Quantification of incoming all-wave radiation in discontinuous forest canopies with application to snowmelt prediction

Snowmelt dynamics in forested areas are largely driven by net radiation, which is controlled by solar elevation angle and spatial variations in canopy structure. Understanding of the spatiotemporal radiative regimes in discontinuous forests during the snow season is limited. To improve understanding of radiation dynamics in discontinuous forests, a theoretical, spatially explicit, all-wave irradiance model was developed. The model was tested using detailed measurements collected with radiometer arrays consisting of 16 pyranometers and 14 pyrgeometers installed in a small forest canopy gap. The model was used to simulate incoming radiation over the snow season at mid-latitude (47 degrees N), level forest canopy gaps with diameters ranging from one to six times the surrounding tree heights (H). Theoretical results indicate that radiative regimes in small canopy gaps may be distinctly different from continuous open areas and closed-canopy forests, especially at low solar elevation angles. At low sun angles, spatial radiative minima can occur in gaps due to low incoming shortwave radiation caused by canopy shading, coupled with minimal longwave radiation enhancement from surrounding forests. At 47 degrees N latitude on 1 February, simulated incoming all-wave radiation in canopy gaps was less than both open and forested areas over 100 to 33% of north-south transects ranging from 25 to 150 m in 1H to 6H gaps, respectively. By 1 May, the zone of radiative minima in 1H to 6H gaps ranged from 44 to 8%, respectively. The outcome of this work is an enhanced understanding of radiation variability in discontinuous forest canopies during the snow season. Copyright. (C) 2011 John Wiley & Sons, Ltd.