The sea-breeze circulation

1. The sea-breeze circulation Part I: Development w/o Earth rotation

3. Mesoscale features in Florida animation • Horizontal convective rolls over land • Sea-breeze front penetrations on all coasts • Thunderstorm outflows • “Lake shadow” downwind of Lake Okeechobee • Interactions among features

10. Lightning frequency(flashes/sq. km/year)

11. Top left: Cross-shore flow w/ no mean flow. Bottom left: buoyancy for same case. Right: displacements over several hours.

12. How is a sea breeze formed?First, look at a generic circulation forced by temperature differences

13. Pressure

14. Pressure

15. Pressure

16. Pressure

17. Pressure Same mass of water would only be 18.5 feet deep

18. Temperature affects thickness

19. Temperature affects thickness

20. Temperature differences make pressure differences

21. Temperature differences make pressure differences

22. Pressure differences make winds

23. Pressure differences make winds Sea-breeze is not this deep…

24. Ahrens’ text Textbook description: circulation starts from top down

25. Is that how it really works?

26. dtdm < input_sbf_norolls.txt &experiment casename = 'sbf.noroll.nowind.nonanel', $ &grid_run timend = 9000., plot = 300., $ &surface_flux ishflux = 1, tdelt = 12., icoast = 90, cdh = 7.2e-3, irand = 0, $

27. DTDM simulation t=0 h t=2.5 h Vertical profile of  over land

28. Making that plot ga-> set lev 0 4 ga-> set vrange 298 314 ga-> set t 1 ga-> set x 210 ga-> d th ga-> draw xlab potential temperature ga-> draw ylab height (km) ga-> set t 7 ga-> d th ga-> set t 13 [etc…]

29. Perturbation potential temperature (colored); cross-shore horizontal velocity (contour) coastline scripts/sbf_devel.gs scripts/sbf_devel_movie.gs

30. The horizontal wind isn’t blowing from land to sea first…

34. Onshore flow always stronger; Vertical scale grows with mixed layer

35. Animation

36. Perturbation pressure (colored); cross-shore horizontal velocity (contour) scripts/sbf_devel2.gs scripts/sbf_devel_movie2.gs

41. Animation

42. Pressure perturbation 5 km inland H t=50 min L t=5 min L at surface; local H above, decreasing farther aloft

43. Analysis where At the rigid surface dw/dt = 0, therefore B > 0 for the heated surface, therefore perturbation pressure increases with height

44. Analysis, continued Why low perturbation pressure at surface? -- Far above heated surface, atmosphere undisturbed, thus ’ ~ 0 there -- If ’ increases with height and approaches zero, surface ’ must be negative

45. Analysis, continued Why low perturbation pressure at surface? Another (essentially similar) view… -- Hydrostatic eqn before & after pert analysis -- Note this implies dw/dt = 0 everywhere -- ’ = 0 at model top, all (initial) ’ > 0, so vertical gradient > 0 thus L at surface needed -- neither explanation tells us why local H pressure above the heated layer…

46. Why does’ overshoot 0, creating perturbation H pressure at z ~ 1 km? • Solve an example 1D version of anelastic ’ equation [demonstrated soon] • Invoke mass continuity in anelastic limit H L

47. Anelastic continuity equation Traditional form Integrate over a 2D column -- depth from z=0 to z=Z. But… …since rigid top and surface, w(z=0) = w(z=Z) = 0, so this term vanishes

48. Anelastic constraint Left with… Indefinite integration yields… If onshore winds are produced in a column, compensating offshore winds must also exist This requires an offshore directed PGF to exist aloft since onshore flow generated near surface

49. Result of anelastic constraint Recall mean density decreases w/ height