Lectures 5 & 6 Flashcards
What is conditional instability?
When an air parcel cools faster than the atmosphere but then cools slowly after condensation
What does the depth of frictional influence depend on?
Surface roughness
Thermal forcing
What is the top of the boundary layer (Zg)?
Where ū (mean windspeed) becomes approximately constant with height
Are vertical gradients of windspeed greater or lesser over smooth terrain?
Greater over smooth terrain - less forced convection
Open country windspeed
Rapid change of windspeed with height
City centre windspeed
More effective mixing so less rapid change in windspeed
Forced convection (in the boundary layer wind field)
Turbulence due to the production of shear stress by surface roughness
Free convection (in the boundary layer wind field)
Turbulence due to sensible heat working against gravity (producing buoyancy)
Which convection has stronger vertical components in eddies?
Free convection (not constrained to ground surface)
Momentum flux
change in momentum
Will a downdraft cause an increase or decrease in velocity and momentum of windspeed?
an increase
Will an updraft cause an increase or decrease in velocity and momentum of windspeed?
A decrease
Stability
A measure of the tendency of air tto move vertically
How is free convections stability assessed?
Via changing temperature with height - the heat flux density
How is forced convections stability assessed?
Via changing horizontal windspeed with height - momentum flux density
What is combined stability
the ratio between difference in temperature with height vs difference in horizontal windspeed with height
How is combined stability commonly compared
Richardson number
What is the leading edge effect?
The boundary line between the 2 climatically different surface types
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?
Internal boundary layer
Speed of leading edge
Slow - gains 100-300m fetch for every 1m increase in vertical.
Speed of internal boundary layer
Fetch of 10-30m for every 1m increase in the vertical
Advection fog
typically formed by advection of air across water of very different temperatures
cold water advection fog
Warm air flows over cold water. if air temp cools to dew point temp, water vapour condenses and fog forms
warm water advection fog
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.
Topographic features causing precipitation
orographic precipitation
Night time topoclimate
Solar angle less important, LW radiation more important
Cold air accumulation depends on
-catchment size from which cold air drains
-duration of drainage
-screening of horizon: sky view factor
Radiation frost
radiation cooling leads to temperatures <0°C. can be significant in valleys
advection frost
air is already cold, and transported in. enhanced by altitude
which direction are succulents likely to face?
towards the equator
Equator facing slope characteristics
higher SW radiation input, higher temps = more evaporation
