A radiation and energy balance study of mature forest and clear-cut sites

Abstract

Components of the radiation and energy balances were measured over a clear-cut area and a mature, mixed forest during the summer of 1981 at the Petawawa National Forestry Institute, Chalk River, Ontario. The work concentrated on the clear-cut site which supported a canopy layer composed primarily of bracken fern and logging remnants.Forty days ofradiation data were collected at the clear-cut site. After the first four weeks ofmeasurements (the 'green' season), most of the ferns quickly died, and their foliage changed appearance from a green to brownish colour (the 'brown' season). The daily mean reflection coefficient of solar radiation determined over the 'green' season was 0.20 and decreased to 0.13 for the 'brown' season. The corresponding value for the forest was 0.13, based on a limited amount of data. The clear-cut site received 11% and 21% less net radiation than the forest on a 24-hr and daylight-hours basis, respectively, as a consequence of the higher reflection coefficient and larger daytime longwave radiation emission.A reversing temperature difference measurement system (RTDMS), incorporating ten-junction thermopiles was employed at each site in order to determine Bowen ratios (/I) via differential psychrometry. Both systems performed well, especially the RTDMS over the forest which was capable of resolving very small differences of temperature, typically less than 0.2 "C over a height of 3 m. The mean hourly Bowen ratio, calculated from values from 0800 to 1600 hr, varied from 0.2 to 1.0 for the forest and from 0.4 to 0.8 for the clear-cut site in the 'green' season.A significant canopy heat storage component of the energy balance, Q,, was found at the clear-cut site. In the early morning, a portion of the available energy was used to heat the biomass materials and air within the canopy layer. The stored heat within the canopy was released later in the day, increasing the available energy total.The daily mean value of the Priestley-Taylor coefficient a (Priestley and Taylor, 1972) for the 'green' season at the clear-cut site was 1.14, and individual values tended to increase during wet surface conditions and decrease when the surface dried. The daylight mean a value during dry canopy conditions at the forest was 1.05, and much higher values occurred when the canopy was wet. The enhancement of a for the wet forest was a result of the evaporation of intercepted rain (which is not limited by stomata1 resistance) and the concomitant transfer of sensible heat to the forest.

Surface energy exchanges and interactions with thunderstorms during the Maritime Continent Thunderstorm Experiment (MCTEX)

“…This value varied from site to site with the Savanna site having the highest evaporative fraction of 0.66 (Table 1), which is comparable to other evaporative fractions for Grassland (0.58–0.83) [ Kim and Verma , 1990]. The Forest site had a slightly lower evaporative fraction of 0.59, which was comparable to mixed forest (0.68) [ McCaughey , 1985], pine forest in Sweden during August (0.6) [ Lindroth , 1985] and spruce forest (0.71) [ Tajchman , 1972]. The Grassland site was the driest and had the lowest evaporative fraction of 0.47.…”

Meso-scale climate change due to lowland deforestation in the maritime tropics

“…The results of the measuring campaign are quite in agreement with other studies of land use change if we consider the effects of albedo change. A range of albedo of 0.08 to 0.14 is reported for a wide range of forest types and for pastures values of 0.17 to 0.20 (Jarvis et al,1976, Monteney and Gosse, 1976, McCaughy, 1985, Culf et al, 1995, McCaughy, 1985, Waterloo, 1994. However, the flux results are quite different from other studies, where midday latent heat fluxes are typically 400 W m -2 over tropical rain forests (Shuttleworth et al, 1984, Wright et al, 1992, with consequently Bowen ratios smaller than 1.…”

Comparison of Different Algorithms for Stomatal Ozone Flux Determination from Micrometeorological Measurements

“…An important discrepancy between results obtained by using the two PM variants appears for both instrumental and theoretical reasons, among which the non-closure of the energy balance hypothesis (Equation 11). The non-closure of the surface energy balance is a general problem encountered in micrometeorological measurements (Culf, Foken, & Gash, 2004;Foken, Richter, & Wichura, 1993;McCaughey, 1985). Errors in determining soil heat fluxes are due to the lack of information on soil heat conductivity and its dependence on soil water content.…”