THE CRAU FIELD EXPERIMENT - TURBULENT EXCHANGE IN THE SURFACE-LAYER UNDER CONDITIONS OF STRONG LOCAL ADVECTION

Turbulent fluctuations of atmospheric properties near the soil-vegetation-atmosphere interface have been investigated in a situation of strong local advection. To this end, an analysis has been made of data from the 1987 field experiment in 'la Crau' (France), where an extensive dry terrain with sparse vegetation was situated upwind from an irrigated field completely covered with grass. The change in surface roughness was negligible, the main effect being a change in surface heat flux. In the experiment, turbulence measurements were made of horizontal and vertical wind speed, temperature and humidity at various locations over both terrains. In this study, 12 runs of 1 h are analysed, roughly half of which were made when an advective inversion was present over the downstream terrain. Both vertical and horizontal velocity fluctuations are almost unaffected by the change in terrain. Temperature fluctuations are influenced by the fact that over the downstream terrain production of variance almost ceases and advection of variance becomes important. This results in a deviation of the sigma(theta)/theta* = f(z/L) relation found for homogeneous situations. Also, the temperature spectra of the unstable runs are affected, showing a different inertial subrange roll-off rate. Humidity fluctuations adjust rapidly to the new surface and, for all but the lowest frequencies, behave as if measured over homogeneous terrain.