Area‐averaged surface fluxes and their time‐space variability over the FIFE experimental domain

Abstract

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.

Radiosonde boundary layer budgets above a boreal forest

“…The mean surface Bowen ratio /3• of 1.05 for the boreal forest in summer is much higher than the value of 0.4 over the FIFE grassland [Smith et al, 1992]. The analysis is subject to large random errors, and it was necessary to average the budgets over periods of 15 days or more to produce credible flux estimates.…”

“…This mean daytime summer/3, near 1.0 over the heterogeneous boreal forest is in sharp contrast to, say, the FIFE grassland, for which daytime summer/3, is •0.4 [Smith et al, 1992]. Table 3 shows the time-of-day variation in the budget estimates for H, and ,•E,.…”

Climatic impacts of historical wetland drainage in Switzerland

“…In this case, the evaporation does not primarily depend on soil moisture controls on vegetative conductance. Smith et al (1992) could show that above a soil moisture threshold of 25% by mass, the evaporative fraction defined as the ratio between evaporation (QE) and available energy (= net radiation -QG) was nearly constant. In our study area, the soil moisture is close to or above that threshold for most of the simulated days also for present conditions.…”

Observations of Neutral Profiles of Wind Speed and Specific Humidity Above a Gently Rolling Landsurface

“…Yet, even if the stations for these measurements are placed densely over the terrain, it is still not clear how a true value for regional fluxes can be derived from the local measurements under heterogeneous conditions (e.g., Brutsaert and Sugita 1992;Crago and Brutsaert 1992;Smith et al 1992). One possible way to scale up the extent to which local measurements are representative is to consider the structure of the atmospheric boundary layer (ABL).…”