Energy budget from large aperture scintillometry during AGRISAR2006

In the lowest part of the atmospheric boundary layer known as the surface layer, interaction between the atmosphere and the earth's surface takes place in the form of turbulent exchange of energy and mass. The surface-atmosphere exchange can be described as follows: R_n = H + L_vE +G_s

In the surface energy balance equation Rn represents the amount of energy (or net radiation) available for various different land surface processes. One of these processes is the warming of the air by the underlying surface, known as the sensible heat flux density H (or shortly sensible heat flux). Part of the net radiation will be used to evaporate water present at the surface and sub-surface and is symbolized by LvE, the water vapor flux or latent heat flux density (shortly latent heat flux, or evapotranspiration). Another portion of Rn is transferred into the soil defined by the soil heat flux density Gs.The evaporative term shows the linkage of the energy balance to the hydrological water balance. Both atmospheric and surface properties (such as surface type and status) determine the size of each flux: in dry areas H and Gs consume most of the available energy, while in wet areas LvE is dominant. It is well known now that the atmospheresurface interactions, which take place at various scales, play an important role in atmospheric processes such as the growth of the ABL, terrestrial and atmospheric moisture content, cloud formation, and consequently precipitation patterns. But also in other scientific fields such as agriculture, bioclimatology and hydrology these interactions are of importance.

In order to advance our understanding of land-atmosphere exchanges of water and heat in space and time over heterogeneous land surfaces, in-situ measurements of these exchanges as well as of the thermal dynamic states of the atmosphere, soil and vegetation were carried out over two representative land cover units in the area in the period 2 – 10 July 2006. The areas covered comprised of a winter wheat field and a corn field. In the winter wheat field a complete energy balance station, comprised of a 5 meter high meteorological tower equipped with a set of meteorological instruments to measure 4-component net radiation (i.e. incoming and outgoing solar radiation, incoming and outgoing thermal infrared radiation), as well as air temperature, relative humidity and wind speed at two different levels and wind direction was setup, combined with soil heat fluxes measured at several positions and depths as well as soil temperature. In addition, sensible heat flux was measured by installing a receiver of a Large Aperture Scintillometer (LAS) on the meteorological tower and a transmitter situated at 420 meters distance in south-eastern direction in the same wheat field. The sensible heat fluxes over the corn field were monitored by a separate LAS set. Finally, evapotranspiration rates were determined as a residual from the energy balance equation.

This contribution describes the derivation and development of the different components in (1) over time and space for the Goermin test site, as observed during the third intensive measurement period of the AgriSAR 2006 Campaign. Results show that the available energy in the observation period is mainly used to evapotranspirate water even during very hot and dry days.