Spatial Structure of Non-Stationary Stable Boundary Layers

The stable boundary layer is generally non-stationary. We analyze measurements of the nocturnal boundary layer from three contrasting sites after the early evening rapid cooling period. The variation of temperature due to motions on time scales of tens of minutes to an hour is often substantially larger than the diurnal trend. Non-stationarity of temperature and turbulence in the stable boundary layer has often been viewed as intermittent shear generation of turbulence and downward transport of warmer air, leading to warm turbulent patches. However, the transitions from warm to cold air (cold microfronts) are found to be of greater amplitude compared to transitions from cold air to warm air. The turbulence following the cold microfront may increase due to increased shear or may decrease due to increased stratification. The different types of structures associated with cold microfronts are explored on different scales and contrasted with the warm microfronts. Depending on the sampling threshold, the microfronts, and their parent circulations on time scales of 10 min or more, can occupy a significant fraction of the total record.

The spatial coherence of microfronts and less sharp variations of temperature and turbulence are both examined in the time-space domain using a network of sonic anemometers. Our analyses must recognize that with low wind speeds in the nocturnal boundary layer, the vertical divergence of the fluxes can be large near the surface. The vertical divergence of the fluxes can be augmented by even weak surface heterogeneity. This height dependence invalidates similarity theory and leads to errors in the estimation of the surface fluxes from measurements at traditional flux levels.