METEOROLOGY

1. METEOROLOGY GEL-1370

2. Chapter Seven Atmospheric Circulations

3. Goal for this Chapter We are going to learn answers to the following questions: • What are eddies? How are these eddies formed? • How are sea breezes and land breezes formed • How are monsoons are formed? • What are chinook? How they are formed? • What kind of weather sea breeze and chinook bring? • Why & how winds blow around the world the way they do? • How heat is transported from equatorial regions poleward? • What are El Nino? How are they formed

4. Scales of Atmospheric Motion • Winds: Workhorse of weather, moves storms and large fair weather systems around the globe; transports heat, moisture, dust, insects/bacteria, pollen, etc. • Circulations are arranged according to their sizes; hierarchy of motion is called scales of motion --- tiny gusts to giant storms • Microscale: Eddies constitute the smallest scale of motion; few meter in diameter; form by convection or by the wind blowing an obstruction; short-lived (few minutes) • Mesoscale (Meso: middle): Size from few km to ~100 km in diameter; lasts from minutes to a day; include local winds, thunderstorms, tornadoes, and small trophical storms

5. Scales of atmospheric motion; tiny microscale motions constitute a part of the larger mesoscale motions and so on

6. Scale of atmospheric motion with the phenomena’s average size and life span

7. Eddies • Synoptic scale: Weather map scale; extend from 102-103 kms; life time: days to weeks • Planetary (global) scale: Largest wind pattern; wind pattern extend over the whole earth; • Macroscale: synoptic + planetary scales • Eddies: When wind encounters a solid object, eddy forms on the object’s downwind side; size and shape of eddy depend on the size of the object and speed of the wind; wind flowing over a building produces a larger eddies that can be size of the building • Mountain Wave Eddy: Strong winds blowing over a mountain in stable air produce a mountain wave eddy on the downwind sie, with a reverse flow near the ground

8. Eddies – contd. • Wind Sheer: Rate of change of wind speed or wind direction over a given surface • Clear air Turbulence (CAT): Turbulence produced in a clean air • Sea breeze: A coastal local wind that blows from the ocean to the adjoining land; leading edge of the breeze is called sea freeze front • Breeze pushes the warmer, unstable humid air to rise and condense, producing rain showers • Thermal circulations: Air circulation primarily resulting from the heating and cooling of air • No horizontal variation in pressure --- no pressure gradient --- no wind (Fig.a)

9. Air flowing past a mountain range creates eddies eddies many km downwind from the mountain

10. Thermal circulation produced by heating & cooling of the atmosphere near the ground

11. Thermal circulations • If the atmosphere is cooled in the North & warmed to the south, isobars bunch close together in the North while in warmed south, they spread apart (Fig.b); this dipping of the isobars produces PGF aloft that causes the air to move from higher pressure to lower pressure • After the air aloft moves from S to N, air piles up in the northern area; surface air pressure in the south decreases and north increases; PGF is established at the earth’s surface from north to south and surface winds begin to blow from north to south • When cool surface air flows southward, it warms & becomes less dense; warm air slowly rises, expands, cools, and flows out the top at an elevation of ~1 km above the surface; at this level, air flows horizontally northward toward lower pressure and the circulation is completed by sinking & flowing out the bottom of the surface high

12. Formation of clear air turbulence along a boundary of increasing wind speed shear

13. Turbulent eddies forming downwind of a mountain chain in a wind shear zone produce these billow clouds

14. Sea & Land Breezes Sea Breeze is a type of thermal circulation; uneven heating of land & water causes these mesoscale coastal winds; are strongest during the afternoon when the temperature contrast between land & ocean occurs Sea Breeze: A coastal local wind that blows from the ocean onto the land. The leading edge of the breeze is called Sea breeze front Land Freeze: A coastal breeze that blows from land to sea, usually at night, when land cools more quickly than the water; temperature contrasts are much weaker are at night hence land breezes are usually weaker than sea breeze

15. Development of a sea breeze and a land breeze

16. Land Breeze – weaker & occurs during night time

17. Sea & Land Breezes – contd. • Some coastal cities experience the sea breeze by noon & their highest temperature usually occurs much earlier than in inland cities • Sea breeze in Florida help produce state’s abundant summertime rainfall • In UP in Michigan, afternoon clouds and showers are brought to the land by breezes while lakeshore areas remains sunny, cool and dry

18. Monsoon – Seasonally changing winds • Monsoon – derived from Arabic word ‘Mausim’ means seasons • Monsoon Wind system: One that changes direction seasonally, blowing from one direction in summer and from the opposite direction in winter • During winter, air over the continent becomes much colder than the air over the ocean; a large, shallow high-pressure area develops over Siberia, producing a clockwise circulation of air that flows out over the Indian Ocean and South China Sea; hence winter monsoon means clear skies, with winds that blow from land to sea

19. Annual wind flow patterns associated with winter Asian Monsoon

20. Monsoon – contd. • In summer, air over the continents become much warmer than air above the water; shallow thermal low develops over the continental interior; heated airrises; moisture bearing winds sweeping into the continent from the ocean; humid air converges with a drier westerly flow, causing it to rise; lifting air masses cool and the air reaches the saturation point, resulting in heavy showers and thunderstorms • Summer monsoon of southeastern Asia (June – September) is wet, rainy weather season with winds blowing from Sea to Land

21. Changing annual wind flow patterns associated with summer monsoon

22. Monsoon – contd. • Strength of Indian monsoon related to the reversal of surface air pressure that occurs at regular intervals about every 2-7 years at opposite ends of the tropical South Pacific Ocean • El Nińo: During this event, surface water near the equator becomes much warmer over the central and eastern Pacific; over this region near equator, we find warm rising air, convection, and heavy rain; west of the warm water (over the region influenced by the summer monsoon) , sinking air prohibits cloud formation and convection --- During El Nino period, monsoon is likely to be deficient

23. Monsoon – contd. • Summer monsoon on the southern hills of the Khasi hills in northeastern India, Cherrapunji, average annual rainfall is 1080 cm (425 inch) • Monsoon wind systems can exist if large contrasts in temperature develop between oceans and continents • Southwestern US (Arizona and New Mexico), monsoonlike circulation exists • Valley Breeze: A local wind system of a mountain valley that blows uphill during the day • Mountain Breeze: A local wind system of a mountain valley that blows downhill at night • Katabatic Wind: Any wind blowing downslope, usually cold

24. Valley Breeze

25. Mountain breeze

26. Mountain slopes warm during the day, air rises and often condenses into cumuliform clouds

27. Other wind systems • Chinook Wind: A warm, dry wind on the eastern side of the Rocky Mountains; source of warmth for a chinook is compressional heating, as warmer (and drier) air is brought down from aloft • Foehn: A warm, dry wind in the Alps • Santa Ana Winds: A warm, dry wind that blows into southern California from the east off the elevated desert plateau; Its warmth is derived from compressional heating • Haboob: A dust or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into the air

28. Other wind systems – contd. • Haboobs are most common in the African Sudan & in the desert southwest of the US (e.g. southern Arizona) • Whirlwinds or dust devils: The spinning vortices so commonly seen on hot days in dry areas • Difference between dust devil and Tornadoes: Circulation of a tornado descends downward from the base of a thunderstorm; circulation of a dust devil begins at the surface, normally in sunny weather, although some form beneath convective-type clouds

29. City near the warm air-cold air boundary can experience sharp temperature changes

30. Conditions that may enhance a chinook

31. A chinook wall cloud forming over the Colorado Rockies

32. Santa Ana conditions in January; downslope winds blowing into Southern California raised temp into the upper 80s; elsewhere much lower

33. Formation of a dust devil; On a hot, dry day, the atmosphere next to the ground becomes unstable; air rises, wind blowing past an obstruction twists the rising air

34. A dust devil forming on a clear, hot summer day just south of Phoenix, Arizona

35. Global Winds • General Circulation: It represents the average air flow around the world; caused by unequal heating of the earth’s surface • What we have learnt: • Incoming Solar radiation = outgoing earth radiation • Energy balance is not maintained for every latitude • Tropics experience a net gain in energy & Polar regions suffer a net loss Atmosphere & Ocean transport warm air poleward and cool air equatorward

36. General Circulation of the Atmosphere • General Circulation Models: Single-cell Model & Three cell Model • Single-cell Model Assumptions: • Earth’s surface is uniformly covered with water (differential heating between the earth & ocean is eliminated) • Sun is always directed over the equator (winds will not shift seasonally) • Earth does not rotate (No Coriolis force and only force is PGF) A huge thermally driven convection cell in each atmosphere Hadley Cell: A thermal circulation proposed to explain the movement of the trade winds; consists of rising air near the equator & sinking air near 30° latitude

37. General circulation of air on a nonrotating earth uniformly covered with water & with the sun directly above the equator

38. Names of different regions and their latitude

39. Single-cell Model • Excessive heating of the equatorial area produces a broad region of surface low pressure, while at the poles excessive cooling creates a region of surface high pressure; closed loop with rising air near the equator, sinking air over the poles, and equatorward flow of air near the surface, and a return flow aloft. In this manner, some of the excess energy of the tropics is transported as sensible and latent heat to the regions of energy deficit at the poles • Limitations: Too simplistic, Coriolis force does deflect the southward-moving surface air in the Northern Hemisphere to the right, producing easterly surface winds

40. Idealized wind and surface pressure distribution over a uniformly water-covered rotating earth

41. Three-cell Model • Features: Tropical regions receive an excess of heat & poles a deficit • In each hemisphere, three cells redistribute energy • Polar Cell: Circulation from the pole to ~60° {cold air aloft sinks and reaches the surface & flows back toward the polar front) • Ferrel Cell: Midlatitude cell from ~30° to ~60° • Hadley Cell: From equator to ~30° • A surface high-pressure area is located at the poles & a broad trough of surface low pressure exists at the equator • Hadley Cell is driven by latent heat released by cumulus clouds and thunderstorms produced by warm air rising in the equatorial region • Doldrums: Region near the equator characterized by low pressure and light, shifting winds

42. Three-cell model contd. • Subtropical Highs: Rising air in the equatorial region reaches the tropopause, which acts like a barrier, causing the air to move toward the pole and this air mass gets deflected by the Coriolis force providing westerly winds aloft in both hemispheres; this air mass converges due to radiational cooling at the midlatitudes; convergence aloft leads to increase in the mass of air above the surface; convergence of air aloft produces of belts of high pressure called subtropical highs • Converging dry air leads to compressional warming; subsiding air produces clear skies & warm surface temp --- major deserts of the world

43. Three-cell model – contd. • Horse Latitudes: Belt of latitude ~30-35° where the winds are dry & predominantly light and the weather is hot and dry • Trade Winds: Winds that occupy most of the tropics and blow from the subtropical highs to the equatorial low (provided an ocean route to the New World) • InterTrophical Convergence Zone (ITCZ): The boundary zone separating the northeast trade winds of the Northern Hemisphere from the southeast trade winds of the Southern Hemisphere • Westerlies: Winds that blow in the midlatitudes on the poleward side of the subtropical high-pressure areas

44. Names of surface winds & pressure systems over a uniformly water-covered rotating earth

45. Generalized wind distribution • From TX to Canada – commonly winds blow out of the west, than from the east • Polar Front: A semipermanent, semicontinuous front that separates tropical air masses from polar air masses • Subpolar Low: A belt of low pressure located between 50° and 70 ° (consists of Aleutian low in the North Pacific & Icelandic low in the North Atlantic in the Northern Hemisphere) • Polar Easterlies: A shallow body of easterly winds located at high latitudes poleward of the subtropic low • Generalized Picture: At the surface, 2 major high (~30° & poles) and low pressure areas (~60° & equator)

46. Wind distribution – contd. • Summary contd (generalized picture of surface winds): • Trade winds extend from subtropical high to the equator • Westerlies from the subtropical high to the polar front • Polar easterlies from the poles to the polar front Comparison of three-cell model with observations: Upper level winds blow from west to east Middle cell suggests an east wind aloft as air flows equatorward – does not agree with observations Model agrees closely with winds & pressure distribution in the surface

47. Average surface winds and Pressure • Four semipermanent pressure systems in the Northern Hemisphere during January: • Bermuda high in the Eastern Atlantic (between 30° & 35 °) • Pacific high in the Pacific (between 25° & 35 °) • Icelandic Low (in North Atlantic, covers Iceland & Southern Greenland) • Aleutian Low (over Aleutian Islands in the N. Pacific) Other non semipermanent: Siberian high (formed because of intense cooling of the land)

48. Sea-level pressure & Surface wind-flow patterns in January

49. Sea-level pressure & Surface wind-flow patterns in July

50. Formation of Monsoon • During summer, land warms --- thermal lows are formed (July map, thermal lows are seen over desert southwest of US, plateau of Iran & north of India) --- warm, moist air from the ocean is drawn, producing the wet summer monsoon • Between January & July, maximum surface heating shifts seasonally ---major pressure systems, wind belts and ITCZ shift toward the north in July & toward south in January • Abundant rainfall where air rises and little where air sinks --- areas of high rainfall exist in the tropics where humid air rises & at 40-55° where midlatitude storms and the polar front force air upward • Areas of low rainfall occur near 30° in the vicinity of subtropical highs and in polar regions where the air is cold & dry