Lecture 5: Micrometerology

Question 1

On what timescale do planetary, synoptic, mesoscale and microscale phenomena occur? Give examples.

Answer 1

Planetary - 5000-40,000 weeks+ - Westerlies, trade winds
Synoptic - 100 to 5000 km days to weeks - cyclones
Mesoscale - 1 to 100km minutes to hours - tornadoes
Microscale - <1km, mins - wind gusts, turbulence

Question 2

Sea breeze is an example of Meso/Micro scale local circulation, but how does it form?

Answer 2

Cool air flows from sea, land heats much faster than sea so rises, drifts back to sea cooled by adiabatic expansion and sinks in high pressure.

Question 3

Describe the variation in mountain and valley flows over a day.

Answer 3

Air rises upslope after sunrise, upslope and up valley at midday, down slope after sunset and down slope and down valley at night so fills with cold air.

Question 4

The constant flux layer, aka boundary layer, lies 10s of meters thick at the bottom of the atmosphere. Describe movement within this.

Answer 4

Variation of vertical turbulent flux with altitude is less than 10% of its magnitude. Flux is not perfectly uniform with height, but is assumed to be for theoretical approaches.

Question 5

What is surface storage of energy equal to?

Answer 5

Total radiative flux - latent heat flux - sensible heat flux - horizontal heat flux.

Question 6

Thermal conductivity of soil is low, what does this mean for ground heat flux?

Answer 6

Very little heat stored in ground and little temperature change below 10cm, therefore the temperature of surface layers closely follows solar energy diurnal cycle.

Question 7

What is net radiative flux equal to?

Answer 7

net solar radiation + net longwave radiation

Question 8

What is sensible heat?

Answer 8

The energy required to change the temperature of a substance, without changing its phase.

Question 9

What is latent heat?

Answer 9

The energy absorbed or released during the phase change of a substance without changing temperature.

Question 10

Where is the energy associated with sensible and latent heat fluxes exchanged?

Answer 10

Just above the surface of the object via convection. On the earth, this is between the surface layer and the atmosphere by small scale convective turbulence.

Question 11

What is the time mean sensible (/latent) heat flux equal to?

Answer 11

specific (/specific latent) heat capacity of air X air density X vertical velocity X temperature change.

Question 12

What is key to determining sensible / latent heat transfer using the bulk transfer formulae?

Answer 12

The difference between surface property (temperature for sensible, and specific humidity for latent) and the same property at the reference level.

Question 13

Why is the balance between sensible and latent heat flux important?

Answer 13

Helps us to understand surface temperature

Question 14

What is the Bowen ratio?

Answer 14

Sensible heat flux / Latent heat flux

Question 15

How does the Bowen ratio vary over wet / dry surfaces?

Answer 15

For wet surfaces, it is around 0.4 at 10 degrees and near zero at 30 degrees. If the surface is dry it is greater than 1.

Question 16

What measurements are needed to determine the bowen ratio experimentally?

Answer 16

The surface and reference level temperature and specific humidity.

Question 17

Where is net radiative heating greatest and what is it balanced by?

Answer 17

At the tropics (weakest at poles) and balanced by latent heat flux.

Question 18

Where is sensible heat flux at its maximum and why?

Answer 18

In the northern hemisphere, around 30 degrees, due to greater land extent.

Question 19

Where is horizontal energy flux observed?

Answer 19

In the oceans due to currents

Question 20

What characterises diurnal variations in surface energy balance in dry conditions?

Answer 20

Radiative flux primarily balance by sensible heat loss and some ground storage.

Question 21

What characterises diurnal variations in surface energy balance in wet conditions?

Answer 21

Radiative flux is primarily balanced by evaporative latent heat loss and cooling of air above surface in the late afternoon. No ground storage.

Question 22

What are the two defining points of annual surface energy balance?

Answer 22

Heat storage is generally small

relative strength of LE and SH depend on the availability of surface moisture.

Question 23

In what three ways is ocean surface energy balance different to land?

Answer 23

Heat storage does not have to be negligible
Heat transport can be a significant contribution to energy balance
Latent heat flux is not limited by the availability of moisture.

Question 24

Describe the atmospheric boundary layer (ABL).

Answer 24

It is the portion of the atmosphere closest to the earth’s surface most affected by surface properties. Around 1 k thick, representing 10% of the total mass of the atmosphere.
Shows large daily variations in wind, temperature, turbulence and stability.

Question 25

How does the surface influence the ABL and what clouds predominate in the ABL?

Answer 25

• Influenced by friction and heat fluxes at the ground.

- Fair-weather cumulus, stratocumulus and fog

Question 26

The ABL is characterised by turbulence, generated by wind shear, but how does wind vary across the ABL? What influence does temperature have?

Answer 26

Wind is approximately geostrophic at the top and zero at surface.
Temperature gradients can either generate or suppress this turbulence.

Question 27

What difference is there between variations in temperature with height during the day and night?

Answer 27

during the day, temperature declines with height until around 1.5km, where it blips upwards before declining again.
The same is almost true for night, except temperature increase with height for a short while, until roughly 0.5km, from when it follows the same pattern.

Question 28

How do winds, temperature and humidity vary within the ABL with altitude and what is significant about vertical fluxes of heat and momentum?

Answer 28

Vary rapidly and logarithmically

Fluxes are approximately constant (constant flux layer)

Question 29

How much hotter is a million city than rural areas?

Answer 29

1-3 degrees

Question 30

What four things are urban heat islands caused by?

Answer 30

• Buildings increasing surface roughness
• Concrete surface prevent latent heat cooling and increase surface albedo
• Air pollution may add to warming
• Heating generated by human activities

Question 31

How do urban heat islands affect communities?

Answer 31

• Increase peak summertime energy demand
• Cost of air conditioning
• Air pollution and GHG emissions
• Illness and mortality related to heat
• water quality