# Presentation: 407704

## NIGHT STORAGE AND ADVECTION OF CO2 IN A SLOPING FOREST

- 	The team : Aubinet Marc
- 		          Heinesch Bernard
- 		          Longdoz Bernard
- 		          Yernaux Michel

##  Are direct measurements of horizontal and vertical  advection of CO2 on a local scale feasible ?

-  Are direct measurements of horizontal and vertical  advection of CO2 on a local scale feasible ?
-  Can it lead to improvement of NEE estimations   during non-turbulent nights ?
-  Can it improve our understanding of process at   work during non-turbulent nights

- Aim

## Mass conservation equation of CO2

- Mass conservation equation of CO2

- I

- II

- III

- IV

- I : Turbulent flux
- II : Storage
- III : Horizontal advection
- IV : Vertical advection

## Experimental set-up

- Campaign summer 1999

- Campaign summer 2002

**Notes:**
Our site is situated in a hilly region and present a uniform gentle slope of 3%.
Because we were suspecting the existence of drainage flows, we restrict ourselves to a 2D approach (vertical and direction of slope).
Two campaigns were made. A first attempt was made in summer 1999. Wind velocities were measured above the canopy by our sonic devoted to EC and inside the in the canopy and the trunk space by home made 2D sonic anemometers. Only two points were used for horizontal CO2 gradients and 3 for vertical CO2 profiles.

A new campaign is running during this summer. The main improvements are the number of CO2 points (20 points - 8 on the tower, 4 upslope, 4 downslope and 4 on an horizontal transect) and the quality of the analyser and the sampling frequency.

The results of 1999 are confirmed by the new campaign and new informations are added.

## Air flow pattern : drainage flows

- During stable nights (z/L > 0.3) :
-   occurrence of drainage flows

**Notes:**
Resultant wind vector computed for 8 direction classes of the 40m height wind direction and for 3 classes of atmospheric stability.

For example, when the wind above the canopy is blowing from the North, wind below the canopy is blowing from the North too (the scale is different). There is a coupling between above and below canopy wind regimes. 

But when we consider only the stable atmospheric situations, whatever direction the wind is blowing above the canopy, the wind near the soil surface is blowing from the South-East, wich is the downslope direction.

The wind regime below the canopy is  nearly completely decoupled from those above the canopy. This is the signature of gravity flows. The catabatic origin of this flow is asses by a strong correlation with Rnet.

## Air flow pattern : drainage flows

- During stable nights (z/L > 0.3) :
-   occurrence of drainage flows

**Notes:**
Moreover, this cold air flowing downslope (gravity flow) initiates , by a process of entrainment, a vertical air motion down into the canopy.

On the x axis is the horizontal velocity below the canopy and on the y axis is the vertical velocity above the canopy.
This is a bin averaged graph. One point represents one hundred measurements. Error bars are standard errors of the mean.

There is a strong relationship between the two velocities. When gravity flows develops, the vertical velocity becomes more and more negative..

## Example

**Notes:**
Example of three consecutive nights :
1 : Calm, no drainage flow, storage important, flush of CO2 at the end of the night
2 : calm, drainage flow, storage limited, no eddy flux
3 : combination of 1 and 2

## [CO2] : Horizontal gradient

- [CO2] : Horizontal gradient

**Notes:**
After caracterysing the air flow pattern inside the forest, we tried to measure the horizontal CO2 gradient.

This is the frequency distribution of the horizontal concentration differences (taken at 1m high, the two points being 50m  away from each other) . 

Due to good mixing during the day (red curve), the horizontal CO2 concentration is supposed to be zero. The frequency maximum at 2 ppm corresponds likely to an instrumental systematic error.

The horizontal CO2 gradient is more often negative at night than during the day.Then during stable nights, concentrations upslope are lesser than concentration downslope. This unexpected result is probably due to the combination of horizontal and vertical advection. Air coming from the top of the canopy is poor in CO2 and mixes with the horizontal air. It results a dilution of the CO2 that cannot be compensated by the respiration of the ecosystem.

Other possible explanations :
- A sink in the ecosystem (what could it be ?)
- Contamination of the horizontal gradient by the vertical gradient
- short distance heterogeneity

## Importance of advection terms in NEE

- During stable nights,
-  Horizontal and vertical advection are two fluxes of opposite sign  (risk of over-correction when including only vertical advection  as in Lee, Baldocchi, Paw U)
-  Horizontal and vertical advection are two fluxes affected   by a large variability (natural or not ?)

**Notes:**
Finally, we can try to estimate the vertical and horizontal advection fluxes of CO2.

This is the mean diurnal course of the turbulent flux (black symbols), the storage (black triangles), the vertical advection (blue) and the horizontal advection (green). This is the mean of thirty stable nights that occured during the summer period.

The important thing to note is that the horizontal and vertical advection are fluxes of opposite sign. Vertical advection is positive and horizontal advection is negative (this is due to the dilution effect). The order of magnitude is correct wich suggest that the mechanism is well identified.

The neglecting of the horizontal advection, as supposed by Lee in his procedure implies that there is no CO2 gradient along the slope or in other words, that the dilution effect is exactly compensated by the source term. This is not justified. Then, introducing only vertical advection in the budget equation would lead to an overcorrection of the night fluxes. 
A night flux correction based on advection must take both components into account.

To be honest, the evaluation of horizontal advection is subject to caution beacause it has been evaluated only at one height (1m) and to overcome the vertical integration of this term, we choosed a « scaling height » (20 m in this case). The value of hor advection is, of course,  highly dependant on this scaling height. The new campaign has already shown that the hor gradient greatly decrease with height and that we are probably overestimating the hor adv in this graph. 

Of course, they are both subject to large variability. This variability could correspond to some natural variablility of the flux but could also reflect the high sensitivity of the flux estimation to the velocity and the CO2 concentration errors.

A careful analysis of error and their propagation (sensitivity analysis) should be done.

It's not yet possible to deduce an accurate estimation of NEE during the night by using these results.

## Storage

- Recommandations

**Storage**
-  spatial variability
-  soil : dependance on turbulence (pressure pumping) and           soil porosity (and its fluctuation with SWC)
-  influence of sampling on [CO2] measurements

**Notes:**
Now, I will review the three terms of the carbon balance I am interested in (storage, vertical and horizontal advection) and make comments or recommandations. These part reflects the conclusions of the carboeuroflux workshop on night-time fluxes thas was held in Sweden in October  2002. Some points are illustrated by an example coming from our data.

For the storage term, we need to investigate about
its spatial heterogeneity and the use of several mast on the same site allow us to try such a study,
The storage of CO2 in the soil should also be taken into account. It would be important to get a better understanding of this term and its dependence on turbulence (pressure pumping), soil porosity (and its fluctuation with SWC) for example by the use of sensors that do not need air pumping (- GMD20 of Vaisala) not use to estimate the soil flux storage itself because of the great spatial heterogeneity of the [CO2] in the soil.
Stability is more important than absolute accuracy for the storage term but we need an analysis of storage measurements uncertainties to help storage system designers. For example, influence of [CO2] sampling frequency and duration is important

## Storage : influence of sampling

- Influence of sampling on [CO2] measurements :

- Uncertainty on [CO2] with 10 samplings in one half hour :
-  2 molmol-1

**Notes:**
The question is : « how well the half hour mean [CO2] obtained with sampling is representative of the half hour » In order to asses the uncertainty on [CO2] due to sampling duration and frequency, we compared at one point the half hourly mean of [CO2] obtained with and without sampling. It is quite easy to do. We stored the raw data (at 1 Hz) of the analyser during a week. We computed the averaged half hour concentrations, on one hand, by using all the raw data and on the other hand, by making artificial sampling on the raw dataset. The comparison of the two methods lead to the results above.
For the stable night conditions
On the y axis is the standard deviation of the residuals of the comparison with and without sampling.

On x axis is the frequency of sampling (the duration of sampling was fixed - 5 measurements.

For example, in stable conditions, if we make 10 sampling during each halh hour, the standard deviation of the residuals is 2 umolmol-1. This is the uncertainty on [CO2] due to sampling. This exercise can help the designers of storage system designers. 

## Recommandations

**Vertical advection**
-  importance of a good spatial resolution of vertical CO2 profiles
-  coupled with horizontal advection
-  error analysis on w necessary (systematic and random)

**Notes:**
For the evaluation of vert. advection, it is stressed that spatial resolution in [CO2] measurements is more important than stability.
The mass flow creates an horizontal flow divergence and vertical advection of CO2 creates horizontal gradients of [CO2] that are ignored by the Lee approach. In other words, horizontal and vertical advection are coupled and must be studied together (site specific conclusion ?)

## Vertical advection : error analysis on w

- Systematic error on w :

- The offset of 2° is an instrumental one
- The impact of an error on  is more important for important wind speed (w2  U).

**Notes:**
We encountered problems when using the tilt angle correction proposed by Wilczack - Lee.
This is the tilt angle in function of the azimuthal angle. The sinusoidal pattern due to the inclination of the sonic is well identified but we have also an offset of 2 ° towards the positive angle. This means a mean vertical velocity of several (7) cms-1 directed upwards. This flow motion cannot be obviously explained by the topography of the site and cannot be attributed to a thermally driven local circulation as the measurements used to establish the relationship were selected in order to retain near-neutral periods (as suggested by Finnigan). Therefore, we concluded that this offset was due to the anemometer and we included it in the correction function.

Some uncertainty on w results from this error. But we can shown that a systematic error on the tilt angle induce an error on w that is proportionnal to the horizontal component velocity. Consequently, the error on w is not too important during the stable nights.

## 

**Horizontal advection**
-  contamination of horizontal gradients by vertical gradients
-  footprint problems
-  velocity profile between the  soil and the point where u  decays (in the crown space)

- better to work with an horizontal profile rather than two points of measurements

- Recommandations

## Horizontal gradient during the day : footprint problem ?

- Horizontal advection : footprint problems

- Rose wind of horizontal gradient
- (cd-cu in molmol-1, day conditions)

- cd

- cu

- 300 m

##  Better spatial and temporal resolution of the   measurements

-  Better spatial and temporal resolution of the   measurements
-  Simultaneity of the horizontal and vertical velocity  measurements
-  analysis of the horizontal heterogeneity
-  study of the difference between sites, preferably by   using the same experimental device
- =>  CARBOEUROFLUX proposal of a mobile advection       system.

- Future