Wind Flashcards
Roughness length (Zo) is greater in what environment?
Urban (or one with greatest friction)
Wind speed over a high roughness length environment takes
longer to reach gradient wind speed
Boundary layers depth is shallower in
high pressure scenarios
Boundary layer depth is greater in
low pressure/unstable scenarios
Main result of surface interacting with atmosphere is
Turbulence
Hard to model low level turbulence due to
the scale (very large - microscopic) and observations are difficult to make.
Laminar sub-layer
layer immediately in contact with surface (mm) non turbulent and laminar.
Turbulent mixed layer
Between laminar sub-layer and top of boundary layer. Mean flow and turbulent eddies.
Eddies are result of
Mechanical turbulence (Wind shea and surface irregularities) and also Buoyancy
Diameter of eddies are proportional to
height above surface. Smaller nearer surface.
l = kz
l = mixing length or diameter of eddies z = height above surface. k = Von Karmans constant (approx 0.4)
Surface friction creates a
momentum gradient in the boundary layer. Turbulence is the method by which it is transferred down to the surface.
Momentum flux
rate of change of momentum per unit area per second.
Roughness length
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
Roughness length formula is only meaningful above a minimum height (Zo) because
as Z–> 0 then u–> - infinity which is impossible
Lowest roughness length is found in
Calm open seas
Highest roughness length in
forests or urban environments
height of obstacle
roughness length x 10
Zero plane displacement (d)
is the adjusted starting point of the zero plane. Approx 2/3 x h of vegetation where the drag is greatest.
Stable eddies
more horizontal then vertical. Thinner boundary layer. Stronger winds aloft.
Neutral eddies
Circular.
Unstable eddies
more vertical than horizontal. Deeper boundary layer. Stronger and often gusty surface winds as momentum flux increased. Loss of wind strength aloft.
Nocturnal Jet
Stable conditions associated with inversion. Clear skies and light gradient winds Sudden increase in winds at height.
Ekman Spiral
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.
Frictiona Velocity represents which aspect of boundary layer winds?
Representative of gusts and lulls in the wind as seen on an anemograph trace
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.
- Forest, 2. Crops, 3. Short Grass, 4. Soil, 5. Snow, 6. Calm open sea.
Describe the effect of an increasingly unstable boundary layer on the vertical wind profile of the boundary layer?
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).
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?
Roughness length is where the wind profile extrapolates to…
ZERO
How does a boundary layer change diurnally under clear skies…
Shallow nocturnal stable boundary layer becomes a much deeper convective boundary layer in the daytime. Then stable layer redevelops from the surface around sunset.
