Near-ground free convection conditions and energy balance closure over complex terrain – results from the surface turbulence network during COPS

Experimental data of the surface energy balance and turbulence network installed during the comprehensive COPS (Convective and Orographically-induced Precipitation Study) field campaign, which took place in south-western Germany and eastern France from June 1^st to August 31^st 2007, are presented. The network consisted of sixteen stations set up over different land use types in the valleys and on the mountain tops of the middle and northern Black Forest, in the Upper Rhine valley and in the lee of the Black Forest. The stations were equipped with eddy-covariance (EC) systems for measuring surface turbulent fluxes of momentum, sensible and latent heat. Additionally, soil and radiation measurements as well as standard surface meteorological data were recorded at most of the sites. A huge data set of boundary layer profiling measurements is also available.

After the application of a comprehensive and uniform turbulence data processing and quality control effort, some typical features of the calculated surface fluxes could be identified. It was found that the flux values are primarily determined by the land use type and that the location of the site within the COPS region plays a minor role.

A focus of the present study is on the investigation of the occurrences of near-ground free convection conditions (FCCs). FCCs occur during situations of high buoyancy fluxes which coincide with a strong decrease of the wind speed during a change of the local circulation system. These situations can be detected by the EC system by calculating a stability parameter. An influence of the land use type on the occurrence of FCCs is obvious. During FCCs, buoyancy-driven turbulence dominates over shear-driven turbulence near the ground, which results in an effective vertical transport mechanism of heat and moisture into upper parts of the atmospheric boundary layer. The application of spectral analysis methods confirmed the large-scale character of the turbulence near the ground during periods of FCCs. Moreover, increased upward vertical wind speeds in Sodar measurements could be related to FCCs.

A further issue of the present study is the investigation of the non-closure of the surface energy balance. The average residuum of the surface energy balance was in the order of 20% for most of the sites in the COPS region. The oasis effect, which could be observed on the highly evapotranspirating maize fields, was found to increase the residuum, presumably due to enhanced advective flux components not caught by the EC measuring system. In the present literature of the surface energy balance closure discussion, the landscape heterogeneity, which causes advective and low-frequency flux components and flux contributions of secondary circulations not seen by the standard EC measurements, is considered to be the main reason for the residuum. We anticipate further insights into this problem with the present study. At one EC station, an additional installed large aperture scintillometer (LAS) gives the opportunity to compare the EC point measurements with area-averaged flux values. Sensible heat fluxes measured with the LAS are often up to 50 Wm^-2 higher compared to the EC fluxes on many days.