Study of the non-stationary behaviour of the Stable Boundary layer in SABLES 98: spectral and wavelet methods

The Stable Boundary Layer (SBL) is characterized by the unstationarity of the events that occur in it. The absence of a sustained forcing generating turbulence (except for windy cases leading to weakly stable startification), the later can not be the major event controlling the SBL behaviour and allows other processes to become crucial. Internal gravity waves or inhomogeneity generated circulations can happen in an intermittent manner, and are difficult to study using the classical statistical method, apt for sustained turbulence cases, such as the convective or the shear-driven boundary layers.

Some nights of the SABLES 98 campaign -focussed on the mid-latitude SBL, Spain, September 1998-, have been explored in order to locate and characterize events of the kind described. Special attention is devoted to the characterization of internal waves. First, the classical methods based on Fourier analysis and linear wave theory are applied and discussed. Afterwards, wavelet analysis is made and, taking advantage of its capabilities in the study of non-stationary processes, a preliminary characterization of some events is achieved.