Lectures 5 & 6

Question 1

What is conditional instability?

Answer 1

When an air parcel cools faster than the atmosphere but then cools slowly after condensation

Question 2

What does the depth of frictional influence depend on?

Answer 2

Surface roughness
Thermal forcing

Question 3

What is the top of the boundary layer (Zg)?

Answer 3

Where ū (mean windspeed) becomes approximately constant with height

Question 4

Are vertical gradients of windspeed greater or lesser over smooth terrain?

Answer 4

Greater over smooth terrain - less forced convection

Question 5

Open country windspeed

Answer 5

Rapid change of windspeed with height

Question 6

City centre windspeed

Answer 6

More effective mixing so less rapid change in windspeed

Question 7

Forced convection (in the boundary layer wind field)

Answer 7

Turbulence due to the production of shear stress by surface roughness

Question 8

Free convection (in the boundary layer wind field)

Answer 8

Turbulence due to sensible heat working against gravity (producing buoyancy)

Question 9

Which convection has stronger vertical components in eddies?

Answer 9

Free convection (not constrained to ground surface)

Question 10

Momentum flux

Answer 10

change in momentum

Question 11

Will a downdraft cause an increase or decrease in velocity and momentum of windspeed?

Answer 11

an increase

Question 12

Will an updraft cause an increase or decrease in velocity and momentum of windspeed?

Answer 12

A decrease

Question 13

Stability

Answer 13

A measure of the tendency of air tto move vertically

Question 14

How is free convections stability assessed?

Answer 14

Via changing temperature with height - the heat flux density

Question 15

How is forced convections stability assessed?

Answer 15

Via changing horizontal windspeed with height - momentum flux density

Question 16

What is combined stability

Answer 16

the ratio between difference in temperature with height vs difference in horizontal windspeed with height

Question 17

How is combined stability commonly compared

Answer 17

Richardson number

Question 18

What is the leading edge effect?

Answer 18

The boundary line between the 2 climatically different surface types

Question 19

The adjustments are not immediate, generated at the surface and diffusing upwards. What is the layer of the air whose properties have been affected by the new surface called?

Answer 19

Internal boundary layer

Question 20

Speed of leading edge

Answer 20

Slow - gains 100-300m fetch for every 1m increase in vertical.

Question 21

Speed of internal boundary layer

Answer 21

Fetch of 10-30m for every 1m increase in the vertical

Question 22

Advection fog

Answer 22

typically formed by advection of air across water of very different temperatures

Question 23

cold water advection fog

Answer 23

Warm air flows over cold water. if air temp cools to dew point temp, water vapour condenses and fog forms

Question 24

warm water advection fog

Answer 24

Forms when very cold dry air is transported across warmer water body. The very low layer of air warms and evaporates water from surface. Warm, moist air next to surface is unstable. convects moisture into cooler air and air quickly becomes condensated.

Question 25

Topographic features causing precipitation

Answer 25

orographic precipitation

Question 26

Night time topoclimate

Answer 26

Solar angle less important, LW radiation more important

Question 27

Cold air accumulation depends on

Answer 27

-catchment size from which cold air drains
-duration of drainage
-screening of horizon: sky view factor

Question 28

Radiation frost

Answer 28

radiation cooling leads to temperatures <0°C. can be significant in valleys

Question 29

advection frost

Answer 29

air is already cold, and transported in. enhanced by altitude

Question 30

which direction are succulents likely to face?

Answer 30

towards the equator

Question 31

Equator facing slope characteristics

Answer 31

higher SW radiation input, higher temps = more evaporation