On surface energy balance over the northern wetlands: 1. The effects of small‐scale temperature and wetness heterogeneity

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Abstract
A model is presented to calculate the surface energy balance over land surfaces covered by alternating small‐scale dry‐warm and wet‐cool patches, as might be found on northern wetlands during summer days. The model considered the effect of local heat advection, which is responsible for reallocation of a portion of surface available energy between the patches, in addition to the spatial change in primary partition of available energy between sensible and latent heat fluxes due to different physical characteristics of the patches. The modeled results showed that neglecting the effect of local heat advection could cause significant errors in calculations of both local and cumulatively regional surface heat and moisture fluxes. The predicted errors ranged up to 200% for evaporation and 67% for Bowen ratio when dry and wet patches are small and unequal in size, or up to about 35% for evaporation and 47% for Bowen ratio when dry and wet patch sizes are equal. Patch sizes with estimated nonnegligible error, i.e., with dimensions less than 100 to 200 m, are not uncommon in northern wetlands. This suggests that the currently used surface schemes (for mesoscale and global circulation models) should incorporate the effect of local heat advection for complete consideration of the sub‐grid‐scale surface heterogeneity, in order to improve their applicability to landscapes like northern wetlands. As a consequence the improvement of image resolution in aircraft and satellite remote sensing technology, for identification of sub‐grid small‐scale surface heterogeneity, seems necessary for accurate determination of surface heat and moisture flux distributions.