tropical squall lines as convectively coupled gravity waves: why do most systems travel westward?...
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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
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
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
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
West African “Disturbance Lines”
• Hamilton and Archibald (1945; QJRMS; No previous articles referenced!) • Eldridge (1957; QJRMS; 2 articles referenced)
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
The Thunderstorm Project (1947; USA)
Newton (1950; J. Meteor.) “Structure and mechanisms of the prefrontal squall line”
The Thunderstorm Project (1947; USA)
Newton (1950; J. Meteor.) “Structure and mechanisms of the prefrontal squall line”
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
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”
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”
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
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
TOGA-COARE (1992; Eq. west Pac.)
2-day wave composite evolution
Haertel and Johnson (1998)
TOGA-COARE (1992; Eq. west Pac.)
2-day wave composite evolution
Haertel and Johnson (1998)
~ 1500 km
TOGA-COARE (1992; Eq. west Pac.)
2-day wave composite evolution
Haertel and Johnson (1998)
16 m/s
TOGA-COARE (1992; Eq. west Pac.)
Takayabu et al. (1996)
2-day wave vertical cloud evolution
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?
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?
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
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
Space-time spectral analysis: Previous work
Wheeler and Kiladis (1999)
Power Spectrum of OLR (symmetric component)
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)
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
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 rainfall spectrum
96 days
3 days
1.7 days
Looking at smaller scales
96 days
12 hrs
1 day
Looking at smaller scales
Sharp diurnal peak
96 days
12 hrs
1 day
Looking at smaller scales
Sharp diurnal peak
hn ~ 20-40 m
96 days
12 hrs
1 day
Looking at smaller scales
Sharp diurnal peak
cn ~ 14-20 m/s
96 days
12 hrs
1 day
Looking at even smaller scales
96 days
6 hrs
12 hrs
Looking at even smaller scales
~ 6-hr periods &~ 400-km wavelengths
96 days
6 hrs
12 hrs
Where are these signals most active?
“WIG” filter window
96 days
6 hrs
12 hrs
Map of WIG-filtered variance (Boreal Summer JJA)
Focus on N. Africa (JJA)
Focus on N. Africa (JJA)
Hovmollers of rainfall over N. Africa (7.5-12.5N)
2005 2006 2007
Hovmollers of rain over N. Africa (7.5-12.5N)
2005 2006 2007
How do these systems relate to objectively identified squall lines?
AMMA 2006 Field Experiment (ROP: July 5 – Sept 27)
Analysis of Niamey Radar Data
Rickenbach et al. (2009; JGR) “Radar-observed squall line propagation…”
Rain Hovmoller + Radar Identified Squall Lines
Linear convective bands during TOGA COARE?
Rickenbach and Rutledge (1998)
Linear convective bands during TOGA COARE?
Rickenbach and Rutledge (1998)
Hovmoller of CLAUS Tb during TOGA COARE (Cruises 2 and 3)
Hovmoller of CLAUS Tb during TOGA COARE (Cruises 2 and 3)
Inclusion of EIG-filtered rainfall
Inclusion of EIG-filtered rainfall
What is the typical evolution of these disturbances?
Strategy:
Lagged linear regression of WIG-filtered rainfall to construct statistical composites
Location of base point
Base point (2E, 10N)
Composite WIG rain evolution (2E,10N)
Note: data averaged between 7.5-12.5 N
Composite WIG rain evolution (2E,10N)
18 m/s
Note: data averaged between 7.5-12.5 N
Composite WIG rain evolution (2E,10N)
18 m/s
~2 day period
Note: data averaged between 7.5-12.5 N
Composite WIG rain evolutionPlan views at lags: -12,0,12 hr
+12 hr 0 hr -12 hr
Comparison to the west Pac.
Composite WIG wave evolution (155E, 5N)
Note: data averaged between 2.5-7.5 N
Composite WIG wave evolution (155E, 5N)
18 m/s
Note: data averaged between 2.5-7.5 N
Composite WIG wave evolution (155E, 5N)
18 m/s
~2 day period
Note: data averaged between 2.5-7.5 N
Side by side comparison
West Pacific West Africa
Side by side comparison
West Pacific West Africa
Side by side comparison
West Pacific West Africa
Side by side comparison (Plan view at lag 0)
West Pacific West Africa
Side by side comparison (Plan view at lag 0)
West Pacific West Africa
Side by side comparison (Plan view at lag 0)
West Pacific West Africa
Oceanic WIG waves as traveling “V”s or “U”s
West Pacific
Takayabu (1994)
Oceanic WIG waves as traveling “V”s or “U”s
West Pacific
Takayabu (1994)
And squall lines too!
West PacificZipser (1969)
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
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
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
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
Further details
• Model: WRF (most recent version)
• Forcing: Spatially uniform radiative-like cooling to drive deep convection
• SST: Zonally uniform; peaked at eq.
Further details
• Model: WRF (most recent version)
• Forcing: Spatially uniform radiative-like cooling to drive deep convection
• SST: Zonally uniform; peaked at eq.
Nesting strategy: 3 grids
dx, dy = 27 km
8000 km
9900
km
Equator
45 N
45 S
Grid 1
Nesting strategy: 3 grids
dx, dy = 27 km
8000 km
9900
km
Equator
45 N
45 S
PeriodicPeriodic
Grid 1
Nesting strategy: 3 grids
dx, dy = 27 km
8000 km
9900
km
Equator
45 N
45 S
PeriodicPeriodic
Rigid wall
Rigid wall
Grid 1
Nesting strategy: 3 grids
dx, dy = 9 km
8000 km
15 N
15 S
45 N
45 S
3300
km Grid 2
Nesting strategy: 3 grids
8000 km
15 N
15 S
PeriodicPeriodic
45 N
45 S
Grid 2
dx, dy = 9 km
Nesting strategy: 3 grids
8000 km
15 N
15 S
PeriodicPeriodic
45 N
45 S
dx, dy = 9 km
Grid 2
Nesting strategy: 3 grids
8000 km
15 N
15 S
45 N
45 S
5 N
5 S
Grid 3dx, dy = 3 km
• Coriolis force acts only on perturbation winds (about the zonal mean)
• Prevents the formation of unwanted zonal jets and tradewinds
One last detail
Results
Rain hovmoller: No shear
Rain hovmoller: No shear
Rain spectrum: No shear
Rain hovmoller: Shear
Rain hovmoller: Shear
Rain spectrum: Shear
Other shear profiles
Hovmoller for shear reversal
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
Implications of “V” pattern
Radar 1
Radar 2
Implications of “V” pattern
Radar 1
Radar 2
Implications of “V” pattern
Radar 1
Radar 2
Implications of “V” pattern
Radar 1
Radar 2
Implications of “V” pattern
Radar 1
Radar 2
Fast-mover;Shear perpendicular
Slow-mover;Shear parallel
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?
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
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.)
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.)
Typical (18-day window) power spectrum of SMS Tb observed during GATE
Hovmoller of SMS Tb (<250 K) during GATE
Squall line dates reported by Houze and Rappaport (1984)
Hovmoller of CLAUS Tb during TOGA COARE (Cruises 1 and 2)
Hovmoller of CLAUS Tb during TOGA COARE (Cruises 1 and 2)
1
Hovmoller of CLAUS Tb during TOGA COARE (Cruises 1 and 2)
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