Standard ISFS sensors are available for the following measurements: momentum, sensible heat, and water vapor fluxes by eddy-correlation; vertical profiles of wind, temperature, and humidity; radiation fluxes; and soil temperature, soil moisture, and soil heat fluxes. The ISFS sensors are a mix of commercial instruments and in-house developments. Whenever possible, each sensor has a microprocessor that provides sensor output in calibrated engineering units through a serial communications link.
User-supplied or non-standard sensors can be readily accommodated by ISFS. Power and mounting are provided and the various data streams are easily integrated into the system.
Flux measurements made by ISFS utilize the eddy-correlation technique. Momentum flux is determined from the covariance between the vertical and horizontal velocity fluctuations measured by the 3-dimensional sonic anemometers. Sensible heat flux is determined from the covariance between measured vertical velocity and temperature fluctuations. Latent heat and carbon dioxide flux is determined from the covariance between measured vertical velocity and humidity or carbon dioxide fluctuations.
Measurement of fluxes of trace chemical species are obtained from correlating fluctuations from chemical species sensors with those from the ISFS velocity sensors. ISFS has several varieties of CO2 sensors for this purpose. Other chemical species measurements require user-supplied sensors which typically can be readily interfaced to ISFS. In chemical flux determination, delay times through inlet tubes can be measured and compensated for in the processing.
Tilt corrections for the sonic anemometer are applied in the software. Over uniformly flat terrain, the tilt correction puts the anemometers into a coordinate system parallel to the surface. In complex terrain, signals from two-axis level sensors are used to orient the sonic anemometers to gravity. Note also that the sonic anemometers make a virtual temperature measurement, Tc, reported in degrees Celsius. We maintain software to remove the moisture contribution to Tc from the sensible heat flux computation based on colocated latent heat flux measurements.
Mean temperature and humidity measurements are made with a commercial solid-state temperature/relative humidity sensor in an aspirated radiation shield developed by us to operate at relatively low power.
Measurement of the surface energy balance is achieved using up-looking and down-looking pairs of short and long-wave radiometers (either individual component or in an integrated 4-component radiometer) in tandem with soil heat flux plates and soil temperature sensors. Soil heat capacity also is measured to complete the calculation of heat storage above the soil heat flux plates.
