Sam Miller Judd Gregg Meteorology Institute

1. SEA BREEZES Sam Miller Judd Gregg Meteorology Institute Dept. of Chemical, Earth, Atmospheric and Physical Sciences

2. OVERVIEW What is the sea breeze? Some ancient history Getting things moving The sea breeze is a complex system Sea breeze response to synoptic-scale wind Forecasting Problems The Central New England Sea Breeze Study Forecasting the Sea Breeze at Logan Airport

3. WHAT IS THE SEA BREEZE? The sea breeze is a mesoscale wind driven by local differences in near-surface air temperature above adjacent land and sea surfaces. (“Mesoscale” means larger than 2 km and smaller than 2,000 km.)

4. WHAT IS THE SEA BREEZE? The sea breeze is a mesoscale wind driven by local differences in near-surface air temperature above adjacent land and sea surfaces. Not a synoptic-scale wind off the water. “Synoptic scale” means greater than 2,000 km

5. Surface Analysis 14 Nov 2006 1500Z

6. With low pressure centered to the south of New England, a wind out of the east to southeast is implied for the coast. L Surface Analysis 14 Nov 2006 1500Z

7. With low pressure centered to the south of New England, a wind out of the east to southeast is implied for the coast. THIS IS NOT A SEA BREEZE. L Surface Analysis 14 Nov 2006 1500Z

8. ANCIENT HISTORY Classical Greeks knew about sea breeze. HOW?

9. ANCIENT HISTORY Classical Greeks knew about sea breeze. HOW? They were fisherman! Of course they knew about the sea breeze!

10. ANCIENT HISTORY 480 BC: General Themistocles defeated Persians using his knowledge of the sea breeze.

11. ANCIENT HISTORY 480 BC: General Themistocles defeated Persians using his knowledge of the sea breeze. Drew the larger Persian ships into a narrow channel between the Greek mainland and the island of Salamis. Began battle at the moment the sea breeze started. Smaller Greek ships outmaneuvered Persians.

12. ANCIENT HISTORY 480 BC: General Themistocles defeated Persians using his knowledge of the sea breeze. Drew the larger Persian ships into a narrow channel between the Greek mainland and the island of Salamis. Began battle at the moment the sea breeze started. Smaller Greek ships outmaneuvered Persians. (Persians had a bad day.)

13. ANCIENT HISTORY First scientist to write about the wind in general was Aristotle. Meteorologica: Written 150 years after the sea battle at Salamis.

14. ANCIENT HISTORY First scientist to write about the wind in general was Aristotle. Meteorologica: Written 150 years after the sea battle at Salamis. Described the prevailing wind in Athens as being from the north or south.

15. ANCIENT HISTORY First scientist to write about the wind in general was Aristotle. Meteorologica: Written 150 years after the sea battle at Salamis. Described the prevailing wind in Athens as being from the north or south. The northerly wind occurs after the Summer Solstice. Called the Etesian Wind.

16. ANCIENT HISTORY 30 years later Theophrastus wrote De Ventis. “At about the time of the Etesians, winds arise counter to the north wind because of a cycling back, so that ships move in the opposite direction. These are called reverse north winds.” ETESIANS ETESIANS REVERSE NORTH WINDS

17. ANCIENT HISTORY “Everywhere at noon the winds die down because of the Sun’s actions, and arise again in the late afternoon.”

18. ANCIENT HISTORY “Everywhere at noon the winds die down because of the Sun’s actions, and arise again in the late afternoon.” THAT’S THE RIGHT ANSWER.

19. GETTING THINGS MOVING (THEOPHRASTUS IN MODERN LANGUAGE) 1. Sun heats Earth’s surface during the day.

20. GETTING THINGS MOVING (THEOPHRASTUS IN MODERN LANGUAGE) 1. Sun heats Earth’s surface during the day. 2. Land surface temperature increases more rapidly than sea surface temperature.

21. GETTING THINGS MOVING (THEOPHRASTUS IN MODERN LANGUAGE) • 1. Sun heats Earth’s surface during the day. • 2. Land surface temperature increases more rapidly • than sea surface temperature. • As density of air over land decreases, an area • of relatively low pressure is created.

22. GETTING THINGS MOVING (THEOPHRASTUS IN MODERN LANGUAGE) • 1. Sun heats Earth’s surface during the day. • 2. Land surface temperature increases more rapidly • than sea surface temperature. • As density of air over land decreases, an area • of relatively low pressure is created. • Landward-pointing thermally-driven pressure • gradient force (Thermal PGF) is created.

23. GETTING THINGS MOVING (THEOPHRASTUS IN MODERN LANGUAGE) • 1. Sun heats Earth’s surface during the day. • 2. Land surface temperature increases more rapidly • than sea surface temperature. • As density of air over land decreases, an area • of relatively low pressure is created. • Landward-pointing thermally-driven pressure • gradient force (Thermal PGF) is created. • Mass begins flowing toward the lower pressure • on land.

24. GETTING THINGS MOVING (THEOPHRASTUS IN MODERN LANGUAGE) • 1. Sun heats Earth’s surface during the day. • 2. Land surface temperature increases more rapidly • than sea surface temperature. • As density of air over land decreases, an area • of relatively low pressure is created. • Landward-pointing thermally-driven pressure • gradient force (Thermal PGF) is created. • Mass begins flowing toward the lower pressure • on land. • Return flow develops aloft, conserving mass.

25. GETTING THINGS MOVING (THEOPHRASTUS IN MODERN LANGUAGE) • 1. Sun heats Earth’s surface during the day. • 2. Land surface temperature increases more rapidly • than sea surface temperature. • As density of air over land decreases, an area • of relatively low pressure is created. • Landward-pointing thermally-driven pressure • gradient force (Thermal PGF) is created. • Mass begins flowing toward the lower pressure • on land. • Return flow develops aloft, conserving mass. • Sea-breeze circulation is established.

26. Bjerknes Circulation Theorem is often used to describe sea breeze. IDEA: Switch on a 10 dC temperature difference between land and sea, and a landward acceleration immediately appears. With “realistic” values of pressure, temperature and scale, BCT predicts wind speeds of 50 knots after one hour.

27. Bjerknes CirculationTheorem is often used to describe sea breeze. THAT’S THE WRONG ANSWER. 5 – 10 knots is more realistic speed for sea breeze.

28. Bjerknes Circulation Theorem ignores friction, time-variance of temperature, heat diffusion, stability, Coriolis force, momentum diffusion, topography, shape of coastline, and the ambient wind. Need to abandon Bjerknes for a much more complete model if you want to get something approaching the right answer.

29. That means abandoning the simple model noted above. Actual initiation of sea breeze is highly non-linear, and has been shown to require non-hydrostatic atmosphere as mechanism for establishing mesoscale pressure gradient.

30. SEA BREEZE IS A COMPLEX SYSTEM

31. Sea-breeze circulation (SBC) • Sea-breeze gravity current (SBG) • Sea-breeze front, in two parts: • Thermal front (SBFth) • Kinematic front (SBFkn) • Sea-breeze head (SBH) and Undular Bores • Kelvin-Helmholtz Billows (KHBs) • Convective Internal Boundary Layer (CIBL) • Pre-frontal phenomena

32. SEA-BREEZE CIRCULATION (SBC) Largest-scale portion of sea-breeze system, including landward flow near surface, seaward flow near 900 hPa, and vertical currents at either end. (Incorrectly described by Bjerknes Circulation Theorem)

33. SEA-BREEZE GRAVITY CURRENT (SBG) Landward flow of cool marine air near the Earth’s surface. Varies in depth from a few hundred to a few thousand meters.

34. SEA-BREEZE FRONT (SBFth) Leading edge of SBG. May be marked by sharp horizontal contrast in temperature and dew point.

35. SEA-BREEZE FRONT (SBFkn) Leading edge of SBC Marked by 1) low-level convergence between synoptic (ambient) wind and landward moving SBC, and 2) strong upward vertical motion.

36. SEA-BREEZE FRONT (SBFkn) Vertical velocities of 1.0 to 1.5 ms-1. Can cause convective clouds, including thunderstorms.

37. SEA-BREEZE FRONT (SBFkn) Can cause convective clouds, including thunderstorms. Example from July 26, 2006.

38. Pretty quiet in the Northeast. Weak ridge of high pressure. Southerly flow in low levels.

39. Mesoscale analysis showed warm air inland, cool air over the Gulf of Maine. Temps in the mid-80’s Temps in the high-50’s/low-60’s

40. Strong cross-shore temperature gradient indicated good potential for a sea breeze

41. At 1800 Z (1400L), surface winds indicated good onshore flow reaching interior Maine and New Hampshire

42. Clear area along coast indicated area of marine air not yet destabilized by surface heating.

43. Enhanced cumulus clouds were further inland, indicating location where marine air converged with continental air.

44. Composite radar image indicated presence of showery precipitation at the leading edge of marine air

45. Lightning detection network indicated that these were thunderstorms.

46. SEA-BREEZE HEAD (SBH) Raised leading edge of SBG. Height may be twice the mean depth of the SBG.

47. UNDULAR BORES During late evening SBH can separate from the feeder flow behind the SBF. Creates a cut off roll vortex ~1 km high and ~20 km across in the cross-shore direction. CONTINENTAL AIR ROLL VORTEX MARINE AIR LAND SEA

48. UNDULAR BORES During late evening SBH can separate from the feeder flow behind the SBF. Creates a cut off roll vortex ~1 km high and ~20 km across in the cross-shore direction. Vortex contains marine air. At ground level, approach is marked by an abrupt increase in pressure and temperature, and change in wind direction. May transport pollutants inland.

49. UNDULAR BORES Famous example is the Australian Morning Glory. Associated with sea breeze of the previous day. One or more roll clouds ~1 km high and many km long.

50. KELVIN-HELMHOLTZ BILLOWS (KHBs) Smaller waves on upper surface of SBG. CAUSED BY STRONG DIRECTION AND SPEED SHEAR ON UPPER SURFACE OF SBG Propagate seaward. Heights of ~100 m, wavelengths of ~1 km.