Convective Initiation Ahead of the Sea-Breeze Front

Robert FovellUCLA Atmospheric & Oceanic Sciences

rfovell@ucla.edu

Overview

Sea-breeze circulation (SBC) and Horizontal Convective Rolls (HCRs)

Interaction leading to convective initiation 3D idealized cloud model

– “pseudo-Florida”

– no Coriolis, 2nd coast, inland water, coastline variations, topography or precip development

– resolution: 500 m horizontal, ≥ 150 m vertical

sea landSBF

sea-breeze circulation (SBC)

Horizontal Convective Rolls (HCRs)

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sea land

rolls

note: no initial cross-shore flow

Convective initiation

Three simulations (Fovell & Dailey 2001)

– Roll only run

– Sea-breeze only (SBO) run

– Control run (with SBC and HCRs) Only the Control run possessed deep

convection Convection initiated ahead of SBF

Control run

Items of interest

Roll spawns deep convection as SBF approaches

SBF cloud becomes suppressed at first– Propagation speed increases by 60%

SBF itself spawns deep convection prior to roll contact– Two vigorous updrafts - single cloud shield– Brief yet strong downdraft appears in between– Dramatic slowing of SBF during this time

t = 1:20 PM

2

4

6roll cloud

90 110100 120 130 140

t = 1:30 PM

t = 1:40 PM

2

4

6

2

4

6

x (km)

t = 1:50 PM

2

4

6

Effect of latent heating on SBF propagation

Suppression of SBF cloud results in propagation speed increase

Hypothesis for roll cloud formation SBC substantially modifies upstream

environment (over land)– midtropospheric moistening– horizontal flow perpendicular to SBF/coast– necessary, but not sufficient

Rolls provide the spark– moist plumes above roll updrafts– obstacle effect gravity waves

SBC influence on inland environment

Highlighted trajectory: ~0.025 m/s(60 km inland from coast)

180 km

SBO run at local noon

horizontal velocity and vapor perturbations

vertical velocity and temperature perturbations

SBO run

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Conditions ahead of SBF

Conditions ahead of SBF

SBO run

Black contours: 0.5 g/kg cloud water isolines

Analysis

Control, SBO runs similar until deep convection onset– Control run fields

– SBO run fields

– Difference fields• Isolate effect of rolls on SBC

α α

α

RH = 100%

12:35 local timeControl run

SBO run

Difference Control - SBO

80 km

roll cloud

30 km

SBO run: perturbation horizontal velocity

added: vertical velocity from control run

top of mixed layer

added: Control - SBO vapor difference

added: SBO leading anvil

RH = 100%

RH = 100%

added: origin of roll cloud and obstacle gravity waves

roll cloud

gravity wavephase lines

30 km

(a) 12:53 PM (b) 12:59 PM

(c) 1:04 PM (d) 1:09 PM

Difference Control - SBO

roll cloud

Where did the roll cloud air come from?

1 PM

10 AM

1:30 PM

1:00 PM

0.1 g/kg

t = 1:20 PM

2

4

6roll cloud

90 110100 120 130 140

t = 1:30 PM

t = 1:40 PM

2

4

6

2

4

6

x (km)

t = 1:50 PM

2

4

6

Suppression and reintensification of the SBF updraft/cloud Use small moisture blob to mimic roll cloud in

SBO run Two gravity waves excited in response

– Deep subsidence responds to latent heating in “roll cloud”

– Shallower mode starting as adiabatic cooling beneath the “roll cloud”

– First wave suppresses SBF cloud, second reinvigorates it

Dynamical effect of shear?

A quick gravity wave primer

Newconvective

heating

A quick gravity wave primer

Newconvective

heating

Initialenvironmental

response

warm

A quick gravity wave primer

warm

A quick gravity wave primer

warm

warmwarm

cool

A quick gravity wave primer

Nicholls et al. 1991; Mapes 1993; Fovell 2002

A quick gravity wave primer

A quick gravity wave primer

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moist perturbationinserted here

“modified SBO” experiment

RH = 100%

Control run

SBO run

80 km

Analysis

Analysis will employ– Original SBO run fields

– Modified SBO run fields “MSBO”

– Difference fields• Isolate effect of “roll cloud” on SBF & cloud

Colored field: vertical velocity

Colored field: MSBO-SBO temperature difference

White contour: cloud water difference

Contoured: MSBO vertical velocity

Colored field: water vapor difference

Arrows: horizontal velocity difference

Summary Deep convection occurs only in run having both

SBC and HCRs Roll convection excited prior to SBF-HCR

merger SBC upstream effects necessary, not sufficient

to support deep convection Roll-associated moist plume, obstacle-effect

gravity waves provide spark Once established, roll convection modulated

SBF convection through additional gravity waves

(a) 12:53 PM (b) 1:02 PM

100 110 100 110

2

4

6

120 120

SBF cloud outline

<<< -0.5 0 0.5 1.0 1.5 2.0 2.5 >>>

modified SBO run

w, TVPT, qc outline ∆�, ∆qc outline

localized cooling

"r oll cloud"outline

( ) 1:02 c PM w, ∆� , qc outline ( ) 1:06 d PM ∆w, ∆� , ∆qc outline

2

4

6

( )x km

MSBO SBF updraft

secondary gravity wave

1:02 PM

1:05 PM

1:11 PM

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40 km

zoom

SBF

Dailey and Fovell (1999)