Impact of longwave enhancement by forests on snow cover in a global climate model

Todt, Markus (2019) Impact of longwave enhancement by forests on snow cover in a global climate model. Doctoral thesis, Northumbria University.

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Abstract

Boreal forests cover about a fifth of seasonally snow-covered land over the Northern Hemisphere. Skill in modelling snow cover has been shown to be lower for forested than for open areas, which was attributed to more complex processes in forests. One of these processes is the enhancement of longwave radiation beneath forest canopies, which has been found to impact the surface energy balance and rates of snowmelt. Single-layer vegetation schemes, as used in the Community Land Model version 4.5 (CLM4.5), have been found to overestimate diurnal cycles in vegetation temperature and sub-canopy longwave radiation. However, simulation of longwave enhancement and its impact on snow cover has not yet been assessed in a global climate model. Forest stand-scale forcing was used for the simulation of sub-canopy longwave radiation by CLM4.5 and to drive SNOWPACK, a snow model featuring a two-layer canopy module, as a benchmark model for CLM4.5. Simulated sub-canopy longwave radiation and longwave enhancement were subsequently assessed using measurements from forest stands located within seasonally snow-covered regions, which vary in vegetation density and cover the range of boreal plant functional types in CLM4.5. CLM4.5 was found to overestimate the diurnal range of sub-canopy longwave radiation and longwave enhancement, and simulation errors increased with decreasing cloudiness and increasing vegetation density. Implementation of a parameterization of heat storage by biomass reduced simulation errors, but only marginally affected the amplitude of diurnal ranges. In contrast, SNOWPACK simulated a small diurnal range of sub-canopy longwave radiation across the range of vegetation density. A simple correction was derived from stand-scale simulations and implemented in global simulations of CLM4.5 in order to scale diurnal cycles of sub-canopy longwave radiation and assess the consequent impact on snow cover. Overestimated diurnal cycles of sub-canopy longwave radiation were found to result in underestimated averages of longwave enhancement over the snow cover season, as nighttime underestimation outweighed daytime overestimation. This underestimated energy input to snow resulted in an underestimation of snow temperatures and a general delay of meltout across snow-covered forests. However, the impact of overestimated diurnal cycles on daily average longwave enhancement was found to change throughout the snowmelt season, due to increasing insolation and day length, which results in spatial differences in its impact on meltout. These findings indicate that multiple vegetation layers are indispensable for accurate simulation of longwave enhancement and its impact on snow cover, thereby contributing to the growing evidence of limitations in modelling vegetation as a single layer.

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