Measurement and modelling of sub-canopy radiation to seasonal snow in alpine forests

Webster, Clare (2017) Measurement and modelling of sub-canopy radiation to seasonal snow in alpine forests. Doctoral thesis, Northumbria University.

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Abstract

The overlap of seasonal snow and forests contributes significantly to global snowmelt, largely driven by net radiation, the combination of shortwave and longwave radiation. In sub-canopy longwave radiation models, unavailability of forest canopy temperature measurements often results in locally measured air temperatures being used to represent the temperature of the emitting canopy. The validity of this assumption was tested using extensive field measurements in three sub-canopy mid-latitude alpine environments, and resulted in improvements to model simulations of sub-canopy incoming longwave radiation. Three different radiometer configurations (stationary distributed, stationary linear, and moving linear) were simultaneously compared in a spruce forest in the eastern Swiss Alps, capturing the annual range of sun angles and sky conditions. The two linear configurations showed the greatest similarity in shortwave transmissivity, and the measurements of longwave enhancement were largely similar between all three configurations. Simulation of incoming longwave radiation at a point commonly partitions the up-looking hemispherical view between radiation coming from the sky (the sky-view fraction, Vf) and the forest canopy (1 - Vf). In this two-part model, using air temperature as a proxy for tree temperature at three forested sites with Vf < 0.3 resulted in model underestimations, representing canopy temperatures elevated above air temperature. Importantly, these errors were largest during sunny, clear-sky conditions, particularly along sun-lit canopy discontinuities. However, in denser canopies, measured tree trunk temperatures were cooler than local air temperature. Model estimations were improved by applying a bulk offset to air temperature (+3�C), reducing error from 11 Wm-2 to 4 Wm-2. Within sun-lit discontinuous forests, measured canopy temperatures varied between 5-25�C above air temperature and errors of the two-part model were as high as 40 Wm-2. Point-scale simulations of longwave radiation were improved by explicitly accounting for tree trunk temperatures within the canopy-view fraction, creating a three-part model (sky, trunk and canopy). Modelled estimates of incoming longwave radiation beyond the point-scale were improved through extensive canopy temperature measurements using infrared thermal imagery around forest gaps. A parametrisation to estimate canopy temperature using sub-canopy shortwave radiation and air temperature was developed for modelling sub-canopy incoming longwave radiation using the two-part model along canopy discontinuities. These findings provide a framework for incorporating sub-canopy longwave radiation within larger scale snowmelt models. Particularly, it is important to represent both canopy discontinuities, as well as canopy temperatures at sub-daily time steps. Exclusion of these factors could lead to inaccurate estimations of snowmelt initiation and runoff rates.

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