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 phenomenas
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 objects 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 earths 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 states
  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 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

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 Nino 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 earths 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
• Earths 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

51
Major pressure systems idealized air motions
(heavy blue arrows) precipitation patterns
(blue abundant rainfall)
52
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
53
Average annual precipitation for Los Angeles
Atlanta
54
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)

55
Jet stream swiftly flowing current of air
colder air lies to the north warmer air to the
south
56
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

57
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
58
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

59
Major ocean currents Blue cold currents Red
warm currents 1 Gulf Stream 2 North Atlantic
Drift 6 Canary current 16 California current
60
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

61
Average position of the polar front jet stream
subtropical jet stream in winter both jet
streams are flowing into the page
62
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

63
Ordinary condition - Higher pressure over the
southeastern Pacific lower pressure near
Indonesia
64
El Nino condition Atm pressure decreases over
the eastern Pacific and rises over the W.
Pacific trade winds weaken or reverse direction
thermocline changes
65
SST during non El Nino conditions upwelling
along the equator and Peru coast keeps the water
cool (blue color) in the tropical eastern Pacific
66
SSTs upwelling is greatly diminished, and warm
water (red color) from the Western Pacific has
replaced the cool water
67
Regions of climatic abnormalities due to ENSO
months in black during the same year red
following year
68
El Nino its impacts

• In the eastern equatorial Pacific, as high as 6C
  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 Americas 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

69
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

70
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!!

71
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

72
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