Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2008
Publication Date: 5/1/2008
Citation: Pomeroy, J., Rowlands, A., Hardy, J., Link, T., Marks, D.G., Essery, R., Sicart, J., Ellis, C. 2008. Spatial Variability of Shortwave Irradiance for Snowmelt in Forests. Journal of Hydrometeorology, 9:1482-1490.
Interpretive Summary: Solar radiation is important to snowmelt, but is difficult to estimate or measure below forest canopies. An experiment using 12 radiometers was conducted to quantify the spatial and temporal variability of solar radiation below different forest canopies, a discontinuous, natural canopy, a homogeneous plantation stand and a dense boreal spruce forest. Variability was much greater during clear than during overcast conditions, and greater in the natural canopies than in the plantation forest over periods less than one hour. For longer periods, this was reversed. For daily averaging intervals the spatial variability of homogeneous stands was lower than that for discontinuous stands. The spatial variability of irradiance for daily intervals was compared to the mean radiation transmitted through to the snow surface for various canopy types. The plantation canopy caused lower spatial variability than spruce and spatial variability increased with as the canopies became more open. This work will improve how snow models account for forest effects on snowmelt.
Technical Abstract: Shortwave irradiance is a major component of snowmelt energy under forest canopies. Its spatial variability strongly influences the spatial variability of melt energy. The spatial variability of melt energy can be important to snow covered area depletion calculations. To explain the factors governing spatial variations in snow melt, the variability of shortwave irradiance under homogeneous and discontinuous evergreen canopies during the snowmelt period in the US Rocky Mountains was measured, analysed and then compared to observations from mountain and boreal forests in Canada. For short sample averaging intervals, the spatial variability of sub-canopy shortwave irradiance on overcast days was much lower than that on clear days. The spatial variability of irradiance for both overcast and clear conditions declined dramatically as the sample averaging interval increased from one minute to one day. At hourly averaging intervals variability was still much higher for clear than for overcast conditions, however at daily averaging intervals there was little influence of cloudiness on the variability of sub-canopy irradiance. Variability was higher under the homogeneous stand than under the discontinuous stand for short time intervals, however for time intervals longer than one hour this reversed. For daily averaging intervals the spatial variability of homogeneous stands was lower than that for discontinuous stands. The spatial variability of irradiance for daily intervals was compared to the mean diffuse radiation transmissivity for various canopy types. Pine caused lower spatial variability than spruce and spatial variability increased with transmissivity.