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

Turbulent fluctuations of atmospheric properties near the soil-vegetat ion-atmosphere interface have been investigated in a situation of stro ng local advection. To this end, an analysis has been made of data fro m the 1987 field experiment in 'la Crau' (France), where an extensive dry terrain with sparse vegetation was situated upwind from an irrigat ed field completely covered with grass. The change in surface roughnes s was negligible, the main effect being a change in surface heat flux. In the experiment, turbulence measurements were made of horizontal an d vertical wind speed, temperature and humidity at various locations o ver 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 fluctuat ions are almost unaffected by the change in terrain. Temperature fluct uations are influenced by the fact that over the downstream terrain pr oduction of variance almost ceases and advection of variance becomes i mportant. This results in a deviation of the sigma(theta)/theta = f(z /L) relation found for homogeneous situations. Also, the temperature s pectra of the unstable runs are affected, showing a different inertial subrange roll-off rate. Humidity fluctuations adjust rapidly to the n ew surface and, for all but the lowest frequencies, behave as if measu red over homogeneous terrain.