Meteorology gel-1370

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

• 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

Major pressure systems & idealized air motions (heavy blue arrows) & precipitation patterns (blue: abundant rainfall)

Pacific high moves northward; sinking air in eastern margin causes dry weather; in the western margin of Bermuda high, southerly winds bring humid air leading to abundant rainfall

Average annual precipitation for Los Angeles & Atlanta

Westerly winds & Jet stream

• Jet Streams: Relatively strong winds concentrated within a narrow band in the atmosphere

• Several hundred miles long, less than several hundred miles wide, less than a mile thick; wind speed can exceed 100 knots (100-200 knots); usually found at the tropopause at 10-14 km

• In the Northern hemisphere, situated along the boundary layer where cold, polar air lies to the north & milder, subtropical air lies to the south; sharp contrast in temp produces rapid horizontal pressure changes --- steep pressure gradient ---- PGF causes the jet stream

• N-S temp contrast along the front is strongest in winter and weakest in summer --- seasonal variations -- Winds blow stronger in winter and jet moves farther south; in summer, jet stream is weaker and is usually found farther north (such as southern Canada)

Jet stream – swiftly flowing current of air; colder air lies to the north & warmer air to the south

Jet streams – contd.

• There are two jet streams, located in the tropopause gaps, where mixing of tropospheric & stratospheric air takes place

• Subtropical jet stream: 13 km above the subtropical high

• Polar front jet stream: 10 km above & near the polar front

• Jet streams play a major role in the global transfer of heat; they tend to meander; pollutants are transported to farther distances by jet streams

Position of polar stream & subtropical jet stream at 300-mb during March 10, 1998; solid gray lines: Isotachs Heavy lines: position of jet stream; Heavy blue lines: direction of cold air southward; heavy red: direction of warm air

Global wind patterns & the oceans

• Wind causes the surface water to drift --- moving water piles up, creating pressure differences within water itself

• In North Atlantic, Gulf Stream, a warm water current, flows northward along the east coast of US, carries warm, tropical water into the higher latitudes; Gulf stream provides moisture and heat for developing mid latitude cyclones

• As Gulf Stream moves toward Europe, it merges with North Atlantic Drift current system; other part flows southward as the Canary Current equatorward;

• Atmospheric and ocean circulation are closely linked; wind and ocean transport heat to higher latitude; leads to energy balance

Major ocean currents: Blue: cold currents; Red: warm currents; 1: Gulf Stream; 2: North Atlantic Drift; 6: Canary current; 16: California current

Winds & Upwelling

• When wind blows over the ocean, surface water is set in motion; it bends slightly right due to Coriolis effect; water drifts away from coast in California current system; cold, nutrient-rich from below rises – upwelling

• Benefits of upwelling: food for fish

• Link between Ocean – Atmosphere - pocketbook:

• Once 2-7 years, surface atmospheric pressure pattern break down (WHY??), as air pressure over western Pacific and falls over the eastern Pacific ---weakens trades and during strong pressure reversals, east winds are replaced by west winds

• A warm current of nutrient-poor tropical water moves southward, replacing the cold, nutrient-rich surface water – El Nino (spanish for boy child) referring to Christ child

Average position of the polar front jet stream & subtropical jet stream in winter; both jet streams are flowing into the page

El Nino & Southern Oscillation

• During El Nino event, large numbers of fish & marine plants may die; dead fish & birds litter the beaches of Peru

• El Nino of 1972-1973 reduced Peruvian anchovy catch from 10.3 million metric tons in 1971 to 4.6 million metric tons in 1972 --- fishmeal production dropped in 1972 --- Animal feed prices went up --- poultry prices went up by 40%

• Southern Oscillation: See-Saw pattern of reversing surface air pressure at opposite ends of the Pacific Ocean; pressure reversals and ocean warming are more or less simultaneous --- El Nino/Southern Oscillation or ENSO

Ordinary condition - Higher pressure over the southeastern Pacific & lower pressure near Indonesia

El Nino condition: Atm pressure decreases over the eastern Pacific and rises over the W. Pacific; trade winds weaken or reverse direction; thermocline changes

SST during non El Nino conditions – upwelling along the equator and Peru coast keeps the water cool (blue color) in the tropical eastern Pacific

SSTs: upwelling is greatly diminished, and warm water (red color) from the Western Pacific has replaced the cool water

Regions of climatic abnormalities due to ENSO; months in black: during the same year; red: following year

El Nino & its impacts

• In the eastern equatorial Pacific, as high as 6°C than the normal has been observed

• Warm water along the coastal areas of Ecuador & Peru chokes off the upwelling that supplies cold, nutrient-rich water to South America’s coastal region

• Warm tropical water fuels the atmosphere with additional warmth and moisture --- additional storminess & rainfall

• Certain regions of the world experience too much rainfall & other regions have very little

• Over the warm tropical central Pacific, the frequency of typhoons increases

Effects of El Nino

• Tropical Atlantic, between Africa and Central America: fewer hurricanes

• Summer monsoon conditions tend to get weaker

• Drought is felt in Indonesia, southern Africa, Australia

• Heavy rains & flooding in Ecuador & Peru

• Storms in to California

• Heavy rain into the Gulf Coast states

• La Nina: Cold surface water moving to Central and eastern Pacific & warm water confined to western tropical Pacific

What Causes El Nino

• Within the changing of seasons, especially the transition periods of spring & fall

• Winter monsoon plays a major role in triggering a major El Nino event

• ENSO and monsoon system are linked

• Linked to out pocket books!!

chapter –7- Summary

• Micro and macro-scale motion

• Wind sheer; sea breeze and land breeze

• Monsoon depression; development, causes

• Valley breeze, katabatic wind; chinook wind, Santa Ana winds

• Dust devil

• ITCZ, location of Detroit, Chicago, Barrow, Honolulu – what types of wind system

• Semi-permanent high and low pressure areas

• Converging/diverging along polar front

• Three- and one-cell general circulation model-driest areas

• Westerlies and easterlies

• Hadley, Ferrel cells

• Subpolar lows, doldrums, horse latitudes

• Where we do see deserts

Summary – contd.

• Polar front jet stream, jet stream blow direction

• Upwelling

• North Atlantic Drift, Gulf Stream current, California current, Labrador

• Major currents that flow parallel to the coast of North America

• El Nino; where does the warming occur; Southern Oscillation