Deep Valley Radiation and Surface Energy Budget Microclimates. Part I: Radiation

Abstract

Solar and longwave radiation data are presented for five sites in Colorado's 650 m deep semiarid Brush Creek Valley (39°32?N, 108°24?W) during September 1984. During the sunlit period of a nearly clear day, individual sites received 0.73?0.81 of the theoretical extraterrestrial solar radiation. Incoming solar radiation increased with elevation in the valley. Direct radiation made up 0.86? 0.88 of the downward shortwave flux. On average, 0.12?0.21 of the incoming shortwave radiation was reflected at the individual sites. Strong variations in reflected solar radiation and outgoing longwave radiation occurred from site to site. Because of the large direct beam component, aspect and inclination angles of the valley surfaces had a strong effect on the solar radiation received. Contrasts between a southwest- and northeast-facing sidewall were significant. Shading from surrounding topography produced inter-site differences in both instantaneous and daily total radiation. Inter-site differences in most daily totals on a clear day were larger than standard deviations of the daily totals at a valley floor site computed over a 16-day period of variable weather. The ridgetop site, on account of its unobstructed view of the sky, had a higher average positive net radiation during the day and a higher average negative net radiation during the night than the valley stations. Observations averaged over a 15-day period of variable weather illustrated the general effect of cloudy weather in reducing contrasts in radiation climate among sites. A simple theoretical correction for radiation measured on a horizontal surface provided a useful estimate of net and global radiation on the underlying sloping surface.