Boundary layer structure: how our ideas have changed in the last 40 years

Studies of the boundary layer have evolved since the 1970s, when I was a graduate student working in Lilly's group at NCAR, until now. In the 1970s, the boundary layer was viewed as turbulent, and studies focused on average fluxes and spectra within the boundary layer. With the emergence of large eddy simulations, satellite images of boundary-layer clouds, and clear-air radar studies of the boundary layer, it became obvious that the motions in the boundary layer could be organized into roll vortices or mesoscale cellular convection. Still later, the emergence of better tools to study the boundary layer allowed us to explore the effects of the heterogeneous surface (soil moisture, vegetation, terrain) on boundary layer evolution and structure. In the 1990s, such studies were finally recognized as important for both weather and climate. Since the 1980s, there has been an interest in the interaction of the boundary layer with clouds, and, with large eddy simulation becoming both more widespread and sophisticated, such studies are becoming more common. Indeed, improved in situ and remote sensing instrumentation, as illustrated by observations in RICO, promise to increase our knowledge of boundary layers topped with cumulus clouds.