In 1987 the Surface Flux Group of the first ISLSCP Field Experiment (FIFE) operated 22 stations at 20 sites. In 1989, 13 sites were instrumented. A variety of sensors were employed to calculate the fluxes of mass and energy. An effort was made throughout the FIFE campaign to compare sensors. A series of papers in this special issue present these group studies and efforts. These papers principally report the 1987 campaign, although two papers report station intercomparison during 1989. Additional papers examine the time‐space variability of heat, moisture, and momentum fluxes, as well as analyses of the properties of the CO2 fluxes and their relationships to water stress. In this overview paper we describe the basic methodologies of the measurements, provide details on the sensor systems used by members of the Surface Flux Group, and provide a summary of the flux articles appearing in this special issue.
The underlying mean and variance properties of surface net radiation, soil heat flux, and sensible‐latent heat fluxes are examined over the densely instrumented grassland region encompassing the First ISLSCP Field Experiment (FIFE). Twenty‐two surface flux stations at 20 sites were deployed during the four 1987 intensive field campaigns (IFCs). Flux variability is addressed together with the problem of scaling up to area‐averaged fluxes. Successful parameterization of area‐averaged fluxes in atmospheric models is based on accounting for internal spatial and temporal scales correctly. Mean and variance properties of fluxes are examined in both daily and diurnally averaged frameworks. Results are compared and contrasted for clear and cloudy situations and checked for the influence of surface‐induced biophysical controls (burn and grazing treatments) and topographic controls (slope factors and aspect ratios). Examination of the sensitivity of domain‐averaged fluxes to different averaging procedures demonstrates that this may be an important consideration. The results reveal six key features of the 1987 surface fluxes: (1) cloudiness variability and ample rainfall throughout the growing season led to near‐consistency in flux magnitudes during the first three IFCs; (2) burn treatment, grazing conditions, and topography have clearly delineated influences on the diurnal cycle flux amplitudes but do not alter the evaporative fraction significantly; (3) cloudiness is the major control on flux variability in terms of both mean and variance properties but has little impact on the Bowen ratio or evaporative fraction; (4) spatial weighting of fluxes based on a biophysicaltopographical cross stratification generates a measurable bias with respect to straight arithmetic averaging (up to 20 W m−2 in available heating); (5) structure function analysis demonstrates significant underlying spatial autocorrelation structure in the fluxes, but the observed distance dependence is due to cloudiness controls, not surface controls; (6) Monte Carlo analysis of high resolution vegetation indices obtained from SPOT satellite measurements suggest that the mean domain amplitudes of the diurnal sensible and latent heat flux cycles can be biased up to 30–40 W m −2 by repositioning the 20 site locations within the experimental domain.
Net all‐wave radiation was observed at 22 surface sites during the 1987 observation year of the First ISLSCP Field Experiment (FIFE). Eight groups of investigators employed seven different designs of net radiometer by five different manufacturers. To establish true differences in received net radiation among sites requires knowledge of differences due to instrument performance. After careful Sun/shade calibration, side‐by‐side comparison revealed daytime differences as large as 5 to 7% for instruments of the same manufacture and 10 to 15% between manufacturer. The largest differences are between instruments with so‐called “thin windows” and “thick windows” and between instruments of “double‐dome” and “single‐dome” design. Comparisons with four‐component reference net radiation observations reveal that the double‐dome and thick window instruments have substantially lower sensitivity to longwave (thermal) net radiation than to shortwave (solar) net radiation. The magnitude of the sensitivity difference is greater when the sky is clear than when cloudy. Observations with thin window instruments agreed more closely with the reference component net radiation. Field observations made with double‐dome radiometers can be corrected when net shortwave radiation is separately measured. Such a correction is shown to reduce the systematic root‐mean‐square differences among instruments to between one half and one quarter of those shown by uncorrected measurements. When net shortwave radiation is not available, correction according to a regression comparison against a “standard” net radiometer may be used. This reduced the systematic root‐mean‐square differences by up to one half of their uncorrected values. From this analysis it is estimated that the regression corrected daytime net radiation observations reported to the FIFE Information System include systematic‐root‐mean‐square instrumental differences of 15 to 35 W m−2.
During FIFE 1987, surface energy fluxes were measured at 22 flux sites by nine groups of scientists using different measuring systems. A rover Bowen ratio station was taken to 20 of the flux stations to serve as a reference for estimating the instrument‐related differences. The rover system was installed within a few meters from the host instrument of a site. Using linear regression analysis, net radiation, Bowen ratio, and latent heat fluxes were compared between the rover measurements and the host measurements. The average differences in net radiation, Bowen ratio, and latent heat flux from different types of instruments can be up to 10, 30, and 20%, respectively. The Didcot net radiometer gave higher net radiation while the Swissteco type showed lower values, as compared to the corrected radiation energy balance system (REBS) model. The four‐way components method and the Thornthwaite type give similar values to the REBS. The surface energy radiation balance systems type Bowen ratio systems exhibit slightly lower Bowen ratios and thus higher latent heat fluxes, compared to the arid zone evapotranspiration systems. Eddy correlation systems showed slightly lower latent heat flux in comparison to the Bowen ratio systems. It is recommended that users of the flux data take these differences into account.
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