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Titre : |
Canopy albedo and transmittance in a spruce hemlock forest in mid-september
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Titre de série : | Agricultural and Forest Meteorology, Vol 59 - n°3-4 |
Auteur(s) : |
T.F. Eck, Auteur
D.W. Deering, Auteur |
Type de document : | Tiré à part |
Sujets : | Insolation ; Bilan énergétique |
Résumé : |
The short-wave radiation balance of a forest stand of predominantly spruce and hemlock in central Maine during 2 days in mid-September was investigated. Pyranometer measurements of insolation, reflected solar radiation from the top of the canopy, radiation incident on the forest floor, and radiation reflected from the forest floor were taken under both cloudless sky conditions and overcast cloud conditions. Measurements were made on two consecutive days to insure minimal change in forest canopy characteristics. The canopy albedo computed from half-day radiation totals, and thus representative of a daily value, was 10.8% on the clear day and 9.8% on the overcast day. The instantaneous albedo on the clear day ranged from 8.7% at 43° solar zenith angle (solar noon) to 20.1% at 78° solar [...] The short-wave radiation balance of a forest stand of predominantly spruce and hemlock in central Maine during 2 days in mid-September was investigated. Pyranometer measurements of insolation, reflected solar radiation from the top of the canopy, radiation incident on the forest floor, and radiation reflected from the forest floor were taken under both cloudless sky conditions and overcast cloud conditions. Measurements were made on two consecutive days to insure minimal change in forest canopy characteristics. The canopy albedo computed from half-day radiation totals, and thus representative of a daily value, was 10.8% on the clear day and 9.8% on the overcast day. The instantaneous albedo on the clear day ranged from 8.7% at 43° solar zenith angle (solar noon) to 20.1% at 78° solar zenith angle, and on the overcast day the albedo was relatively constant as a result of the diffuse irradiance condition. Canopy transmittance for the daily period was 2.9% on the clear day and 4.2% on the overcast day. The canopy transmittance was nearly constant throughout the day on the overcast day at about 4.2%, whereas sunfleck penetration on the clear day resulted in a higher transmittance of 4.5% from 43 to 47° solar zenith angles, which dropped to 1.8% at 68–72° solar zenith angles. Thus over the entire day, for the range of solar zenith angles measured, diffuse radiation on the overcast day penetrated further down into the forest canopy than the predominantly direct radiation on the clear day. It should be noted, however, that during the summer, when the solar zenith angle is lower at solar noon, the reverse may be true as a result of direct solar radiation penetration into the canopy gaps. The forest floor albedo was similar on both days at about 19%, resulting in about 2.4% of the total insolation being absorbed by the ground below the forest canopy on the clear day, and 3.4% on the overcast day. |
Date de publication : | 1992 |
Format : | pp. 237-348 / graph., tabl., réf. |
Langue(s) : | Anglais |
Lien vers la notice : | https://infodoc.agroparistech.fr/index.php?lvl=notice_display&id=148401 |
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