Background Science

Global Climate

Global climate results from the complex interactions between the energy from the sun, the exchange and processing of this energy within the earth’s gaseous atmosphere, and the exchange of energy and gases between the earth’s land, vegetation and ocean surface.

The dynamic nature of these interactions can easily appear to be chaotic because it is hard to understand how all the interactions can be followed. However with the availability of powerful computing it is now possible to simulate the energy exchanges in every few square kilometres of the earth’s surface. Hence forecasts of all the daily weather variables, temperature, humidity, wind and rain can be generated and used to inform the more detailed local weather forecasts. These computer simulations are called General Circulation Models or GCM’s.

The GCM’s need some critical current information about the surface and atmosphere conditions, and there is also a very important need for on-the-ground observations to check that the GCM outputs are accurate. Much of the information about surface conditions can now be gathered from an array of satellites orbiting the earth which give good information for very large areas, but we know that there is a lot of local variation that is important.

The largest uncertainty that still exists is associated with the exchange of energy, water vapour and gases including carbon dioxide from the land. Unlike the oceans, land is highly variable. It changes in height, in colour, in whether it is wet or dry, and the kind of vegetation that is present. To improve our understanding of these processes, detailed measurements are needed to characterise the energy, water vapour and carbon dioxide exchanges between different land ecosystems and the atmosphere. These measurements can be supplemented with other data gathered using Unmanned Aerial Vehicles that helps to describe the vegetation distribution, density, colour and condition.

Land surface interactions

Air moves over land surfaces in a non-linear way, through a chaotic array of circulating eddies which each have rotating horizontal and vertical components. Parcels of air can have different concentrations of gases, temperature, pressure and humidity and the turbulence of eddies increases as does the roughness of the surface (e.g. due to vegetation). When studying the exchange of gases, energy and water between the atmosphere and the earth it is the net flux which is important. Eddy covariance (EC) is a highly technical approach, using specialised equipment, to measure and calculate net exchanges in turbulent conditions. Click here for more information on eddy covariance.

The flux of CO2 determined from eddy covariance (EC) measures and calculations is a net value because sequestration by photosynthesising vegetation (mainly green leaves) and emission from respiration within a soil plant ecosystem occurs concurrently. The total of this flux over a day is called the net ecosystem exchange (NEE). Partitioning EC determined NEE, to quantify the contributions of sequestration and emission of CO2, is challenging. Ideally, independent measures of both daytime and night-time ecosystem respiration are needed to make reliable estimates of net ecosystem productivity (NEP), the amount of Carbon retained in the ecosystem. However, very few independent measures of respiration are made and so other methods of estimation are used. Efforts are underway through the Calperum site to develop methods to improve the estimate of NEP.

At night when photosynthesis is not occurring, the flux of CO2 from the ecosystem into the atmosphere is the measure of ecosystem respiration. However the often calm night-time atmospheric conditions are not ideal for measures of CO2 flux to and from an ecosystem. Quality EC measures depend on adequate mixing of the atmosphere moving over the soil plant system. This is generally not an issue during daylight hours when near surface atmospheric mixing is usually large (Burba 2013). During the night however, air within the vegetation layer cools and decouples from the layer of air above the plant canopy. With little turbulent mixing, sensors at the top of the EC tower do not fully indicate near surface fluxes. When this happens the estimate of night time CO2 flux will be an unreliable measure of ecosystem respiration (ER). To minimise the bias that this measurement limitation may induce in full day exchange values, various data filters are applied. Most commonly, minimum thresholds for average half hourly values of friction velocity, that are quite directly related to the strength of wind turbulence, are used for removing measurements when it is deemed that there is insufficient mixing.

Flux Monitoring

The presence of eddies in the lower atmosphere ensures that changes in the composition of the air in and around the vegetation are reflected in the measurements at the top of the EC tower. The measurement that is especially important is the velocity of the air that is moving in a vertical direction, either away from the ground and vegetation surfaces, or that coming from the atmosphere down to the vegetation layer. To measure the vertical fluxes of water vapour and carbon dioxide, it is important to understand how eddies move within an air mass. To do this requires a 3D sonic anemometer, collecting data at fractions of a second. Coupled with this eddy profiling data, an Infra-Red Gas Analyser (IRGA) measures the chemical composition of the air that is being mixed and moved by the eddies.

This link to Wikipedia – Eddy Covariance has a brief explanation of eddy covariance with a diagrammatic representation of eddies in the atmosphere

By including an array of monitoring devices for meteorology and solar energy balances, valuable data is collected which can be used to verify various models, such as global climate, weather, biogeochemical and ecological models, and to provide ground reference data for remote sensing estimates.

Ground-truth Data

The instrumentation at the OzFlux site provides measurements of the minute to minute changes in all of the weather variables i.e. atmospheric pressure, intensity and amount of sunshine, temperature, wind, rain and humidity. The tower-sourced data are supplemented with measures in the soil of temperatures, water content and salinity. As a special component, instruments on the tower also measure the concentration of carbon dioxide (CO2) and water vapour in the atmosphere, and a wind direction measurement indicates whether the CO2 and water vapour are moving up or down, away from or towards, the vegetation. From these measures it is possible to calculate how much water the vegetation is using (evaporation) and how much carbon is being stored (sequestered) or lost (emitted) from the vegetation and soil.

At the same time as the OzFlux site is taking these measurements, earth orbiting satellites are taking an extensive range of light frequency measurements of the earth’s atmosphere and surface. These frequency measures are processed to indicate characteristics of the atmosphere and the earth surface. For the atmosphere, factors such as cloud cover, temperature and clarity can be inferred while for the surface, its temperature, colour, reflectance, water content, vegetation cover and density can be inferred. From these inferred values rates of evaporation (water use) and carbon uptake can be derived. It is these derived values that need to be checked and validated using the ground based measures obtained from the OzFlux sites.

As a further development, Global Climate Models (GCM’s) are now commonly used to assist projections of short term weather forecasts, and to assess likely changes as the chemical composition of the earth’s atmosphere changes (from increased CO2 and other ‘greenhouse’ gases). The accuracy of the GCM’s is strongly influenced by the accuracy with which the transfer of water vapour and CO2 to and from the land to the atmosphere is known and represented. The network of land based flux towers like the Calperum OzFlux site is providing the best ‘ground truth’ information on the transfer of these gases and hence the interaction between the land, its vegetation and the air passing over it. Without the ground truth data it would be very hard to improve the GCM’s and establish that they are accurate.