Thermal Roughness of the Fen Surface

The surface roughness lengths for momentum, z(0u), and heat, z(0T), are key parameters for modeling the momentum, mass and energy exchange between underlying surface and the atmosphere. Established approach predicts the dependency of the ratio C = ln (z(0u)/z(0T)) on the roughness Reynolds number Re-s, confirmed earlier for a number of surface types. This paper for the first time evaluates such a relationship based on the eddy covariance data collected on a boreal fen dominated by moss and sedge vegetation. It is shown that the best parameterization is provided by two dependencies: C = 0.359 Re-s(1/4) + 0.711 and C = 0.032 Re-s(1/2) + 1.706. The significant sensitivity of these dependencies to longwave emissivity was revealed. We generalized this issue to linear analysis of parameter uncertainties in the MOST formulas. This analysis suggested, that the surface temperature errors contribute more to C evaluation compared to heat flux uncertainties, the former being affected also by mismatch between footprints of radiation and eddy covariance measurements. The practical solutions to minimize the issue are proposed. Comparison of our measurement data and obtained dependencies for C with known functions C( Res) for other types of surface from literature demonstrated that grassland and cropland surface types are the closest to fen in terms of z(0u)/z(0T).