The Energy Balance Experiment EBEX-2000. Part III: Behaviour and quality of the radiation measurements

Abstract

An important part of the Energy Balance Experiment (EBEX-2000) was the measurement of the net radiation and its components. Since the terrain, an irrigated cotton field, could not be considered homogeneous, radiation measurements were made at nine sites using a variety of radiation instruments, including pyranometers, pyrgeometers and net radiometers. At several of these sites multiple instruments were employed, which enabled us to compare instruments and assess accuracies. At all sites the outgoing longwave and shortwave radiation and the net radiation were measured, while the incoming radiation was supposed to be uniformly distributed over the field and was therefore measured at three sites only. Net radiation was calculated for all sites from the sum of its four components, and compared with the direct measurement of net radiometers. The main conclusions were: (a) the outgoing shortwave radiation showed differences of up to 30 W m −2 over the field; the differences were not clearly related to the irrigation events; (b) the outgoing longwave radiation showed differences of up to 50 W m −2 ; the differences increased during the periods of irrigation; (c) the net radiation showed differences of several tens of W m −2 across the field, rising to 50 W m −2 or more during the periods of irrigation; (d) the net radiation is preferably to be inferred from its four components, rather than measured directly, and (e) attention should be paid to the characteristics of pyranometers that measure the outgoing radiation, and thus are mounted upside down, while they are commonly calibrated in the upward position. The error in the net radiation at EBEX-2000 is estimated at max (25 W m −2 , 5%) per site during the day and 10 W m −2 at night.

Available energy and energy balance closure at four coniferous forest sites across Europe

“…It performed well almost within the accuracy of their reference net radiation that was ±3.5% and ±10 W m −2 (whichever was the largest). Kohsiek et al (2007) found that CNR1 and Schulze-Däke yielded similar values (within 20 W m −2 ) like the reference sum of four components.…”

“…The measurement of net radiation has been subject of several studies (e.g. Brotzge and Duchon 2000;Duchon and Wilk 1994;Field et al 1992;Halldin and Lindroth 1992;Kohsiek et al 2007;Vogt et al 1996;Vogt 2000). The evaluation of the accuracy of the net radiation measurements is complicated as there is no accepted standard for net radiation measurements (e.g.…”

“…Kohsiek et al 2007;Vogt et al 1996). More recently, it was recommended to measure the individual components separately (Brotzge and Duchon 2000;Halldin 2004;Kohsiek et al 2007).…”

“…Data pairs with global radiation ≤5 W m −2 were excluded for this purpose. Information found in the literature (Halldin and Lindroth 1992;Kohsiek et al 2007) was included to determine uncertainty bounds for net radiation.…”

“…Information found in literature was used to estimate the uncertainty ranges. The applied net radiometer types have been subject of several inter-comparison studies (Brotzge and Duchon 2000;Halldin and Lindroth 1992;Kohsiek et al 2007;Vogt et al 1996). The Schulze-Däke type was investigated by Halldin and Lindroth (1992).…”