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Title: Convective Initiation Ahead of the Sea-Breeze Front


1
Convective Initiation Ahead of the Sea-Breeze
Front
  • Robert Fovell
  • UCLA Atmospheric Oceanic Sciences
  • rfovell_at_ucla.edu

2
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

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Horizontal Convective Rolls (HCRs)
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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

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Control run
11
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

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Effect of latent heating on SBF propagation
Suppression of SBF cloud results in propagation
speed increase
14
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

15
SBC influence on inland environment
Highlighted trajectory 0.025 m/s (60 km inland
from coast)
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SBO run
18
Conditions ahead of SBF
19
Conditions ahead of SBF
20
SBO run
Black contours 0.5 g/kg cloud water isolines
21
Analysis
  • Control, SBO runs similar until deep convection
    onset
  • Control run fields
  • SBO run fields
  • Difference fields
  • Isolate effect of rolls on SBC

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roll cloud
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Where did the roll cloud air come from?
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0.1 g/kg
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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?

36
A quick gravity wave primer
New convective heating
37
A quick gravity wave primer
New convective heating
Initial environmental response
38
A quick gravity wave primer
39
A quick gravity wave primer
40
A quick gravity wave primer
Nicholls et al. 1991 Mapes 1993 Fovell 2002
41
A quick gravity wave primer
42
A quick gravity wave primer
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Analysis
  • Analysis will employ
  • Original SBO run fields
  • Modified SBO run fields MSBO
  • Difference fields
  • Isolate effect of roll cloud on SBF cloud

45
Colored field vertical velocity
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Colored field MSBO-SBO temperature difference
48
White contour cloud water difference
49
Contoured MSBO vertical velocity
50
Colored field water vapor difference
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Arrows horizontal velocity difference
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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

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Dailey and Fovell (1999)
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