Transcript Lesson 14 - University of Maryland, College Park

AOSC 200
Lesson 14
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 midlatitudes, 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.