Transcript Document

Convective Initiation Ahead of
the Sea-Breeze Front
Robert Fovell
UCLA Atmospheric & Oceanic Sciences
[email protected]
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-breeze circulation (SBC)
sea
SBF
land
Horizontal Convective Rolls (HCRs)
QuickTime™ and a V ideo decompressor are needed to see this picture.
rolls
sea
land
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
roll clou d
6
4
2
t = 1:30 PM
6
4
2
t = 1:40 PM
6
4
2
t = 1:50 PM
6
4
2
90
100
110
x ( km)
120
130
140
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)
SBO run at local noon
vertical velocity and temperature perturbations
horizontal velocity and vapor perturbations
180 km
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
12:35 local time
Control run

RH = 100%
SBO run
Difference Control - SBO

80 km

roll cloud
SBO run: perturbation horizontal velocity
30 km
added: vertical velocity from control run
top of mixed layer
added: Control - SBO vapor difference
added: SBO leading anvil
RH = 100%
added: origin of roll cloud and obstacle gravity waves
gravity wave
phase lines
roll cloud
RH = 100%
Difference Control - SBO
(a) 12:53 PM
(b) 12:59 PM
roll cloud
(c) 1:04 PM
(d) 1:09 PM
30 km
Where did the roll cloud air
come from?
1:30 PM
1:00 PM
1 PM
0.1 g/kg
10 AM
t = 1:20 PM
roll clou d
6
4
2
t = 1:30 PM
6
4
2
t = 1:40 PM
6
4
2
t = 1:50 PM
6
4
2
90
100
110
x ( km)
120
130
140
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
New
convective
heating
A quick gravity wave primer
Initial
environmental
response
New
convective
heating
A quick gravity wave primer
warm
A quick gravity wave primer
warm
A quick gravity wave primer
warm
warm
warm
cool
Nicholls et al. 1991; Mapes 1993; Fovell 2002
A quick gravity wave primer
A quick gravity wave primer
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“modified SBO” experiment
Cont rol ru n
RH = 100%
SBO run
moist perturbation
inserted he re
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
modified SBO run
w, TVPT, qc outline
( a) 12:53 PM
² ²qc outline
( b) 1:02 PM
"roll cloud"
outline
6
SBF cloud
outline
4
2
localized cooling
MSBO SBF updraft
w, ² •, qc outline
( c) 1:02 PM
² w, ² •, ²qc outline
( d) 1:06 PM
6
4
2
secondary
gravity wave
100
110
120
<<<
-0.5
0
100
x ( km)
0.5
1.0
1.5
2.0
110
2.5
>>>
120
1:02 PM
1:05 PM
1:11 PM
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Sorenson Video 3 decompressor
are needed to see this picture.
zoom
SBF
40 km
Dailey and Fovell (1999)