Convective initiation ahead of squall lines Robert Fovell UCLA Atmospheric & Oceanic Sciences (Fovell, Mullendore and Kim 2006, MWR)

Radar image of a squall line

Vertical cross-section

A typical multicellular squall line

Vertical cross-section “discrete convective initiation”

Vertical cross-section

“discrete propagation” X

7 May 1995, early evening

0509Z - Hastings, NE radar 8 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

X = Hays, KS 05 June 2004

X = Hays, KS 05 June 2004

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 clouds influence 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

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 below 8-9 km

Vertical velocity (colored) ~ sunrise

Gravity wave ducting z x Scorer 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*2 is BV frequency modified for moisture.

Ducting: sharp decrease of l 2 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

U zz min

upstream sounding

Trapped waves leading to discrete initiation

6 h ARPS model animation

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 waves beneath anvil

Hypothesis Moisture bands preferred locations for discrete 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