
• Brightness temp.– Shaded every 2.5 K– Negative anomalies
shaded with warm colors
– Positive anomalies shaded with cool colors
– Averaged 10°S-10°N
Low-pass filtered map on 20 September
• 850-hPa winds
• Brightness temperature – 270-280 K is shaded with cyan
– Less than 280 K is shaded with warm colors in 10 K intervals
• Twin tropical cyclogenesis within 4.5 days before or after plot
Low-pass filtered map on 29 September
• 850-hPa winds
• Brightness temperature – 270-280 K is shaded with cyan
– Less than 280 K is shaded with warm colors in 10 K intervals
• Twin tropical cyclogenesis within 4.5 days before or after plot
Low-pass filtered map on 8 October
• 850-hPa winds
• Brightness temperature – 270-280 K is shaded with cyan
– Less than 280 K is shaded with warm colors in 10 K intervals
• Twin tropical cyclogenesis within 4.5 days before or after plot
Low-pass filtered map on 17 October
• 850-hPa winds
• Brightness temperature – 270-280 K is shaded with cyan
– Less than 280 K is shaded with warm colors in 10 K intervals
• Twin tropical cyclogenesis within 4.5 days before or after plot
Low-pass filtered map on 26 October
• 850-hPa winds
• Brightness temperature – 270-280 K is shaded with cyan
– Less than 280 K is shaded with warm colors in 10 K intervals
• Twin tropical cyclogenesis within 4.5 days before or after plot
Low-pass filtered map on 4 November
• 850-hPa winds
• Brightness temperature – 270-280 K is shaded with cyan
– Less than 280 K is shaded with warm colors in 10 K intervals
• Twin tropical cyclogenesis within 4.5 days before or after plot
Summary: Low-frequency convectively generated ER waves
• The MJO could provide a favorable environment for the first three sets of twin tropical cyclones, as in Liebmann et al. (1994)
• No active MJO present for the final set of twins– But a broad area of convection was
associated with the development of equatorial westerlies
Unfiltered• Twin tropical
cyclogenesis– Red-filled symbols
indicate N.H. storms – Blue-filled symbols
indicate S.H. storms• 850-hPa u
– Contours every 3 m s-1
– Westerlies in red– Easterlies in blue– Averaged 4.5°S-4.5°N
• Brightness temp.– 265-290 K shaded
with cyan– Less than 265 K
shaded with warm colors in 25 K intervals
– Averaged 10°S-10°N
15-40 dayband-pass filtered
• Twin tropical cyclogenesis– Red-filled symbols
indicate N.H. storms – Blue-filled symbols
indicate S.H. storms• 850-hPa u
– Contours every 1 m s-1
– Westerlies in red– Easterlies in blue– Averaged 4.5°S-4.5°N
• Brightness temp.– Shaded every 5 K– Negative anomalies
shaded with warm colors
– Positive anomalies shaded with cool colors
– Averaged 10°S-10°N
• Twin tropical cyclogenesis– Red-filled symbols
indicate N.H. storms – Blue-filled symbols
indicate S.H. storms• 850-hPa u
– Contours every 1 m s-1
– Westerlies in red– Easterlies in blue– Averaged 4.5°S-4.5°N
• Brightness temp.– Shaded every 2.5 K– Negative anomalies
shaded with warm colors
– Positive anomalies shaded with cool colors
– Averaged 10°S-10°N
ER-band space-time filtered
Summary: Convectively Coupled Wave Packets
• First two sets of twins appear to be associated with a convectively coupled ER wave packet during an active MJO– Evidence even exists of the anticyclonic phase in the
unfiltered data
• Time-filtered anomalies actually propagate eastward leading up to the third set of twins
• Convectively coupled ER wave signature associated with the final set of twins is probably just a reflection of the tropical cyclones in the filter
Convectively Forced ER waves
• The MJO and convectively coupled ER waves provide favorable regions for twin tropical cyclogenesis
• But what determines when and where the storms actually form within these broad regions?
• Convectively forced ER waves could be one explanation
• Averaged maps of winds and brightness temp. before and after the development of equatorial westerlies may show the influence of convectively forced ER waves
Unfiltered map averaged 23-29 September
• 850-hPa winds• Brightness Temp.
– 270-280 K is shaded with cyan– Less than 280 K is shaded with warm
colors in 10 K intervals
Unfiltered map averaged 29 September to 4 October
• 850-hPa winds• Brightness Temp.
– 270-280 K is shaded with cyan– Less than 280 K is shaded with warm
colors in 10 K intervals
• Twin tropical cyclogenesis locations shown
23 September• Unfiltered Brightness
Temp.– 265-290 K is
shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Unfiltered 1000-hPa:– Winds– Virtual Temp.
• Countoured every 2°C
– Convergence• Shaded in 10-5 s-1
intervals
26 September• Unfiltered Brightness
Temp.– 265-290 K is
shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Unfiltered 1000-hPa:– Winds– Virtual Temp.
• Countoured every 2°C
– Convergence• Shaded in 10-5 s-1
intervals
29 September• Unfiltered Brightness
Temp.– 265-290 K is
shaded with cyan
– Less than 265 K is shaded with warm colors in 25 K intervals
• Unfiltered 1000-hPa:– Winds– Virtual Temp.
• Countoured every 2°C
– Convergence• Shaded in 10-5 s-1
intervals
Unfiltered map averaged 13-19 October
• 850-hPa winds• Brightness Temp.
– 270-280 K is shaded with cyan– Less than 280 K is shaded with warm
colors in 10 K intervals
Unfiltered map averaged 19-22 October
• 850-hPa winds• Brightness Temp.
– 270-280 K is shaded with cyan– Less than 280 K is shaded with warm
colors in 10 K intervals
• Twin tropical cyclogenesis locations shown
Unfiltered map averaged 20-26 November
• 850-hPa winds• Brightness Temp.
– 270-280 K is shaded with cyan– Less than 280 K is shaded with warm
colors in 10 K intervals
Unfiltered map averaged 26-28 November
• 850-hPa winds• Brightness Temp.
– 270-280 K is shaded with cyan– Less than 280 K is shaded with warm
colors in 10 K intervals
• Twin tropical cyclogenesis locations shown
Summary: Convectively forced ER waves
• In each case, convection sustains for 6 days near 10°S leading up to the development of equatorial westerlies
• After westerlies develop, convection intensifies and spreads across the equator
• The first tropical cyclone forms 2-6 days after the development of the equatorial westerlies
• Convection seems to be triggered by a frontal zone before the first ER wave– Other two waves lack obvious triggers
Conclusions
• The MJO created cyclonic regions that were favorable for the first three sets of twin tropical cyclogenesis
• Final set of twins had similar low-frequency convection, but probably not the MJO
• A convectively coupled ER wave packet may have contributed to the first two sets of twins, but probably not the last two
• Convectively forced ER waves might determine when the twin tropical cyclones formed
Future Work
• More complete climatology of twin tropical cyclones is needed to determine common preconditions
• Apply other filters to convectively coupled ER waves• Vertical structures of the convectively forced ER waves
need to be examined• Use idealized modeling to determine how ER waves are
influenced by– Surface friction– Surface heat fluxes– Convective heating– Various background conditions
Acknowledgements• John Molinari• Dave Vollaro, Anantha Aiyyer, Kristen
Corbosiero, Kelly Canavan, Kay Shelton, and Jackie Frank
• All the grad students • Ron McTaggart-Cowan• Chris Thorncroft, Paul Roundy, and all the
faculty• Kevin Tyle & David Knight• Celeste Iovinella, Lynn Hughes, & Sharon
Baumgardner• Mary & My Parents