METEOROLOGY
GEL-1370
Chapter Seven
Atmospheric Circulations
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
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
Scales of atmospheric motion; tiny microscale motions constitute a part of the larger mesoscale motions and so on
Scale of atmospheric motion with the phenomena’s average size and life span
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
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)
Air flowing past a mountain range creates eddies eddies many km downwind from the mountain
Thermal circulation produced by heating & cooling of the atmosphere near the ground
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
Formation of clear air turbulence along a boundary of increasing wind speed shear
Turbulent eddies forming downwind of a mountain chain in a wind shear zone produce these billow clouds
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
Development of a sea breeze and a land breeze
Land Breeze – weaker & occurs during night time
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
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
Annual wind flow patterns associated with winter Asian Monsoon
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 air rises; 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
Changing annual wind flow patterns associated with summer monsoon
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 Niño: 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
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
Valley Breeze
Mountain breeze
Mountain slopes warm during the day, air rises and often condenses into cumuliform clouds
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
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
City near the warm air-cold air boundary can experience sharp temperature changes
Conditions that may enhance a chinook
A chinook wall cloud forming over the Colorado Rockies
Santa Ana conditions in January; downslope winds blowing into Southern California raised temp into the upper 80s; elsewhere much lower
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
A dust devil forming on a clear, hot summer day just south of Phoenix, Arizona
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
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
General circulation of air on a nonrotating earth uniformly covered with water & with the sun directly above the equator
Names of different regions and their latitude
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
Idealized wind and surface pressure distribution over a uniformly water-covered rotating earth
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
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
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
Names of surface winds & pressure systems over a uniformly water-covered rotating earth
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)
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
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)
Sea-level pressure & Surface wind-flow patterns in January
Sea-level pressure & Surface wind-flow patterns in July
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