An introduction to aspects of Flux measurements that are useful in understanding

the AMMA Mk4 and HFS Flux systems

Flux Workshop, Cotonou, Benin: 15-17 February 2006

Colin LloydCentre for Ecology and Hydrology

Wallingford, OX10 8BBUK

Raingauge

T107Soil Temperatures

CS616Soil Moisture

Solent R3-50 or Windmaster Pro

Kipp and ZonenCNR1 Radiometer

Clark Mast

Solar Panels

Zarges Box contains logger boxes + 2x 80Ah batteries

Heat Flux Station (HFS)

orMk4 Hydra

(Mk4)

or Mk4 Head

Sensor measurement List for Flux station

HFS: R3-50 u,v,w,t,dir, u*, z/L, HorMk4 Hydra u,v,w,t,dir, u*,z/L, H, LE, CO2

Common to both systems

CNR1 Swin SWRefl, LWin, LWout

WXT510 U,Dir, T, RH, Pressure, RainfallRaingauge RainfallCS616 Soil moisture at 2 depthsT107 Soil temperature at 4 depths

Vaisala WXT510

AMMA-UK, AMMA-EU & CLASSIC Funded Instrumentation

Grassland Site, Mali supersite - Hombori

Agoufou

Grassland - April

Grassland - August

The other Mali Mesosite flux stations

Wetland Acacia Forest

Gravelly red soil

Hedgerit

Kelema

Bamba

Desert

What is the Flux System trying to measure

This is an idealised view of what the Sonic anemometer is trying to measure. As the “elliptical” eddies pass the anemometer, so the vertical velocity changes.

The relative sizes of the horizontal velocity in the direction of the mean wind (u), the crosswind velocity normal to the horizontal wind (v) and the vertical velocity (w), orthogonal to both u and v.

+u -u

+w

+v

-v

-w

The shape and size of the turbulent eddies is dependent on both the height in the atmosphere that they propagate to and on the general stability of the layer of atmosphere that they find themselves in.

Neutral: Overcast, normal wind, dawn and dusk

Unstable: Sun, low winds, daytime

Stable: Night

But the anemometer does not measure in orthogonal vectors

+u -u

+w

+v

-v

-w

Coordinate rotation

1. Resolve into u2. Rotate so that v is orthogonal to u3. Rotate this pairing so that w=0

Such an exercise can be performed on the vertical windspeed velocity. But this figure illustrates “intermittency” rather than removing any long term trend – as physically at the surface, during an averaging time period, there can be no net movement of air towards or away from the surface

In order to “Correlate” the vertical windspeed with T, H2O or CO2, we need to separate the instantaneous value of T, CO2 or H2O from any longer term variation in these entities.This has been done in this instance by using a high-pass filter to remove the gradual trend seen in the middle trace – leaving the instantaneous values seen in the lower trace.

i.e. 0w

This, then, is the process of Eddy correlation or eddy covariance. Instantaneous values of vertical windspeed are multiplied by scalar measurements of Temperature (as in this diagram), water vapour or CO2 (and other atmospheric scalars) to produce a flux (in this case = Sensible Heat)

30 minute averaging periodEddies are too

small to be resolved Eddies are too

large to be resolved

Eddies are being measured in different places

Fluxloss Factors

The problem of surface homogeneity

It is necessary to remain within the constant flux layer while avoiding the heterogeneity of the Roughness sublayer

When the airflow passes from one surface to another, a new regime is formed which takes time and distance to propagate upwards.

The problem is for the measurement to be in the new boundary layer – which this measurement mast is not. This sensor will measure a flux that is a mixture of the sources from the smooth and rough surfaces.

The formation of a new boundary layer is slower when going from rough to smooth surface – the fetch is therefore longer.

Another problem is isolated roughness elements within an otherwise homogeneous surface – e.g. trees

The flow lines will be deflected around and over the obstruction – and this is relatively smooth in laminar flow as seen here. Vertical windspeed will tend to be over- or underestimated if measured close to these obstructions.

In reality, laminar flow is not common and the more normal state is turbulent flow – which produces a more chaotic windflow pattern downstream of the obstruction. Not only does this affect the measurement of vertical windspeed but it can alter the exchange of water vapour or CO2 from the surface

Trees are usually more porous than the obstruction shown here

The building here has created a turbulent field that has effectively kept the downwind area free of snow

The Source Footprint

Scalar quantities measured by the sensor are a mixture of individual sources upwind from the measurement position.

The relative contribution of these sources is a function of upwind and crosswind position and stability

The position of the measurement in highly heterogeneous terrain e.g. Tiger bush is extremely important.

The effect of stability on fluxes over heterogeneous surfaces is seen here – where the maximum source strength is from the local bush during the day ( z/L = -1.88) but from the sand during the night (z/L = -0.01)

Source: Lloyd, 1995

The average area of Tiger Bush to soil is 0.46 but it is evident that none of the footprint integrations at different measurement heights have achieved this ratio as instability increases. But the average ratio is maintained further into the unstable region with increasing height.

From the 2nd tower – only 50m away, the converse is seen . The footprint analysis shows that the contribution of sand to the measurement dominates until well into the unstable region – thereafter, the bush contribution is again dominant.

Source: Lloyd, 1995

Schmid & Lloyd (1999) presented this diagram which shows the blurring of the perceived surface with decreasing instability. It is apparent that any measurement of scalar quantities is closer to the areal average in the right-hand diagram.

HourlyFootprints2001:YD 217-YD 225

Aug 5 –Aug 13

500 m1000 m

The Webb Correction – strickly the Webb, Pearman and Leuning correction

Moist air is less dense than dry air – and due to this and pressure differences – the rising eddy is less dense and rises fast

The eddy has lost its moisture – is now more dense but encounters higher pressure forces in approaching the ground – the parcel is denser and slower in approaching the ground

This is a problem because the assumption that underpins the eddy correlation method, namely that there is no net flux of air at the lower boundary of the atmosphere is not true. It is true that there is no net flux of dry air at the surface. The presence of moisture and heat in the air affects the overall density of this moist warm air creating an apparent mean non-zero vertical velocity, , which will cause contamination of mass fluxes deduced from devices measuring the density of constituents.

For Water vapour- but true also for CO2

-20 ºα = 0 º

+20 º

Only wind vectors within +/- 20 º of horizontal are within the Solent’s specification

Another source of error and flux loss – the Angle of Attack problem

forest grasscrops

(z-d)/z0

σα

(o

)

neutral

10 1000

10

20

30

unstable

stable

Instantaneous angle of attack

Time (seconds)

0 20 40 60 80 100

Ang

le o

f att

ack

(deg

)

-60

-40

-20

0

20

40

60

peat bog

forest

Loobos Days 224, 225 & 226

Angle of attack (deg)

-80 -60 -40 -20 0 20 40 60 80

Fre

quen

cy o

f occ

uren

ce

0.0

0.1

0.2

0.3

0.4

Horstermeer Days 140, 141 & 152

Angle of attack (deg)

-80 -60 -40 -20 0 20 40 60 80

Fre

quen

cy o

f occ

uren

ce

0.0

0.1

0.2

0.3

0.4

The horizontal wind attack angles are evidently larger for the Forest (Loobos) than for the peat bog (Horstmeer)

Horstermeer Day 140, 141 & 152(excluding CO2 on night of 152)

Angle of attack envelope (+/- deg)

0 20 40 60 80

Cu

mu

lativ

e f

req

ue

ncy

0.0

0.2

0.4

0.6

0.8

1.0

evaporation - - -sensible heat - - --ve CO2 ———

alpha

night-time CO2

Outside specification : angles 3%evaporation18%heat flux 20%- ve CO2 20%+ ve CO2 8%

The area to the right of the 20o line shows the extent that measured fluxes are underestimating the quantities due to the angle of attack effect.

Over forests, just this effect can reduce the flux loss by 10-20%

The angle of attack problem can affect the different fluxes in different ways.

Loobos Days 224, 225 & 226

Angle of attack envelope (+/- deg)

0 20 40 60 80

Cu

mu

lativ

e f

req

ue

ncy

0.0

0.2

0.4

0.6

0.8

1.0

evaporation - - -sensible heat - - --ve CO2 ———

alpha

+ve CO2

Outside specification : angles 12%evaporation45%heat flux 57%- ve CO2 53%+ ve CO2 19%

The quality of measured fluxes is generally quantified by estimating the Energy Balance i.e. Rn-G=H+LE.However, over heterogeneous surfaces, the measurement of Rn-G, being static in space, is often at variance with the dynamic measurement of H and LE. So the placement of the radiation and soil heat flux measurements can be very important if an energy balance is to be constructed

Rn Considerations As with the flux measurements, height of radiation measurement is important. With hemispherical instruments subject to cosine response, it is generally true that 50% of the perceived reflected radiation is coming from an area on the ground whose radius is equal to the height above the surface.The height is, however, constrained by radiative flux divergence which means that at great height (above 15m) the measured values have to be adjusted for this flux loss.

There are also considerations for the measurement of Soil Heat Flux G

Soil heat flux plates are constructed to be as similar in thermal properties as normal European loamy soil – not quite what we find in west Africa.

Also the soil tends to dry out at the surface with a wetting front at some depth. If the Soil Heat flux plate is near the surface (as it should be) then the heat flux plate is measuring a latent heat flux from the wet soil beneath.

In such conditions, it is not always appropriate to rely upon:

G = 0 over 24 hours

It is generally agreed within the Flux Measurement community that Rn-G H+LE is rarely satisfied. For all the reasons given above, H+LE can be typically 80-90% of Rn-G. Only at carefully selected flat extensive homogeneous sites is parity gained. The new analyses with angle of attack, improved coordinate rotation and averaging methods means that we are approaching parity –at last.