Transcript Global Scale Winds Chapter 7 - Atmospheric and Oceanic Science

METO 200
Lesson 7
Fig. 7-1, p. 189
Voyage of Christopher Columbus
• Columbus sailed down the coast of Africa
to pick up the Northeasterly winds (The
trade winds).
• He returned by sailing North and then
picked up the prevailing Westerlies.
• Why is there this change of wind direction
as one goes North?
Fig. 7-2, p. 190
Voyage of MHS Beagle
• This was the second voyage undertaken by
the HMS Beagle, and is noteworthy as the
one on which Charles Darwin was a
passenger.
• The route taken also took advantage of the
prevailing winds at the low latitudes.
Hadley Cell
GLOBAL CIRCULATION
• George Hadley first suggested in 1735 the general
concept of atmospheric circulation to explain the
existence of the trade winds at the surface
• He envisioned a single convective cell which
extended from the equator to the pole.
• Cold air at pole - high pressure at surface. Warm
air at equator - low pressure at surface. Pressure
gradient force at surface will move air from pole
to equator at surface. Return path at high altitudes.
• Coriolis force produces the trade winds.
Global Circulation
• However although Hadley explained the north
easterlies, he could not explain with the same
model, the prevailing westerlies at mid latitudes.
• So his model cannot be complete.
• Another intriguing question – why are the deserts
mainly seen about 30 degrees N and S?
• The answer is that the strength of the upwelling at
the equator in not enough to complete the single
convective cell
• In fact we have three convective cells between the
equator and the poles.
Three cell
model
Fig. 7-6, p. 193
GLOBAL CIRCULATION
• These three cells have boundaries at about
30 and 60 degrees latitude.
• This results in sinking air at 30 N and 30 S.
But sinking air warms, suppressing cloud
development and precipitation. Hence most
of the worlds deserts occur along these
latitudes.
• These latitudes are where one finds the subtropical highs.
• Also known as the Horse latitudes
Global Circulation
• At the equator the Northern and Southern Easterly
winds (the Trade Winds) meet and the warm air
rises.
• The zone where the two winds interact is known
as the Inter-tropical Convergence Zone , ITCZ
• In this region the pressure gradients are weak and
the winds are light, This region is referred to as
the Doldrums.
• The cell between the equator and 30 degrees is
called the Hadley cell.
Composite (clouds, surface temperature (colors)) image. Note
the line of clouds along the ITCZ
Fig. 7.10
Global Circulation
• The cell between 30 and 60 degrees latitude is
called the Ferrel cell.
• At latitudes between 30 and 60 degrees the
circulation at the surface is towards the pole,
which when combined with the Coriolis force
produces the prevailing Westerlies
• At about 60 degrees latitude the air rises, which
produces precipitation.
• Not as much as in the tropics, Why?
• Because the air is colder and can hold less water
vapor.
Global Circulation
• The cell between 60 degrees latitude and the
pole is known as the Polar cell
• The circulation at the ground is from the
pole to 60 degrees, which when combined
with the Coriolis force produces easterly
winds.
• Polar easterlies.
The three cell conceptual model depicted earlier is somewhat
simplistic. Shown above is the actual position of the lows and highs
for the northern winter. The difference from the conceptual model is
because of the differential heating between land and water.
Fig. 7-22a, p. 188
Highs and lows for the northern summer. Note that the
ITCZ has moved northward from its position in the
winter.
Fig. 7-22b, p. 189
Fig. 7-7, p. 194
Conservation of Angular Momentum
• At the Equator the earth rotates eastward at about
1000 miles per hour. On a windless day the air at
the surface rotates at the same speed.
• If the earth were to stop rotating the air would
continue to rotate until friction slowed it down.
• The air keeps moving because it has momentum.
• Linear momentum is the product of the mass (m)
of the object times its velocity (v)
momentum = mv
Conservation of Angular Momentum
• If the body is rotating about a central point
then we define the angular momentum as
the product of its mass, M, times its velocity
, V, times its distance from the center, R, :
Angular momentum = MVR
• If no external force is applied then the
angular momentum of the system does not
change
Fig. 7-8, p. 194
Conservation of Angular Momentum
• The skater or diver decrease their distance R
from the center of rotation by bringing their
arms (in the case of the skater) and their
arms and legs (in the case of the diver), in
toward the body.
• This increases the speed of rotation (VR)
about the center.
Fig. 7.11
Jet Streams
• As the parcel of air moves from the equator
to 30 degrees at high altitudes, its velocity
increases to conserve angular momentum
• At the same time the Coriolis force acts to
produce a strong westerly wind – the subtropical jet stream.
• A similar jet forms at 60 degrees – the polar
jet, or polar front jet
Fig. 7-26, p. 190
Fig. 7-27, p. 191
Fig. 7.9
Cloud band from equatorial Pacific to Florida follows the Subtropical jet
Fig. 7.13
Subtropical and Polar jet streams in relation to the three cells
WESTERLIES
• IN THE UPPER TROPOSPHERE THERE IS
HIGH PRESSURE OVER THE EQUATOR,
AND A LOW PRESSURE OVER THE POLES.
• THIS PRODUCES A NET FLOW FROM THE
EQUATOR TO THE POLES.
• THIS FLOW PLUS THE CORIOLIS FORCE
PRODUCES WESTERLIES.
• WINDS ARE GEOSTROPHIC
• PRESSURE GRADIENT INCREASES WITH
ALTITUDE. THUS SO DOES THE WIND
SPEED
• JET STREAMS ARE PART OF THE
WESTERLIES
Dish-pan Experiment
Fig. 7-14, p. 199
500 mb winds
Fig. 7-15, p. 200
(A) Zonal flow pattern – air flows nearly parallel to latitudes
(B) Meridional flow pattern –
(C) Combination of the two flows
Fig. 7-16, p. 200
Jet Streams on March 11, 1990
Jet streams on March 11, 1990
• The next slide is an image of the total column of
ozone measured from a satellite.
• Ozone can be used to trace the changes in
dynamics of the atmosphere. In this case it can be
used to locate the jet streams.
• Note the undulations within the Jet streams
(Rossby waves).
• Also note the cut off low.
WAVES IN THE WESTERLIES
• DISH PAN EXPERIMENT
• C. G. ROSSBY
• WAVES ALONG THE JET STREAMS ARE KNOWN
AS ROSSBY WAVES
• THREE TO SIX OF THEM AROUND THE GLOBE.
• THE AIR FLOW ALONG THE EDGE OF THE WAVES
CAN BE RAPID, HOWEVER THE WAVES MOVE
SLOWLY - 15 DEGREES PER DAY.
• HIGHER JET STREAM SPEEDS IN THE WINTER.
• JETS SHIFTS SOUTH IN THE WINTER, NORTH IN
THE SUMMER.
Cut-off Low
WESTERLIES AND THE HEAT
BUDGET
• MAJOR FUNCTION OF ATMOSPHERIC DYNAMICS IS TO
MOVE HEAT FROM THE EQUATOR TO THE POLES.
• BUT HOW CAN WINDS MOVE HEAT WHEN THE
PREDOMINATE WIND DIRECTION IS ZONAL (E TO W, OR
W TO E).
• THE MEANDERINGS OF THE JET STREAMS
CONTINUALLY MIX COLD AND WARM AIR, THUS
TRANSPORTING HEAT.
Poleward transport of heat by the oceans and atmosphere
Fig. 7-19, p. 202
Poleward transport of Energy
• We noted before that there is an inbalance between
the energy from the sun received by the tropics and
that received at the Poles.
• There must be a net movement of energy from the
equator to the Poles.
• This transfer of energy is achieved by the atmosphere
and the oceans.
• The atmosphere moves energy at the mid-latitudes,
while the oceans move energy predominately in the
tropics.
Approximate position of the ITCZ in January and July
Fig. 7-20, p. 203
Monsoon
• A monsoon is a weather feature driven by the change
in position of the ITCZ
• In the winter the ITCZ is South of the Equator and
the dominant feature over the Himalayas is a High
pressure system. This brings winds from the North
across India – cool dry air.
• But in the summer that ITCZ is now North of India,
and the dominant weather feature is a Low pressure
system. This brings large amounts of rainfall.
Fig. 7-8, p. 176
Precipitation patterns and topography
Box 7-2, p. 204
Precipitation patterns and Topography
• At the beginning when the air is lifted up the mountain the air
cools at the dry adiabatic lapse rate.
• The dew point temperature also falls.
• At I km altitude the dew point temperature equals the air
temperature – saturation.
• As the air goes further up the mountain it now cools at the wet
adiabatic lapse, and the dew point temperature must equal the
air temperature
• Why? Because the air is saturated.
• At the top of the mountain both the air and dew point
temperature are at -2 C.
• The absolute water vapor pressure of the air is set by the dew
point temperature at the top of the mountain
Precipitation patterns and Topography
• As the air descends its temperature will increase
(adiabatic compression) and automatically the air is
no longer saturated.
• Hence the temperature of the air will increase at the
dry adiabatic lapse rate.
• At the same time the dew point temperature will
increase at about 2 degrees C per kilometer.
• The net effect is to increase the temperature of the air
and decrease the dew point temperature from one side
of the mountain to the other:
• Temperature 20 C to 28 C
• Dew-point temperature 12 to 4 C.
• Example is the island of Hawaii.