Convective Initiation Ahead of the Sea-Breeze Front

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Convective Initiation Ahead of the Sea-Breeze
Front

• Robert Fovell
• UCLA Atmospheric Oceanic Sciences
• rfovell_at_ucla.edu

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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
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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
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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

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SBC influence on inland environment
Highlighted trajectory 0.025 m/s (60 km inland
from coast)
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SBO run
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Conditions ahead of SBF
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Conditions ahead of SBF
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SBO run
Black contours 0.5 g/kg cloud water isolines
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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?

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A quick gravity wave primer
New convective heating
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A quick gravity wave primer
New convective heating
Initial environmental response
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A quick gravity wave primer
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A quick gravity wave primer
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A quick gravity wave primer
Nicholls et al. 1991 Mapes 1993 Fovell 2002
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A quick gravity wave primer
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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

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Colored field vertical velocity
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Colored field MSBO-SBO temperature difference
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White contour cloud water difference
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Contoured MSBO vertical velocity
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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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