Outline of Our Observation

Background

Observational instruments are divided into and managed as two separate types: eddy covariance flux measurements and micrometeorological average values.

The measurement of flux, including carbon dioxide, uses fast-response instruments because wind speed as well as temperature, humidity, and CO2 concentration must be measured. With a sampling rate of five to ten times per second, 12 to 16 micrometeorological elements are measured and stored, creating a data volume of 300MB in 2 weeks. Micrometeorological average values, such as the amount of solar radiation, are measured once every 15 to 30 seconds, added and averaged every 5 to 10 minutes, and then recorded.

The instruments comprise models that are comparatively easily to maintain. However, the radiometer dome is cleaned, water is added to the wet-bulb hygrometer and its gauze is cleaned, and the CO2 gas analyzer is calibrated, once every 1 to 3 weeks.

Specific details, such as the altitude where the devices are installed and observation items, vary by observation site. (Please refer to Observation Items and Observation Site Network pages.)

What is the Eddy covariance method? (Using carbon dioxide as an example)

The carbon dioxide in a forest canopy is transported in a vertical direction with various sized eddies that exist in the atmosphere above the canopy. The micrometeorological technique measures the movement of these eddies and the fluctuations in carbon dioxide concentration and then measures the speed of carbon dioxide absorption and release (flux). The micrometeorological technique includes the gradient and Eddy covariance methods, but the Eddy covariance method has become mainstream because of mechanical improvements and improved calculation processes in recent years.

Carbon dioxide flux measurement with the Eddy covariance method is performed on forest communities using a fast-response three-dimensional ultrasound anemometer and an infrared carbon dioxide gas analyzer. The Eddy covariance method performs sampling at 5 to 10 Hz (5 to 10 samplings per second) and calculates flux from the fluctuation component (difference from the average value). When the concentration of carbon dioxide in the downward vertical wind increases, then air with a high concentration of carbon dioxide is being transported in the downward direction. In other words, the phenomenon of carbon dioxide being absorbed by the forest ecosystem has been captured. By accumulating this type of data, the general carbon dioxide absorption and release speed (flux) of the forest can be found in 30 minute intervals.