Propagation and Effects of a Mesoscale Gravity Wave Over a Weakly-Stratified Nocturnal Boundary Layer During the SABLES2006 Field Campaign

During the SABLES2006 (Stable Atmospheric Boundary Layer Experiment in Spain 2006) field campaign, a gravity-wave episode was observed on the night of July 11 by the microbarometers deployed at the surface and on the 100-m tower. The high-amplitude, low-frequency periodic pressure fluctuations were very well correlated with the wind speed and direction. Data from neighbouring automatic stations showed that the gravity wave was not local, but long-lived and mesoscale. The propagation of the wave over the experimental site had significant effects on the structure of the weakly-stratified nocturnal boundary layer that developed that night: the stability increased, turbulent vertical motions were suppressed, the nocturnal low-level jet was disrupted, and periodic temperature fluctuations of amplitude up to 3–4 K were observed. In this work we analyse the different available data sources (tower data, RASS-SODAR, microbarometric, satellite imagery, automatic stations) to describe the phenomena in depth and to find a suitable explanation for the generation and propagation of the wave. The linear wave theory explains remarkably well most of the observations, and the wave parameters could be estimated by applying a wavelet-based technique to surface microbarometric measurements. We also analyse the vertical structure of the wave and find wave ducting conditions above the surface. Finally, by means of the multi-resolution flux decomposition, we analyse in detail the changes in vertical turbulent fluxes and the spectra of turbulent motions produced by the interaction between the gravity wave and the local flow.