Simulated single-layer forest canopies delay Northern Hemisphere snowmelt

Markus Todt,Nick Rutter,Christopher G Fletcher,Leanne M Wake

doi:10.5194/tc-13-3077-2019

Abstract

Abstract. Single-layer vegetation schemes in modern land surface models have been found to overestimate diurnal cycles in longwave radiation beneath forest canopies. This study introduces an empirical correction, based on forest-stand-scale simulations, which reduces diurnal cycles of sub-canopy longwave radiation. The correction is subsequently implemented in land-only simulations of the Community Land Model version 4.5 (CLM4.5) in order to assess the impact on snow cover. Nighttime underestimations of sub-canopy longwave radiation outweigh daytime overestimations, which leads to underestimated averages over the snow cover season. As a result, snow temperatures are underestimated and snowmelt is delayed in CLM4.5 across evergreen boreal forests. Comparison with global observations confirms this delay and its reduction by correction of sub-canopy longwave radiation. Increasing insolation and day length change the impact of overestimated diurnal cycles on daily average sub-canopy longwave radiation throughout the snowmelt season. Consequently, delay of snowmelt in land-only simulations is more substantial where snowmelt occurs early.

Highlights

Forest canopy cover modulates longwave radiation received by the ground, which differs from atmospheric longwave radiation
Todt et al (2018) found roughly similar magnitudes for daytime overestimations and nighttime underestimations of subcanopy longwave radiation in CLM4.5; this study shows that different durations of day and night over the snow cover season result in a net positive impact of correction on daily averages of sub-canopy longwave radiation
This study assessed the impact of deficiencies in simulated longwave enhancement by forest canopies on snow cover in CLM4.5

Summary

Introduction

Forest canopy cover modulates longwave radiation received by the ground, which differs from atmospheric longwave radiation. This process is called longwave enhancement and has been shown to result in substantial positive net longwave radiation of the surface when snow cover is prevalent, especially under clear skies and during snowmelt (Webster et al, 2016). Meteorological conditions control longwave enhancement, as clear skies increase insolation and thereby vegetation temperature while radiative temperature of the sky is reduced (Sicart et al, 2004; Lundquist et al, 2013; Todt et al, 2018). While observations have shown trunks heating up due to insolation and emission of longwave radiation increasing (Rowlands et al, 2002; Pomeroy et al, 2009), diurnal variations in tree temperatures depend on exposure to insolation and vegetation density (Webster et al, 2016)

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Discussion

Conclusion