Convective Initiation Ahead of the Sea-Breeze Front Robert Fovell UCLA 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

Horizontal Convective Rolls (HCRs)

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

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

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

roll cloud

Where did the roll cloud air come from?

0.1 g/kg

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 New convective heating Initial environmental response

A quick gravity wave primer

Nicholls et al. 1991; Mapes 1993; Fovell 2002

A quick gravity wave primer

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

Dailey and Fovell (1999)