Wavelet analysis of intermittent turbulence in a very stable nocturnal boundary layer: implications for the vertical mixing of ozone

Abstract

Turbulence in the very stable nocturnal boundary layer is weak, patchy and intermittent. Near the surface isolated bursts of turbulent activity, characterised by abrupt changes in vertical velocity variance, have been shown to play an important role in determining the vertical transport of pollutants. However, there is little consensus as to the most appropriate methods for identifying or analysing the characteristics of intermittent turbulence that are of direct relevance to air quality studies. This paper presents an original technique, based on wavelet analysis, to objectively isolate intermittent turbulent ‘bursts’ within vertical velocity time series. The technique permits the quantitative description of global intermittency and can be used to assess the duration and strength of turbulence within a time series. The technique is applied to a dataset from a summer field experiment in the Lower Fraser Valley, British Columbia, 1998. A very stable nocturnal boundary layer was observed in this region of complex terrain during anticyclonic synoptic conditions. During the 11 nights studied turbulent activity was characterised (within each 30-min time series) by three to four individual bursts persisting for less than 10 min in total. The implications of these results for air quality studies are discussed within the context of the vertical mixing of ozone (stored within the residual layer) to the surface. Results show that, despite the complexity of the processes determining nocturnal surface ozone concentration, the strength and duration of turbulent bursts can play an important role in determining local surface concentrations.