Measurement of incoming radiation below forest canopies: A comparison of different radiometer configurations

Ground-based, sub-canopy measurements of incoming shortwave and longwave radiation are frequently used to drive and validate energy balance and snowmelt models. These sub-canopy measurements are frequently obtained using different configurations (linear or distributed; stationary or moving) of radiometer arrays that are installed to capture the spatial and temporal variability of longwave and shortwave radiation. Three different radiometer configurations (stationary distributed, stationary linear and moving linear) were deployed in a spruce forest in the eastern Swiss Alps across a 9 month period, capturing the annual range of sun angles and sky conditions. Results showed a strong seasonal variation in differences between measurements of shortwave transmissivity between the three configurations whereas differences in longwave enhancement appeared to be seasonally independent. Shortwave transmissivity showed a larger spatial variation in the sub-canopy than longwave enhancement at this field site. The two linear configurations showed the greatest similarity in shortwave transmissivity measurements and the measurements of longwave enhancement were largely similar between all three configurations. A reduction in the number of radiometers in each array reduced the similarities between each stationary configuration. The differences presented here are taken to reflect the natural threshold of spatial noise in sub-canopy measurements that can be expected between the three configurations.