tropical squall lines as convectively coupled gravity waves: why do most systems travel westward?...
TRANSCRIPT
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Tropical squall lines as convectively coupled gravity waves: Why do most
systems travel westward?
Stefan Tulich1 and George Kiladis2
1CIRES, University of Colorado, Boulder CO, USA
2NOAA ESRL, Boulder CO, USA
Funding: NSF ATM-0806553
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Objectives
1) Provide evidence that many tropical “squall line systems” are part of a broad family of disturbances that arise through coupling between convection and tropospheric gravity waves
2) Start to address the question of why most of these wave disturbances move westward
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Outline
1) Brief historical review of tropical squall lines - how did we come to know about them; current state of knowledge
2) Analysis of observational data - provide evidence to support the idea
3) Explicit simulations of convection on an equatorial beta-plane - test hypothesis about what causes westward bias
4) Conclusions and future work
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Historical Review of Tropical Squall Lines
If one goes back to the earliest papers by leading authors, they’ll be pointed to two even earlier papers on west African squall lines
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West African “Disturbance Lines”
• Hamilton and Archibald (1945; QJRMS; No previous articles referenced!) • Eldridge (1957; QJRMS; 2 articles referenced)
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West African “Disturbance Lines”
• Hamilton and Archibald (1945; QJRMS; No previous articles referenced!) • Eldridge (1957; QJRMS; 2 articles referenced)
25 deg / 45 hr = 17 m/s
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The Thunderstorm Project (1947; USA)
Newton (1950; J. Meteor.) “Structure and mechanisms of the prefrontal squall line”
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The Thunderstorm Project (1947; USA)
Newton (1950; J. Meteor.) “Structure and mechanisms of the prefrontal squall line”
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The Line Islands Exp. (1967 Cntrl. Pac.)
Zipser (1969; J. Appl. Meteor.) “The role of organized unsaturated downdrafts in the structure and decay of an equatorial disturbance”
15 m/s
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The Line Islands Exp. (1967 Cntrl. Pac.)
Zipser (1969; J. Appl. Meteor.) “The role of organized unsaturated downdrafts in the structure and decay of an equatorial disturbance”
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GATE (1974; Eastern Atlantic)
• Several squall lines sampled as they passed across the IFA
• Barnes and Sieckman (1984; MWR) “The environment of fast- and slow-moving tropical mesoscale convective cloud lines”
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GATE (1974; Eastern Atlantic)
• A number of squall lines sampled as they passed across the IFA
• Barnes and Sieckman (1984; MWR) “The environment of fast- and slow-moving tropical mesoscale convective cloud lines”
Vn > 7 m/s Vn < 3 m/s
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TOGA-COARE (1992; Eq. west Pac.)
• Similar to GATE but satellite data more accessible
• Linear MCS-scale bands dominate total rainfall• Numerous fast-moving “2-day waves” were sampled
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TOGA-COARE (1992; Eq. west Pac.)
2-day wave composite evolution
Haertel and Johnson (1998)
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TOGA-COARE (1992; Eq. west Pac.)
2-day wave composite evolution
Haertel and Johnson (1998)
~ 1500 km
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TOGA-COARE (1992; Eq. west Pac.)
2-day wave composite evolution
Haertel and Johnson (1998)
16 m/s
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TOGA-COARE (1992; Eq. west Pac.)
Takayabu et al. (1996)
2-day wave vertical cloud evolution
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TOGA-COARE (1992; Eq. west Pac.)
Takayabu et al. (1996)
2-day wave vertical cloud evolution
Are 2-day waves just large-scale squall lines?
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TOGA-COARE (1992; Eq. west Pac.)
Takayabu et al. (1996)
2-day wave vertical cloud evolution
Are 2-day waves just large-scale squall lines?
Or are squall-lines mini-versions of 2-day waves?
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Observational Analysis
• Goal: Advance the idea that many tropical squall line systems are part of a broader family of convectively coupled gravity wave disturbances
• Strategy: Space-time spectral (Fourier) analysis of high-resolution satellite data
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Space-time spectral analysis: Previous work
Wheeler and Kiladis (1999)
Power Spectrum of OLR (symmetric component)
Westward Eastward
96 days
3 days
-15 15
1.25 days
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Space-time spectral analysis: Previous work
Wheeler and Kiladis (1999)
Power Spectrum of OLR (symmetric component)
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Space-time spectral analysis: Previous work
Wheeler and Kiladis (1999)
Power Spectrum of OLR (symmetric component)
Kelvin waves (3-10 day) Eq. Rossby waves(6-50 day)
Westward inertia-gravitywaves (1.3-2.5 day)
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Spectral Analysis of TRMM• TRMM 3B42 Rainfall Product
• 1) Global from 50N-50S• 2) 0.25 deg. resolution in space• 3) 3-hourly in time (1999-present)
TRMM TMI CPC Global Merged IR
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Spectral Analysis of TRMM• TRMM 3B42 Rainfall Product
• 1) Global from 50N-50S• 2) 0.25 deg. resolution in space• 3) 3-hourly in time (1999-present)
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TRMM rainfall spectrum
96 days
3 days
1.7 days
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Looking at smaller scales
96 days
12 hrs
1 day
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Looking at smaller scales
Sharp diurnal peak
96 days
12 hrs
1 day
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Looking at smaller scales
Sharp diurnal peak
hn ~ 20-40 m
96 days
12 hrs
1 day
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Looking at smaller scales
Sharp diurnal peak
cn ~ 14-20 m/s
96 days
12 hrs
1 day
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Looking at even smaller scales
96 days
6 hrs
12 hrs
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Looking at even smaller scales
~ 6-hr periods &~ 400-km wavelengths
96 days
6 hrs
12 hrs
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Where are these signals most active?
“WIG” filter window
96 days
6 hrs
12 hrs
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Map of WIG-filtered variance (Boreal Summer JJA)
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Focus on N. Africa (JJA)
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Focus on N. Africa (JJA)
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Hovmollers of rainfall over N. Africa (7.5-12.5N)
2005 2006 2007
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Hovmollers of rain over N. Africa (7.5-12.5N)
2005 2006 2007
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How do these systems relate to objectively identified squall lines?
AMMA 2006 Field Experiment (ROP: July 5 – Sept 27)
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Analysis of Niamey Radar Data
Rickenbach et al. (2009; JGR) “Radar-observed squall line propagation…”
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Rain Hovmoller + Radar Identified Squall Lines
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Linear convective bands during TOGA COARE?
Rickenbach and Rutledge (1998)
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Linear convective bands during TOGA COARE?
Rickenbach and Rutledge (1998)
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Hovmoller of CLAUS Tb during TOGA COARE (Cruises 2 and 3)
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Hovmoller of CLAUS Tb during TOGA COARE (Cruises 2 and 3)
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Inclusion of EIG-filtered rainfall
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Inclusion of EIG-filtered rainfall
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What is the typical evolution of these disturbances?
Strategy:
Lagged linear regression of WIG-filtered rainfall to construct statistical composites
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Location of base point
Base point (2E, 10N)
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Composite WIG rain evolution (2E,10N)
Note: data averaged between 7.5-12.5 N
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Composite WIG rain evolution (2E,10N)
18 m/s
Note: data averaged between 7.5-12.5 N
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Composite WIG rain evolution (2E,10N)
18 m/s
~2 day period
Note: data averaged between 7.5-12.5 N
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Composite WIG rain evolutionPlan views at lags: -12,0,12 hr
+12 hr 0 hr -12 hr
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Comparison to the west Pac.
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Composite WIG wave evolution (155E, 5N)
Note: data averaged between 2.5-7.5 N
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Composite WIG wave evolution (155E, 5N)
18 m/s
Note: data averaged between 2.5-7.5 N
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Composite WIG wave evolution (155E, 5N)
18 m/s
~2 day period
Note: data averaged between 2.5-7.5 N
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Side by side comparison
West Pacific West Africa
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Side by side comparison
West Pacific West Africa
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Side by side comparison
West Pacific West Africa
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Side by side comparison (Plan view at lag 0)
West Pacific West Africa
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Side by side comparison (Plan view at lag 0)
West Pacific West Africa
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Side by side comparison (Plan view at lag 0)
West Pacific West Africa
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Oceanic WIG waves as traveling “V”s or “U”s
West Pacific
Takayabu (1994)
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Oceanic WIG waves as traveling “V”s or “U”s
West Pacific
Takayabu (1994)
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And squall lines too!
West PacificZipser (1969)
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Conclusions thus far
• Tropical squall line systems and linear MCSs appear to be associated (if not synonymous) with convectively coupled gravity wave disturbances
• Westward-moving waves dominate, especially over Africa
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Idealized numerical experiment
• Explicit, nested simulations of convection on an equatorial beta-plane
• Two types of runs:
1) Zonal-mean u-wind relaxed to zero
2) Zonal-mean u-wind relaxed to shear profile
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Idealized numerical experiment
• Explicit, nested simulations of convection on an equatorial beta-plane
• Two types of runs:
1) Zonal-mean u-wind relaxed to zero
2) Zonal-mean u-wind relaxed to shear profile
![Page 70: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/70.jpg)
Idealized numerical experiment
• Explicit, nested simulations of convection on an equatorial beta-plane
• Two types of runs:
1) Zonal-mean u-wind relaxed to zero
2) Zonal-mean u-wind relaxed to shear profile
![Page 71: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/71.jpg)
Further details
• Model: WRF (most recent version)
• Forcing: Spatially uniform radiative-like cooling to drive deep convection
• SST: Zonally uniform; peaked at eq.
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Further details
• Model: WRF (most recent version)
• Forcing: Spatially uniform radiative-like cooling to drive deep convection
• SST: Zonally uniform; peaked at eq.
![Page 73: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/73.jpg)
Nesting strategy: 3 grids
dx, dy = 27 km
8000 km
9900
km
Equator
45 N
45 S
Grid 1
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Nesting strategy: 3 grids
dx, dy = 27 km
8000 km
9900
km
Equator
45 N
45 S
PeriodicPeriodic
Grid 1
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Nesting strategy: 3 grids
dx, dy = 27 km
8000 km
9900
km
Equator
45 N
45 S
PeriodicPeriodic
Rigid wall
Rigid wall
Grid 1
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Nesting strategy: 3 grids
dx, dy = 9 km
8000 km
15 N
15 S
45 N
45 S
3300
km Grid 2
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Nesting strategy: 3 grids
8000 km
15 N
15 S
PeriodicPeriodic
45 N
45 S
Grid 2
dx, dy = 9 km
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Nesting strategy: 3 grids
8000 km
15 N
15 S
PeriodicPeriodic
45 N
45 S
dx, dy = 9 km
Grid 2
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Nesting strategy: 3 grids
8000 km
15 N
15 S
45 N
45 S
5 N
5 S
Grid 3dx, dy = 3 km
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• Coriolis force acts only on perturbation winds (about the zonal mean)
• Prevents the formation of unwanted zonal jets and tradewinds
One last detail
![Page 81: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/81.jpg)
Results
![Page 82: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/82.jpg)
Rain hovmoller: No shear
![Page 83: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/83.jpg)
Rain hovmoller: No shear
![Page 84: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/84.jpg)
Rain spectrum: No shear
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Rain hovmoller: Shear
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Rain hovmoller: Shear
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Rain spectrum: Shear
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Other shear profiles
![Page 90: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/90.jpg)
Hovmoller for shear reversal
![Page 91: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/91.jpg)
Conclusions
• Vertical shear of background zonal wind is essential for producing westward bias in convective wave propagation
• Simulated “V”-pattern in cloudiness consistent with observations of oceanic squall lines and 2-day waves
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Implications of “V” pattern
Radar 1
Radar 2
![Page 93: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/93.jpg)
Implications of “V” pattern
Radar 1
Radar 2
![Page 94: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/94.jpg)
Implications of “V” pattern
Radar 1
Radar 2
![Page 95: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/95.jpg)
Implications of “V” pattern
Radar 1
Radar 2
![Page 96: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/96.jpg)
Implications of “V” pattern
Radar 1
Radar 2
Fast-mover;Shear perpendicular
Slow-mover;Shear parallel
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Open Questions
• Why are two-day periodicities absent from the model?
• Why is low-level shear important?• Role of topography/diurnal forcing?• What determines the “V” vs. N-S line
structure?• Implications of westward bias towards the
QBO?
![Page 98: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/98.jpg)
What about the squall lines observed during GATE?
Going back to the first geostationary satellite IR dataset
(SMS-1; Smith & Vonderhaar 1976, CSU Tech note.)
• hourly at ~ 0.1 deg
![Page 99: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/99.jpg)
What about the squall lines observed during GATE?
Going back to the first geostationary satellite IR dataset
(SMS-1; Smith & Vonderhaar 1976, CSU Tech note.)
![Page 100: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/100.jpg)
What about the squall lines observed during GATE?
Going back to the first geostationary satellite IR dataset
(SMS-1; Smith & Vonderhaar 1976, CSU Tech note.)
![Page 101: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/101.jpg)
Typical (18-day window) power spectrum of SMS Tb observed during GATE
![Page 102: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/102.jpg)
Hovmoller of SMS Tb (<250 K) during GATE
Squall line dates reported by Houze and Rappaport (1984)
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Hovmoller of CLAUS Tb during TOGA COARE (Cruises 1 and 2)
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Hovmoller of CLAUS Tb during TOGA COARE (Cruises 1 and 2)
![Page 105: Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich 1 and George Kiladis 2 1 CIRES, University](https://reader036.vdocument.in/reader036/viewer/2022062407/56649e5c5503460f94b54ca2/html5/thumbnails/105.jpg)
1
Hovmoller of CLAUS Tb during TOGA COARE (Cruises 1 and 2)