Impact of terrain heterogeneity on near-surface turbulence: long-term investigation at SIRTA observatory

On flat terrain, Monin-Obukhov similarity theory predicting vertical profiles of wind and temperature in any stratification compares well to atmospheric measurements and are key relationships for surface-atmosphere coupling in numerical models. In the framework of this theory, the control parameters are mainly the roughness length and height. However, in complex terrain with significant surface heterogeneity, Monin-Obukhov theory fails in predicting accurately the vertical profiles of wind and temperature. Despite the numerous numerical and laboratory experiments on the effect of surface heterogeneity on near-surface turbulence, the experimental investigation in the “real” atmosphere has comparatively been the subject of less attention.

Since April 2005, turbulence measurements have been collected at the SIRTA observatory located at Palaiseau, 20 km south of Paris, France, using two sonic anemometers at 10 and 30 m measuring the three components of the wind and the temperature at 10 Hz on a 30 m tower. The tower is set up on a grassy terrain. At about 100 m to the north and 400 m to the east, there is a forest with 15 m height trees. About 400 m to the south, the laboratory buildings are about 15 m high. There is no obstacle west of the tower.

The study investigates the impact of surface heterogeneity on local turbulence measurement and on near-surface coherent structure dynamics. The study shows that despite the strong impact of topographical elements upstream of the tower on turbulence kinetic energy, fluxes and roughness length, the analysis shows no significant effect on coherent structure properties in terms of (i) occurrence, (ii) duration and (iii) associated heat and moment fluxes.