Atmospheric Motion

Question 1

Define Coriolis force and state its key properties

Answer 1

Result of motion of the atmosphere over a rotating surface (earth).

Apparent force acting at right angles to the desired direction of travel (deflects air left in the southern hemisphere)

Affects large scale motion

Directly proportional to wind speed and lattitude

Question 2

State the key components of the Coriolis force equation

Answer 2

𝑐𝑓 = 2𝜔𝑝𝑉 sin 𝐿𝑎𝑡

𝜔 = angular velocity of earth’s rotation (rpm)
𝑝 = air density
𝑉 = wind velocity
sin 𝐿𝑎𝑡 = sin of the latitude which force acts

cf is greatest at the poles and increases with increasing wind speed

Question 3

Define pressure gradient and state its key properties

Answer 3

Difference in pressure across horizontal distance

Linear relationship with wind speed

First force to act

Degrades over time

Pressure gradient force acts at right angles to isobars and from high to low pressure

Question 4

Define geostrophic wind

Answer 4

Wind that results from a balance of PGF and CF above the friction layer in a straight path.

PGF causes the air to move, once it moves CF force acts proportionally to the w/v.

The resulting vector causes the air to veer away from its intended direction until the PGF and CF are equal.

Air flows parallel to the isobars with PGF acting towards the lower pressure, and CF toward the higher pressure.

Question 5

Define gradient winds

Answer 5

Wind flowing parallel to curved isobars above the friction layer where centripetal force is added to the geostrophic balance.

The force resulting from the imbalance between the PGF and CF as wind flows in a curve is the CPF which acts toward the centre of a pressure system.

Question 6

Gradient wind in anticyclonic systems

Answer 6

Flow is initially a balanced geostrophic wind.

As it moves into the curved isobars, the PGF acts first, rotating anticlockwise (always directly from H-L pressure)

The imbalance created results in a net force which acts to increase w/v. With increase w/v, CF increases.

CF is now > than PGF, which provides the centripetal acceleration which is required to ‘bend’ the wind with the curvature of the isobars.

The stronger CF is now opposite to the PGF and the wind parallel to the curved isobars

Question 7

Gradient wind in cyclonic systems

Answer 7

Flow is initially a balanced geostrophic wind.

As it moves into the cyclonically curved isobars, the PGF rotates clockwise.

Imbalance in forces results in a net force which acts to decrease w/v, decreasing CF.

PGF is now > CF, providing the centripetal acceleration required to pull the wind around the curved isobars.

Question 8

Describe the effects of friction on airflow near the surface

Answer 8

Friction results from the interaction between wind and the surface.

Friction opposes wind and decreases the w/v, Coriolis force therefore decreases, the PGF becomes dominant and the resulting flow tilts toward the low pressure (or away from the left in the southern hemisphere)

The resulting flow is the frictional wind balance.

Question 9

State the factors that determine the strength of the friction layer and any
related deflections

Answer 9

1. roughness of the surface (more rough, deeper friction layer)
2. speed of the wind
   (faster w/v, deeper friction layer)
3. stability of the atmosphere
   (increased stability, increased friction)

Deflection over:

ocean = 10-20°
flat ground = 30°
undulating ground = 40-60°
mountainous terrain = 90°

Question 10

Define and describe the four scales of atmospheric motion

Answer 10

Global - circulations that can last for weeks or months, typically 5000km or more in size (prevailing winds, Hadley)

Synoptic - circulation lasting days to weeks, typically 1500-2000km in size (SIGWX, MSLP, ARFOR)

Mesoscale - circulation lasting days to weeks, typically 20-30km in size (SIGMET, TAF)

Microscale - circulations lasting seconds to minutes (METAR, SIGMET, TAF)

Question 11

Define General Circulation and state its key purpose

Answer 11

the GC describes the global patterns of prevailing winds and surface pressure and the general motion of the atmosphere across the globes regional zones;

polar latitudes (60-90N/S)
mid latitudes (30-60N/S)
tropics (0-30N/S)

Its purpose is to distribute energy, generally an excess in the tropical lats to the polar lats where there is a deficit, regulating global temperatures

Question 12

Explain how the Global Energy Budget is responsible for the General Circulation

Answer 12

GEB creates a excess of energy in the tropical lats and a deficit in the polar lats, resulting in permanent average pressure and temp differences which drive patterns of atmospheric motion