Wind

Question 1

Roughness length (Z_o) is greater in what environment?

Answer 1

Urban (or one with greatest friction)

Question 2

Wind speed over a high roughness length environment takes

Answer 2

longer to reach gradient wind speed

Question 3

Boundary layers depth is shallower in

Answer 3

high pressure scenarios

Question 4

Boundary layer depth is greater in

Answer 4

low pressure/unstable scenarios

Question 5

Main result of surface interacting with atmosphere is

Answer 5

Turbulence

Question 6

Hard to model low level turbulence due to

Answer 6

the scale (very large - microscopic) and observations are difficult to make.

Question 7

Laminar sub-layer

Answer 7

layer immediately in contact with surface (mm) non turbulent and laminar.

Question 8

Turbulent mixed layer

Answer 8

Between laminar sub-layer and top of boundary layer. Mean flow and turbulent eddies.

Question 9

Eddies are result of

Answer 9

Mechanical turbulence (Wind shea and surface irregularities) and also Buoyancy

Question 10

Diameter of eddies are proportional to

Answer 10

height above surface. Smaller nearer surface.

Question 11

l = kz

Answer 11

l = mixing length or diameter of eddies z = height above surface. k = Von Karmans constant (approx 0.4)

Question 12

Surface friction creates a

Answer 12

momentum gradient in the boundary layer. Turbulence is the method by which it is transferred down to the surface.

Question 13

Momentum flux

Answer 13

rate of change of momentum per unit area per second.

Question 14

Roughness length

Answer 14

u(z) = (u* / k) (ln (z/zo)) u(z) =horizontal wind speed at height (z) u* = friction velocity of boundary layer k = Von Karmans constant zo = roughness length

Question 15

Roughness length formula is only meaningful above a minimum height (Zo) because

Answer 15

as Z–> 0 then u–> - infinity which is impossible

Question 16

Lowest roughness length is found in

Answer 16

Calm open seas

Question 17

Highest roughness length in

Answer 17

forests or urban environments

Question 18

height of obstacle

Answer 18

roughness length x 10

Question 19

Zero plane displacement (d)

Answer 19

is the adjusted starting point of the zero plane. Approx 2/3 x h of vegetation where the drag is greatest.

Question 20

Stable eddies

Answer 20

more horizontal then vertical. Thinner boundary layer. Stronger winds aloft.

Question 21

Neutral eddies

Answer 21

Circular.

Question 22

Unstable eddies

Answer 22

more vertical than horizontal. Deeper boundary layer. Stronger and often gusty surface winds as momentum flux increased. Loss of wind strength aloft.

Question 23

Nocturnal Jet

Answer 23

Stable conditions associated with inversion. Clear skies and light gradient winds Sudden increase in winds at height.

Question 24

Ekman Spiral

Answer 24

Backing and slacking towards surface. Assumes atmosphere neutrally stable. Isobars straight and parallel.

Wind speed changes greatest initally then higher up the change in direction is more significant.

Question 25

Frictiona Velocity represents which aspect of boundary layer winds?

Answer 25

Representative of gusts and lulls in the wind as seen on an anemograph trace

Question 26

Put the following list of surface types in order of roughness length from largest to smallest?

Snow, Short Grass, Forest, Calm open sea, Soil, Crops.

Answer 26

1. Forest, 2. Crops, 3. Short Grass, 4. Soil, 5. Snow, 6. Calm open sea.

Question 27

Describe the effect of an increasingly unstable boundary layer on the vertical wind profile of the boundary layer?

Answer 27

Boundary layer deepens, and gradient wind reached further away from the earth’s surface. Momentum flux increased, so that stronger and gustier winds evidence near the surface and lighter winds aloft (compared to stable or neutral stability).

Question 28

Boundary Layer 1 (wind) Exercise

You have the following data:

Von Karman’s constant = 0.4

10 m wind speed = 12 ms-1

Friction velocity = 4 ms-1

Density = 1.2kgm-3

Calculate the following:

a) Mixing length
b) Roughness length
c) Momentum Flux
d) The wind speed at 5m
e) The wind speed at 20m
f) What type of terrain is this?

Answer 28

Question 29

Roughness length is where the wind profile extrapolates to…

Answer 29

ZERO

Question 30

How does a boundary layer change diurnally under clear skies…

Answer 30

Shallow nocturnal stable boundary layer becomes a much deeper convective boundary layer in the daytime. Then stable layer redevelops from the surface around sunset.