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 flux is determined from the covariance between measured vertical velocity and humidity 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 specie measurement requires 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, Tvs, reported in degrees Celsius.
Mean temperature and humidity measurements are made with a hygrothermometer developed at NCAR and previously used with the NCAR Portable Automated Mesonet (PAM). These hygrothermometers are integrated sensors which utilize a platinum resistance thermometer and solid state relative humidity sensor inside an aspirated radiation shield enclosure.
Wind profiles are measured with commercial propeller-vane wind sensors which have been modified at NCAR to obtain a high resolution vane azimuth using an optical encoder. These sensors have also been modified to output a serial data string giving output in either windspeed and direction or orthogonal components.
Measurement of the surface energy balance is achieved using up-looking and down-looking pairs of short and long-wave radiometers in tandem with soil heat flux plates and soil temperature sensors. The chemical environment around ISFF sites can be monitored with sensors which measure ozone, carbon monoxide, as well as condensation nuclei.
