© University of Reading 2008

Atmospheric Science Fieldcourse

September 5 2009

MicrometeorologySurface Layer Dynamics and Surface Energy ExchangeJanet Barlow and Andrew Ross

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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)

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• 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…

Measuring the boundary layer

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Temperature Profile

tropopause

free troposphere

boundary layer

temperatureinversion

stratosphere

April 24 2004

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Humidity Profile

tropopause

inversion

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Sources of Turbulence• 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

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• 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.

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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

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Sensibleheat flux

GroundHeat flux

Shortwaveradiation

Longwaveradiation

Flux densities = rate of transfer of energy across a surface

Sn= S↓- S↑ = (1-α)S↓

Ln=L↓-L↑

G

LE H

Latentheat flux

Rn

Netradiation

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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

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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

**TuCH p TwCH p

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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

**TuCH p TwCH p

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SEE Activity

1. 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

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InstrumentsPyranometer

• Measures the flux of solar radiation (W m-2)

• Two instruments mounted back to back – measurement of downwelling and upwelling radiation

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Pyrgeometer

• Measures flux of infrared radiation (W m-2)

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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!

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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

**TuCH p TwCH p

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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* **TuCH p

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Instruments

15m Mast

• Air temperature and wind speed are measured at 6 levels on the mast

• Also have soil temperature just below surface.

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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

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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

TwCH p **TuCH p

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• 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

u

v

w

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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’

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Eddy mixes some air down & some air up

(K)

Z (

m)

w’ +vew’ -ve

Warmer air is moved upcooler air is moved down

Heat flux

Z (

m)

0Wind speed (m s-1)

Faster moving air is moved down, slower air is moved up

Momentum flux

Turbulent fluxes result from the physical movement of parcels of air with different properties: temperature, humidity, gas concentration, momentum…

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SLD 2 - Turbulent Eddy Measurements

• Use sonic anemometer data to calculate surface heat flux

Sonic Anemometer

• Measures 3D wind components at very short intervals.

TwCH p

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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

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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.

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u

w = 0

Sonic axes tilted off vertical

m

m

u

w1tan

um

wm

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um

wm

wt

ut

cossin

sincos

mmt

mmt

wuw

wuu

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• 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.

'Uw

2122' vwuw

• The wind stress is often represented by the friction velocity

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* u

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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