My work at Carleton University is connected with the learning of the Eddy Covariance method. The idea is to measure vertical fluxes in a atmospheric turbulent system. These fluxes are the exchange of our ecosystem with the atmosphere and are related to the general productivity of the ecosystem.
But lets start at the beginning: Imagine wind moving over a canopy. All along its way are small obstacles on the ground i.e. through leafs or tree trunks creating mechanical turbulence, small air circulations, also called Eddy’s. The second process creating Eddy’s is called convective turbulence where the warming of air at the surface causes a movement due to buoyancy. Now imagine that all these Eddy’s are interacting with each other and that there are not only small Eddy’s with some centimeter size but also large variants of several kilometer size creating low and high pressure areas in weather systems. Its a complex system, continuously changing and oscillating on all orders of spatial and time scales transporting also matter. Due to this high turbulence elements of air i.e. CO2 or water vapor are well mixed.
These elements are part of the ecosystem balance. If plants are active they take up CO2 and produce biomass with photosynthesis while if they are inactive at night when no energy through sunlight is dumped into the system or when temperatures are just too low they just respire CO2 bringing it back to the atmosphere. In a similar sense Water is evaporated or transpired to the atmosphere and the other way around in case of rain water is given back to the ecosystem. (Of course there are also other processes causing exchange between ecosystem and atmosphere)
Eddy’s will transport the components of air upward or downward. Depending on concentration differences the more transport is done in either direction. (I.e. if at day CO2 is taken up by plants due to photosynthesis there is a lower concentration of CO2 at the plant boundary, causing a Net Downward CO2 flux due to concentration difference). Now starts the interesting part: If one could measure all the vertical fluctuations of air and with it the compositions is this related to the ecosystem activity? Yes. (But there are lots of issues. As always 😛 )
An Eddy Covariance system measures all kind of meteorological parameters (like humidity, air temperature, air composition, wind speed, wind direction, incoming and outgoing short and long wave radiation, soil heat flux) and with these it is possible to calculate the mean vertical Flux of the important parameters, like Heat Flux, Evapotranspiration or CO2 as parameter for ecosystemproductivity. For the calculation Reynolds averging rules are used to express the mean over the material flux (mean over the product of airdensity, vertical windspeed and mixing ratio) as the mean of air density multiplied with the covariance of instantaneous deviation in vertical wind speed and instantaneous deviation in gas concentration. (The assumptions of constant air density and mean vertical flow is zero, which is the case at horizontal surfaces, are done for these simplifications).
Next up I will go into some details of the derivation and into problems of the method.