1. Micrometeorology Surface Layer Dynamics and Surface Energy Exchange
Janet Barlow and Andrew Ross
September

2. Aims of exercise Micrometeorology is concerned with:
Interaction of atmosphere with the surface
Turbulent mixing
Exchanges of momentum, heat, moisture…traces gases, aerosol
Radiative energy exchange at the surface
Solar (shortwave)
Infra-red (longwave)

3. Measuring the boundary layer
Lowest part of troposphere
Few 10s of metres to ~2km deep
Interacts directly with surface:
Feels the effect of friction
Heated/cooled by surface
Dynamics are dominated by turbulence
Exhibits large diurnal changes in many properties: depth, temperature…

4. Temperature Profile temperature inversion boundary layer
April
stratosphere
temperature inversion
boundary layer
tropopause
free troposphere

5. Humidity Profile
tropopause
inversion

6. Sources of Turbulence z Friction: mechanical generation of turbulence
Flow over rough surface / obstacles
Small perturbations of the flow act as obstacles to the surrounding flow
Shear in the flow can result in instability & overturning
Turbulence results in a wind speed profile that is close to logarithmic z
Wind speed

7. Convection:
heating of air near the surface (or cooling of air aloft) increases (decreases) its density with respect to the air around it, so that it becomes buoyant.

8. Surface energy budget
Radiative transfer at the Earth’s surface dominates the production or suppression of turbulence in low wind conditions
The heating of the lower layers of the atmosphere is governed by
Heating of the surface itself
Transfer of heat from the surface to the air by four processes:
Absorption and emission of “natural” EM radiation at the surface
Thermal conduction of heat energy within ground
Turbulent transfer of heat energy within the atmosphere
Evaporation of water stored in the surface layer or condensation of water vapour onto surface

9. Flux densities = rate of
Net
radiation
Flux densities = rate of
transfer of energy
across a surface Rn
Shortwave
radiation
Latent
heat flux
Sensible
heat flux
Longwave
radiation LE H
Ln=L↓-L↑
Sn= S↓- S↑ = (1-α)S↓
Ground
Heat flux G

10. Surface energy balance
Considering a thin layer of soil at the surface:
Heat storage = What goes in - what goes out!
For an infinitely thin layer – no heat storage therefore
Rn-G=H+LE
Where Rn = Sn+Ln

11. Ultimate aim of SEE/SLD sessions
There are 3 ways of determining H, the sensible heat flux
1. SEE
Measure Rn and G
Estimate LE from met measurements using Penman-Monteith equation
Residual is H (assuming infinitely thin layer)
2. T profile
Take a logarithmic T profile from the mast
Find friction velocity and friction temperature
3. Turbulent eddy measurements
Measure heat flux due to turbulent eddies using sonic anemometer data

12. Ultimate aim of SEE/SLD sessions
There are 3 ways of determining H, the sensible heat flux
1. SEE
Measure Rn and G
Estimate LE from met measurements using Penman-Monteith equation
Residual is H (assuming infinitely thin layer)
2. T profile
Take a logarithmic T profile from the mast
Find friction velocity and friction temperature
3. Turbulent eddy measurements
Measure heat flux due to turbulent eddies using sonic anemometer data

13. SEE Activity
Real-time calculation of radiation budget (using portable mast)
Estimates of surface albedo, emissivity, response time
2. Components of the surface energy budget for a time period, and link to meteorology (uses Excel worksheets extensively)
3. Estimate H

14. Instruments Pyranometer Measures the flux of solar radiation (W m-2)
Two instruments mounted back to back – measurement of downwelling and upwelling radiation

15. Pyrgeometer
Measures flux of infrared radiation (W m-2)

16. Weather station and ground heat flux
Ground heat flux is also measured using a plate buried below the ground.
Weather station data is used to estimate LE using the Penman- Monteith method.
H can be estimated by balancing the budget!

17. Ultimate aim of SEE/SLD sessions
There are 3 ways of determining H, the sensible heat flux
1. SEE
Measure Rn and G
Estimate LE from met measurements using Penman-Monteith equation
Residual is H (assuming infinitely thin layer)
2. T profile
Take a logarithmic T profile from the mast
Find friction velocity and friction temperature
3. Turbulent eddy measurements
Measure heat flux due to turbulent eddies using sonic anemometer data

18. SLD 1 - T profile method
Calculate and analyse temperature and wind profiles from the mast data for two one hour periods (one stable and one unstable). Use these to calculate H
A standard result: under neutral conditions, surface layer winds and temperatures have a logarithmic form.
H can be estimated using the values of u* and T*

19. Instruments
15m Mast
Air temperature and wind speed are measured at 6 levels on the mast
Also have soil temperature just below surface.

20. Key parts of SLD1 Check offset correction is applied
Plot timeseries of quantities to find a stable and unstable period
Plot logarithmic profiles of u and T to find u* and T*
Calculate H

21. Ultimate aim of SEE/SLD sessions
There are 3 ways of determining H, the sensible heat flux
1. SEE
Measure Rn and G
Estimate LE from met measurements using Penman-Monteith equation
Residual is H (assuming infinitely thin layer)
2. T profile
Take a logarithmic T profile from the mast
Find friction velocity and friction temperature
3. Turbulent eddy measurements
Measure heat flux due to turbulent eddies using sonic anemometer data

22. 3-D air motion is broken down into 3 velocity components (all in m s-1):
u horizontal, along mean wind direction, positive in direction of mean wind
v horizontal, perpendicular to u, positive to left of mean wind direction.
w vertical, positive upwards w v u

23. Eddy averaging
Any quantity can be divided into mean and fluctuating terms:
To look at turbulent fluxes we are most concerned with the fluctuating terms e.g. w’

24. Turbulent fluxes result from the physical movement of parcels of air with different properties: temperature, humidity, gas concentration, momentum…
Heat flux
Momentum flux
Eddy mixes some air down & some air up 
w’ -ve
w’ +ve
Z (m)
Z (m)
Faster moving air is moved down, slower air is moved up
Warmer air is moved up cooler air is moved down
 (K)
Wind speed (m s-1)

26. SLD 2 - Turbulent Eddy Measurements
Use sonic anemometer data to calculate surface heat flux
Sonic Anemometer
Measures 3D wind components at very short intervals.

28. Key parts of SLD2
Calculate u’ and w’ series from 15 minute sonic measurements
Calculate vertical momentum flux
Calculate u* and H
Compare with T profile measurements

29. Summary
The SEE and SLD are linked exercises in which you will use different methods to explore the surface layer.
The three experiments will all estimate the value of H, the turbulent transfer of heat energy from the surface to the lower layers of the atmosphere.
You should assess the quality and reliability of the different techniques and what the changing value of H tells you about the meteorological situation.

30. Sonic axes tilted off verticalum wm  u
w = 0

31. wt ut wm um 

32. For example, the wind stress at the surface (the vertical flux of horizontal momentum) is
where  is air density, and U the wind speed.
More strictly it is
where u is the wind component in the direction of the mean wind direction and v the component perpendicular to the mean wind.
The wind stress is often represented by the friction velocity

33. The turbulent flux of some quantity ‘x’ is determined by averaging the vertical exchange of parcels of air with different values of ‘x’.
Flux of x = 1 (w′1x′1 + w′2x′2 + …w′Nx′N) N
= w′x′
where w′N = wN – w
and an overbar signifies averaging