1 Introduction

It is safe to state that the significance of turbulent flows in engineering and numerous branches of science is not in dispute. The equations describing turbulent flows, the Navier–Stokes equations, have been around since 1845 and yet the mathematical description of turbulence remains complex and often forbidding in practical situations. From the turn of the nineteenth century onwards, reliance on experiments became a necessity to arrive at usable expressions for conveyance in turbulent flows. By the middle of the twentieth century onwards, improved experimentation and data analysis tools (e.g. conditional sampling) began to reveal that aspects of turbulent fluctuations are not entirely irregular, and regularity has been detected in large-scale structures (Kovasznay, Kibens & Blackwelder Reference Kovasznay, Kibens and Blackwelder1970). Experiments have also shown that these large-scale structures dominate the energetics of the flow and contribute to momentum, heat and scalar exchanges (Monty et al. Reference Monty, Stewart, Williams and Chong2007; Marusic, Mathis & Hutchins Reference Marusic, Mathis and Hutchins2010a ). They also have provided tantalizing results about self-similarity in large-scale structures in certain situations (Marusic & Monty Reference Marusic and Monty2019). Undoubtedly, connecting such large-scale eddies to quantities required in practical turbulence problems such as momentum, heat and scalar exchanges between the flow and the underlying boundary refocused interest on the properties of large-scale motion (Hutchins & Marusic Reference Hutchins and Marusic2007; Marusic et al. Reference Marusic, McKeon, Monkewitz, Nagib, Smits and Sreenivasan2010b ). So, what is meant by large- and very large-scale motion (LSM and VLSM) here? How large is large? Do these LSM and VLSM impact turbulence near boundaries? And how can the properties of VLSM or LSM be used to ‘upgrade’ phenomenological theories being used in practice such as Monin–Obukhov surface layer similarity theory (MOST)? MOST was originally introduced to correct for the effect of thermal stratification on flow statistics in a stationary, planar-homogeneous atmospheric flow in the absence of mean vertical advection or Coriolis effects (Monin & Obukhov Reference Monin and Obukhov1954). The desire to answer these questions has led to an ‘up-tick’ in experiments and simulation studies of boundary layer turbulence in highly controlled settings. Unsurprisingly, much of the qualified answers to these questions originate from studies of turbulent boundary layers over smooth walls at very high Reynolds numbers. Experiments have shown that, far from the boundary (a region referred to as the outer layer), the LSM has characteristic dimensions of boundary layer depth $\unicode[STIX]{x1D6FF}$ . Other measurements above the viscous sublayer but not too high up in the boundary layer (a region known as the inner layer) suggest that both LSM and VLSM are present. The size of VLSM appears to be commensurate with $10\unicode[STIX]{x1D6FF}$ or larger. Beyond the niceties of smooth walls, does this emerging picture about LSM and VLSM hold for other situations, such as the atmospheric boundary layer where heating at the surface occurs? This is the essence of the problem considered in the paper by Salesky & Anderson (Reference Salesky and Anderson2018) (hereafter SA18).

Figure 1. Updrafts (positive vertical velocity) in a mildly heated and strongly heated atmospheric boundary layer.