W. M. L. Meijninger

Determination of Area-Averaged Sensible Heat Fluxes with a Large Aperture Scintillometer over a Heterogeneous Surface – Flevoland Field Experiment

To test the applicability of the scintillation method over a heterogeneous area an experiment was carried out in the summer of 1998 in Flevoland (The Netherlands). In the patchy area only four crops were grown namely sugar beet, potatoes, wheat and onions. From eddy covariance measurements it was found that the heterogeneity was mainly caused by differences in thermal properties. No variations in the aerodynamics roughness length were observed. Two large aperture scintillometers were installed at a height of 11.6 and 20.4 m. A good resemblance was found between the sensible heat fluxes derived from both LAS instruments and the area-averaged fluxes obtained from the in-situ eddy covariance measurements. The slight underestimation of the lower LAS could be assessed using a blending height model and an analytical footprint model. The results also indicated that when scintillometer measurements are made below the blending height the violation to Monin–Obukhov Similarity Theory is small and that reasonable fluxes can be obtained from path-averaged structure parameters.

Determination of area averaged water vapour fluxes with large aperture and radio wave scintillometers over a heterogeneous surface - Flevoland field experiment

A large aperture scintillometer (LAS) andradio wave scintillometer (RWS)were installed over a heterogeneous areato test the applicability of the scintillation method.The heterogeneity in the area, whichconsisted of many plots, was mainly caused bydifferences in thermal properties ofthe crops; the variations in theaerodynamic roughness lengthwere small. The water vapour fluxesderived from the combined LAS-RWSsystem, also known as the two-wavelengthmethod, agreed fairly well with the aggregatedwater vapour fluxes derived from in-situeddy covariance measurements. The water vapourfluxes derived from a stand-alone LASare also presented. It was found that a single LASand an estimate of the area averagedavailable energy (using a simple parameterisationscheme) can provide also reasonablearea-averaged water vapour fluxes.

Results from One-Year Continuous Operation of a Large Aperture Scintillometer over a Heterogeneous Land Surface

A large-aperture scintillometer (LAS) was operated continuouslyduring a period of more than one year over a heterogeneous land surface in Central Europeat the transition between marine and continental climates. The LAS measurements of the refractiveindex structure parameter, CN2, were used to estimate the sensible heat flux. Thiswas possible for about 60to 80% of the time under daytime conditions during thesummer, with lower values obtained for the cold season (October to March). Using datafrom a three-week long field experiment, the LAS-based heat flux was compared with a weighedaverage of local heat flux measurements over the main land use classes (forest, agriculture,water) in the area, resulting in reasonable agreement. LAS-based heat fluxes were then used forcomparison with the heat flux values of a numerical weather prediction model. An over-predictionof the model heat flux was found in summer but the modelled values were lower than the LASderived data during the cold season.

Displaced-Beam Small Aperture Scintillometer Test. Part I: The Wintex Data-Set

The friction velocity (u*) and the sensible heat flux density (H) determined with a displaced-beam small aperture scintillometer (DBSAS) and a hot-film eddy correlation system are compared. Random errors in the DBSAS are relatively small, compared to scatter found with two eddy-correlation systems. Assuming that the hot-film system yields the ‘true’ fluxes, theDBSAS appears to overestimate u* when u* is less than ≈0.2 m s-1 and to underestimate u* at high wind speeds. This implies that the DBSAS measurements of theinner scale length of turbulence, l0, a direct measure for the dissipation rate of kinetic turbulent energy, are biased. Possible causes for these results are discussedin detail. A correction procedure is presented to account for effects of random noise and of so-called inactive turbulence or sensor vibrations. The errors in u* cause errors in the DBSAS measurements of the structure parameter of temperature CT2. The derived H appears to be less sensitive to errors in l0 and CT2, because errors in these quantities tend to cancel out.

An extra large aperture scintillometer for long range applications

An incoherent scintillometer with 0.31-m aperture was testedalong a 9.8-km path over grassland. Scintillometer derived heat fluxes were comparedwith in situ eddy covariance measurements. Albeit with considerable scatter, the fluxescompared well during daytime. During nighttime credible fluxes are also obtained. The scintillometerfunctioned satisfactory for 96% of the 7-week period.

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.

Structure parameters for temperature and humidity from simultaneous eddy-covariance and scintillometer measurements

Line-averaged values of the structure parameters of temperature and humidity, CT2 and Cq2, were estimated from simultaneous measurements with an optical and a microwave scintillometer over a path of 4.7 km length at the Meteorological Observatory Lindenberg during the LITFASS-2003 experiment. By cross-correlating the detected signals of the two scintillometers, the temperature-humidity structure parameter, CTq, and the temperature-humidity correlation, rTq, were also derived directly from the measurements. Comparison with corresponding values obtained from local measurements with an eddy-covariance system on a meteorological tower show a consistent behaviour in time (with some exceptions especially for Cq2 and rTq during nighttime). The deviations are of a magnitude between 20 % and 35 % of the typical daytime values of the structure parameters.

Spatially averaged sensible heat fluxes measured over barley

Abstract Measurements made with a large aperture scintillometer were compared to those made using an eddy correlation system over a uniform barley field. In spite of wind profile measurements indicating that conditions were not ideal for the application of Monin–Obukhov theory, good agreement was found between the instruments for measuring sensible heat fluxes as long as an effective value for the displacement height was used in the equations. This effective value was calculated from measurements made using sonic anemometers, although these data indicated a value well outside the normally accepted range. It is suggested that this is a consequence of the changes in atmospheric turbulence produced by a hill upwind of the experimental site.