Convective initiation ahead of squall lines

Robert FovellUCLA Atmospheric & Oceanic Sciences

rfovell@ucla.edu

(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