Does Sub-canopy Longwave Radiation Enhancement Affect Boreal Snowmelt Dynamics?

Climate models currently underestimate the effects of warming air temperatures on springtime snow retreat across the northern hemisphere. This effect is particularly pronounced in boreal forests, where rates of warming are among the most rapid on earth. Inaccuracies in the timing of springtime snow retreat effectively alter land surface albedo and subsequently atmospheric temperatures, contributing to uncertainty in projections of future climate. Enhanced emission of longwave radiation beneath forest canopies is a potential mechanism for inaccurate model snowmelt dynamics. Specifically, the absorption of shortwave radiation and downward subsequent re-emission of longwave radiation by forest canopies is not accounted for in most climate models. Several observational studies have demonstrated enhanced downwelling longwave radiation fluxes beneath forest canopies, yet there has been little research as to whether this phenomenon affects snowmelt dynamics. In this study we examine the rate of decline in snow water equivalent (SWE) per unit temperature increase (i.e. mm °C-1) across the northern high-latitude regions using monthly GlobSnow and CRU data in conjunction with the GLC2000 vegetation map and several geospatial indicators of vegetation cover. We find greater reductions in SWE per unit temperature increase in boreal forests than in tundra. Within boreal forests, SWE reduction per unit temperature increase exhibits a significant positive correlation with forest canopy cover, and this relationship varies between dominant forest types. Greater declines in SWE per unit temperature increase in areas with higher forest canopy cover suggests that enhanced downwelling longwave radiation fluxes are stronger than canopy-shading effects on understory microclimate for determining rates of spring snowmelt across the boreal biome. Accurate process representation of sub-canopy longwave radiation fluxes may help to improve climate model representations of springtime snow cover dynamics.