Claudia Liebethal

Evaporation over a heterogeneous land surface - The EVA-GRIPS project

The representation of subgrid-scale surface heterogeneities in numerical weather and climate models has been a challenging problem for more than a decade. The Evaporation at Grid and Pixel Scale (EVA-GRIPS) project adds to the numerous studies on vegetation-atmosphere interaction processes through a comprehensive field campaign and through simulation studies with land surface schemes and mesoscale models. The mixture of surface types in the test area in eastern Germany is typical for larger parts of northern Central Europe. The spatial scale considered corresponds to the grid scale of a regional atmospheric weather prediction or climate model and to the pixel scale of satellite images. Area-averaged fluxes derived from point measurements, scintillometer measurements, and a helicopter-borne turbulence probe were widely consistent with respect to the sensible heat flux. The latent heat flux from the scintillometer measurements is systematically higher than the eddy covariance data. Fluxes derived from numerical simulations proved the so-called mosaic approach to be an appropriate parameterization for subgrid heterogeneity.

On the Significance of the Webb Correction to Fluxes

For establishing correct mass or energy balances at the Earths surface, detailed and correct measurements of air constituent fluxes are needed. Flux measurements obtained from the eddy covariance technique have to pass several corrections of different relevance in order to give correct flux data. One of these corrections, the ‘Webb correction’, is analysed herein from latent heat flux and CO2 flux data recorded during two field experiments. The significance of this correction for the latent heat flux data varies with the air humidity and the Bowen ratio. The correction changes the latent heat flux values only a little, but significantly (by 2 to 3%). For other air constituents (like CO2), the Webb correction is much more important (20 to 30% of the flux).

On the use of two repeatedly heated sensors in the determination of physical soil parameters

Variables describing the heat transport in soils are usually determined indirectly from soil temperature and/or moisture measurements. Alternatively, they can be measured using repeatedly heated sensors like the self-calibrating heat flux plate HFP01SC and the thermal properties sensor TP01 from Hukseflux (Delft, NL). This study aims to validate the data recorded with these instruments for three variables: The PHILIP correction (factor fp), the soil heat conductivity (λ s ), and the volumetric soil heat capacity (c v ). All of these were measured in a short experiment with the HFP01SC and/or the TP01 sensor and were simultaneously calculated from reference methods using soil temperature and moisture measurements. For the data set on which this study is based, the HFP01SCs self-correction agrees with the PHILIP correction, but the sensor cannot be recommended for measuring λ s . The TP01 seems to underestimate λ s and should not be used to quantify c v .