Modelling Sub-canopy Longwave Radiation in Snow-covered Deciduous and Needleleaf Boreal Forests

Abstract:

In-situ meteorological, tree temperature and forest canopy structure measurements from Arctic European snow-covered boreal forests, were made March through April in Abisko, Sweden (2011) and Sodankylä, Finland (2012). Data were collected to evaluate methods that simulate longwave radiation to sub-canopy snow; an important component of the land surface energy balance influencing the timing and magnitude of snowmelt.

Simple models of sub-canopy longwave radiation to snow worked very well. In continuous forest canopies away from forest edges, although tree trunks receiving direct solar radiation got hot, trunks made a small contribution to total sub-canopy longwave emission because trunk view fraction was small. Other parts of the forest canopy (especially the down-facing elements) were often well ventilated and in equilibrium with sub-canopy air temperature. Although canopies, especially sky-facing elements of canopies, can absorb shortwave radiation, the strength of the modelled fit suggests these elements are not strongly contributing to subcanopy snow. Consequently, the vertical location of air temperature measurements (i.e. above or below canopy) used to drive models and the representation of canopy structure becomes critical.

Sub-canopy air temperature provided the best meteorological forcing for accurate longwave modelling. At Sodankylä there was a decoupling of air temperatures above and below forest canopy when less than -5°C, which was enhanced during night-time (above canopy air temperatures staying warmer than below canopy). Where air temperature does not adequately describe the radiometric temperature of an emitting canopy towards the snow surface, improved knowledge of vertical temperature profiles within canopies that have more complex vertical structures may help improve models of sub-canopy longwave radiation.