Surface energy balance
Surface temperature is interpreted as a surface skin temperature unless the Canopy Heat Capcity model is selected, in which case it is a canopy layer temperature for vegetated tiles. The surface energy balance for each tile includes fluxes of sensible heat and moisture, and latent heat of vaporization for snow-free tiles or sublimation for snow-covered or ice tiles. The heat flux into the ground, combining radiative fluxes below vegetation canopies and conductive fluxes for the unvegetated fraction, is parametrized as a function of the thickness and temperature of the surface soil layer. Radiative canopy fraction is calculated separately if the Canopy Heat Capacity model is selected but is set to zero otherwise. The thermal conductivity is equal to the soil conductivity for snow-free tiles, but is adjusted for insulation by snow of depth.
Resistances and conductances
An aerodynamic resistance for sensible and latent heat fluxes between the surface and the atmosphere over each tile is calculated as a function of the temperature, specific humidity, and windspeed. An additional the surface resistance factor is defined for evaporation to represent the resistance from the saturated humidity value at the surface temperature within the surface (within the leaf or soil matrix), to the surface humidity. For transpiration, this surface resistance is the reciprocal of the canopy conductance, which depends on the photosynthesis.
Canopy Heat Capacity
A canopy heat capacity and radiative coupling between the canopy and underlying ground can be used. It depends on the masses of carbon in leaves and stems per unit area of canopy. The areal canopy heat capacity is calculated assuming different specific heat capacities of leaves and wood, based on values given by Jones (1983) and Moore and Fisch (1986).
Surface evaporation is drawn from soil, canopy and snow moisture stores. Evaporation from saturated parts of the surface (lakes, wet vegetation canopies and snow) is calculated at the potential rate (i.e. subject to an aerodynamic resistance only). Evaporation from transpiring vegetation is controlled by the canopy conductance. The ability of vegetation to access moisture at each level in the soil is determined by root density, assumed to follow an exponential distribution with depth.
The evaporative flux extracted from each soil layer is dependent on the soil moisture availability factor. Bare-soil evaporation is extracted from the surface soil layer for both bare-soil tiles and fraction of vegetated tiles. A fraction of the tile is assumed to be saturated and hence has aerodynamic resistance only. This factor is 1 for lake, ice or snow-covered tiles, and varies for a vegetated tile with canopy moisture content and canopy capacity. The urban tile is also given a small surface capacity.
Jones, H. G., 1983: Plants and microclimate. A quantitative approach to environmental plant physiology. Cambridge University Press.
Moore, C. J., and G. Fisch, 1986: Estimating heat storage in Amazonian tropical forest. Agric. and Forest Meteorol., 38, 147-168.