Climate Lecture 4- Global Atmospheric Circulation and Westher Systems

Question 1

Name three factors which determine wind flow.

Answer 1

1. Pressure gradient force
2. Coriolis effect (force)
3. Surface friction

Question 2

What is wind direction defined as?

Answer 2

The direction from which it flows

Question 3

Discuss the role of pressure gradient on wind speed.

Answer 3

If a pressure gradient in one location is 0.1 mb/km and in another 0.4 mb/km then the second location has a wind speed 4 times greater.

Question 4

What happens in a high pressure system is there is a steep pressure gradient comapred to a shallow pressure gradient?

Answer 4

Steep pressure gradient = strong PGF
Shallow pressure gradient = weak PGF

Question 5

What is Coriolis force?

Answer 5

Arises from earth’s rotation on its axis

Question 6

Discuss the role of CF on deflection in the NH and SH.

Answer 6

NH - deflection to the right from high to low
SH- deflection to the left from high to low

Question 7

What is Coriolis force dependent on?

Answer 7

Wind speed and latitude

Question 8

Discuss geostrophic wind.

Answer 8

In the free atmosphere balance between PGF and CF.
The wind that will be blowing parallel to the isobars, when Coriolis force is balanced by the pressure gradient force

Question 9

What are isobars?

Answer 9

Lines that connect equal points of atmospheric pressure

Question 10

What is the influence of surface friction on surface wind?

Answer 10

Can deflect wind 10-30 degrees
Friction reduces the wind speed, which reduces the Coriolis force. Reduced Coriolis force no longer balances the pressure gradient force, and the wind blows across the isobars
toward or away from the pressure centre.
The pressure gradient force is now balanced by the sum of the frictional force and the Coriolis force.

Question 11

Discuss surface friction in the SH.

Answer 11

Southern Hemisphere: Coriolis force acts to the left rather than the right. This causes the winds of the Southern Hemisphere to blow clockwise and inward around surface lows, and counter clockwise and outward around surface highs.

Question 12

Discuss surface friction in the NH.

Answer 12

Northern Hemisphere: surface winds blow counter clockwise and inward into a surface low, and clockwise and out of a surface high in the Northern Hemisphere.

Question 13

Discuss global pressure and wind patterns.

Answer 13

Surface convergence and ascent at the ITCZ
NE or SE trades wind within the hadley cell - flow towards the equator
Westerlies in the Ferrel cell - flow away from equator
Polar easterlies in the polar cell flowing towards the equator
Alternating low and high pressure starting with high pressure in poles due to polar votex.

Question 14

What does PFJS stand for?

Answer 14

Polar front jet stream

Question 15

What does STJS stand for?

Answer 15

Sub tropical jet stream

Question 16

Discuss seasonal heating and circulation.

Answer 16

ITCZ movement with seasonal change of sub-polar point and zone of maximum heating.

Seasonal movement of subtropical high pressure systems

Polar high- seasonal expansion and contraction of cold air.

Question 17

Where is the seasonal variation of the polar front.

Answer 17

60 degrees N in the summer and 30 degrees N in the winter

Question 18

Does pressure change seasonally?

Answer 18

Yes

Question 19

What is upper flow mostly associated with?

Answer 19

Most westerly

Associated with Hadley, Ferrel and Polar cell

Question 20

Are there jet steams near the equator?

Answer 20

No, they are also not in a straight line

Question 21

What are jet streams?

Answer 21

Jet streams are relatively narrow bands of strong winds in the upper westerlies (troposphere) that blow from west to east at an altitude of 7000 - 15000m.

Question 22

What are jet streams caused by?

Answer 22

Jet streams are caused by a combination of the
planet’s rotation on its axis and atmospheric
heating (by solar radiation).

The jet stream results from latitudinal and vertical large gradients of temperature and pressure at the intersection of a colder air mass from the north with a warmer air mass from the south (Northern Hemisphere case).

Question 23

Where do jets tend to occur?

Answer 23

At about the height of the tropopause

Question 24

What creates the acceleration of winds into the jet stream?

Answer 24

The combination of temperature and pressure
differences in the air mass, plus some help from the coriolis force (coriolis acceleration) creates the acceleration of winds into the jet stream.

Question 25

Where do the upper westerlies flow?

Answer 25

Upper westerlies flow entirely around the globe. They have a tendency to meander eastwards The nature, size and spacing of meanders has important influence on the weather experienced in the mid-latitudes

Question 26

What are the meanders in the upper westerly circulation known as?

Answer 26

Rossby waves

Question 27

Are there ridges and troughs in Rossby waves?

Answer 27

Yes, under the wind there is anti cyclonic spins in the ridge Above the wind there is cyclonic spin in thr trough

Question 28

What happens with jet streams and movement from trough to ridge?

Answer 28

Air speeds up Divergence Rising motion Decreasing surface pressure

Question 29

What happens with jet streams and movement from ridge to trough?

Answer 29

Air slows down Convergence Sinking motion Increasing surface pressure

Question 30

Discuss lower and upper level convergence and divergence.

Answer 30

Air sinks from zone of convergence towards the surface creating an area of High pressure. Therefore down-wind of a ridge and up-wind of a trough subsidence generally leads to good weather in the area of the High pressure system In zone of divergence air moves outwards at the upper level. This air needs to be replaced from the surface creating an area of surface low pressure. Therefore down-wind of a trough or up-wind from a ridge there is ascending air and generally cloudy and rainy conditions

Question 31

What are air masses?

Answer 31

Large volumes of air with homogenous thermal, moisture and stability characteristics

Question 32

How are air masses formed?

Answer 32

Air sitting over an ocean or land surface for several days

Question 33

Name the 6 air mass types.

Answer 33

Arctic or Antarctic (A or AA) Polar Continental (cP) Polar maritime (mP) Tropical continental (cT) Tropical maritime (mT) Equatorial (E)

Question 34

Discuss A or AA air mass.

Answer 34

Extremely cold Formed over poles Very dry due to extreme cold

Question 35

Discuss cP air mass.

Answer 35

Very cold having developed over sub-polar regions Very dry due to the cold and having developed over land

Question 36

Discuss mP air mass.

Answer 36

Very cool because of the high latitude but not cold, due to moderating influence of the sea and the warm ocean currents at these latitudes. Moderately moist because of the cool temperature but not as dry as polar continental air because of evaporation from the water surface.

Question 37

Discuss cT air mass.

Answer 37

Very warm because of the lower sub-tropical latitude of formation. Dry because it formed over land.

Question 38

Discuss mT air mass.

Answer 38

Very warm because of the sub-tropical latitudes at which it forms. Very humid because of the warm tropical waters below.

Question 39

Discuss E air mass.

Answer 39

Hot. Extremely humid, continental is not differentiated from martitime because much of equatorial land is covered with humid tropical rainforests.

Question 40

Discuss cold fronts.

Answer 40

Advancing cold air behind cold front Receding warm air ahead of cold front Cloud development because of frontal lifting of warm moist air

Question 41

Discuss warm front.

Answer 41

Advancing warm air behind warm front Receding cold air ahead of warm front Cloud development beause of frontal lifting of warm air.

Question 42

Discuss weather of cold fronts.

Answer 42

Temp- warm prior passing of front, cooling suddenly in front, cold and getting colder after passing AP- decreasing steadily, leveling off then increasing, increasing steadily. Winds- south to southeast, variable and gust, west to northwest Precipitation- showers, heavy rain or snow or hail sometimes, showers then clearing Cloud- cirrus and cirrostratus changing later to cumulus and cumulonimbus, cumulus and cumulonimbus, cumulus.

Question 43

Discuss weather of warm fronts.

Answer 43

Temp- cool, warming suddenly, warmer then levelling off AP- decreasing steadily, levelling off, slight rise followed by a decrease Winds- south to southeast, variable, south to southwest Precipitation- showers or snow or sleet or drizzle, light drizzle, none Clouds- cirrus and cirrostratus and altostratus and nimbostratus and then stratus, stratus and sometimes cumulonimbus, clearing with scattered stratus and sometimes scattered cumulonimbus.

Question 44

What happens at an occluded front?

Answer 44

Advancing cold air behind occluded front Receding cold air ahead of occluded front Cloud development because of frontal lifting of warm moist air

Question 45

What is cyclogenesis?

Answer 45

Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere Pressure and wind distribution - conveyor belt model.

Question 46

How do mid-latitude cyclones form?

Answer 46

Mid-latitude cyclones form when freezing air masses from the Arctic meet with warm moist air from the tropics. These air masses begin to rotate and increase in energy, thus forming a large low-pressure center or cyclone.