Interpretation of flux observations along the Cabauw 200 m meteorological tower

The CESAR (Cabauw Experimental Site for Atmospheric Research) consortium is founded in 2001 to join the experimental atmospheric research of eight Dutch research institutes at Cabauw, the Netherlands. Within the CESAR research theme Land-Atmosphere Interaction an observational flux profile program has been initiated for the period 2003-2005. The observations include the fluxes of momentum, sensible heat, latent heat and carbon dioxide at 5, 60, 100 and 180 m height. Additionally, the profiles of wind, temperature, humidity and carbon dioxide are measured . The tower flux observations will be used in three CESAR research projects related to: 1) the estimation of regional scale fluxes, 2) the nature of the imbalances in the observed surface energy budget, and 3) night time flux estimates of CO2 with the atmospheric budget method at a relatively simple site. This study focus on some fundamental issues which have to be solved before the observations can be used in the context of these projects. They are: finding the appropriate averaging time for the calculation of the eddy-correlation flux, and establishing the influence of atmospheric advection on the observed flux divergence.

Finding the appropriate averaging time is intimately related to the behaviour of the co-spectrum at low frequencies. In the atmospheric surface layer the magnitude of the low frequency contribution to the vertical fluxes is quite well established. Appropriate averaging times can be defined and corrections for low frequency loss can be applied. For observations above the surface layer the situation is less clear. Recently Vickers and Mahrt (2003) derived a simple parameterisation for the so-called gap-scale, based on aircraft based observations. We present an analysis of the low frequency flux contribution for the observations at different heights.

If we try to relate vertical fluxes observed at elevated levels to the surface flux we have to take into account the rate of change of storage in the profile below the observation level and the influence of atmospheric advection. In our case the storage term can be derived from direct observations however, the magnitude of advection is more difficult to establish. Here we investigate the influence of advection on the observed flux divergence with help of a high resolution limited area atmospheric model.