convective initiation ahead of the sea-breeze front robert fovell ucla atmospheric & oceanic...
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Convective Initiation Ahead of the Sea-Breeze Front
Robert FovellUCLA Atmospheric & Oceanic Sciences
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)
QuickTime™ and aVideo decompressorare needed to see this picture.
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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are needed to see this picture.
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)