Convective Initiation Ahead of the Sea-Breeze Front

1. Convective Initiation Ahead of the Sea-Breeze Front Robert Fovell UCLA Atmospheric & Oceanic Sciences rfovell@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

4. Horizontal Convective Rolls (HCRs)

8. 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

10. 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

13. 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)

17. 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

24. roll cloud

31. Where did the roll cloud air come from?

33. 0.1 g/kg

35. 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 Initial environmental response New convective heating

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

44. 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

47. Colored field: MSBO-SBO temperature difference

48. White contour: cloud water difference

49. Contoured: MSBO vertical velocity

50. Colored field: water vapor difference