convective initiation ahead of squall lines robert fovell ucla atmospheric & oceanic sciences...
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Convective initiation ahead of squall lines
Robert FovellUCLA Atmospheric & Oceanic Sciences
(Fovell, Mullendore and Kim 2006, MWR)
Radar image of a squall line
Vertical cross-section
Vertical cross-section
Vertical cross-section
A typical multicellular squall line
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Vertical cross-section
“discrete convective initiation”
Vertical cross-section
Vertical cross-section
“discrete propagation”
X
7 May 1995, early evening
0509Z - Hastings, NE radar8 July 2003
gust front
0539Z - Hastings, NE radar
new cells ~ 18 km ahead
0549Z - Hastings, NE radar
0609Z - Hastings, NE radar
Animation of Hastings radar
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X = Hays, KS
05 June 2004
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X = Hays, KS
05 June 2004
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21 June 2003, W Oklahoma ~ after midnight
2245Z(545 PM CDT)
2245Z(545 PM CDT)
Ft. Worth
00Z Fort Worth hodograph
Rolls in an ARPS simulation
How do afternoon roll cloudsinfluence nocturnal convection?
By organizing the moisture field;
effect survives rolls themselves
MM5 simulation
• 4 km horizontal resolution; 250x330 pts
• Start 12Z previous day
• Initial/boundary conditions from Eta model
• MRF PBL scheme
MM5 model animation
• 3 hour animation (01-04Z)
• Colored field is 2 m water vapor
• Vertically integrated condensate contoured
• 10 m wind vectors
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MM5 moisture bands
• Remains of convective rolls present in model on previous afternoon
• Rolls are spurious– reflect deficiency of PBL scheme– ~40 km wavelength >> theoretical value– actual roll clouds ~ theoretical value
• Rolls are fortuitous– suggest orientation for the new cell lines
“Action at a distance” mechanism
Trapped internal gravity waves
An ARPS simulation
• 2D & 3D models
• Horizontally homogeneous initial conditions
• ∆x = 1 km, ∆z ≥ 40 m
• Warm rain processes
• Starts late afternoon
Vertical velocity (colored) ~ sunrise
main updraft
cold pool
Vertical velocity (colored) ~ sunrise
gravity waves
20 m/s
Vertical velocity (colored) ~ sunrise
Trapping or ducting below8-9 km
Vertical velocity (colored) ~ sunrise
Gravity wave ducting
z
xScorer parameter
Closer look at Scorer parameter
In mountain wave derivation, we had
Difference: mountain waves presumed steady,therefore = 0 and c = /k = 0. Also, N*2is BV frequency modified for moisture.
• Ducting: sharp decrease of l2 with height
• Here c > U
• Forward anvil as wave duct– decrease in ambient stability
• anvil: warming below, cooling above; saturated
• partially opposed by (U - c) decrease
– jet-like wind profile - curvature shear
upstream sounding
upstream soundingUzz min
upstream sounding
Trapped waves leading to discrete initiation
6 h ARPS model animation
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Discrete initiation by gravity waves alone
Note forward anvil
Discrete initiation by gravity waves alone
Gravity waves trapped beneath anvil
Discrete initiation by gravity waves alone
Wave-relative flow shown(recall c > U)
Transient trapping conditions
Internal gravity waves alone apparently can’t account for the orientation of the new cell bands
Combine gravity waves & moisture bands
Hypothesis
Plane view, looking from above
Moisture bands remaining from earlier roll activity
Hypothesis
Squall line and its forward anvil
Hypothesis
Trapped internal gravity wavesbeneath anvil
Hypothesis
Moisture bands preferred locations fordiscrete initiation
Hypothesis
Expect newest cells farthest away along moisture band
Summary
• A case of discrete initiation has been observed & simulated using variety of models
• New cell lines may be forming along pre-existing moisture bands left by previous roll activity
• “Action at a distance” may be provided by internal gravity waves excited by main storm
• Available observations appear insufficient to confirm or refute this hypothesis