Ekman Lecture: The Planetary Boundary Layer: What We Know Now and Where to Go Next

Since my first involvement in meteorology, in the late-fifties, our understanding of the planetary boundary layer (PBL) has progressed enormously. At that time, the effects of stratification on surface-layer structure in the horizontally homogeneous PBL were just starting to become quantified. Above the surface layer, PBL structure was still somewhat mysterious and unquantified. Now, as a result of advances in theory, numerical and laboratory modeling, and measurement technology, both the surface layer and the rest of the PBL are well understood for unstable stratification, but many questions remain for stable stratification, and for horizontally heterogeneous PBLs. Other problem areas include the entrainment zone at the top of the convective PBL, and interactions of vegetated and urban canopies with the overlying PBL. I will present my view of the current state of knowledge of the PBL, and what I believe are likely to be fertile areas of research for both workable solutions to current practical problems that require knowledge of the PBL and for fundamental advancements in understanding the role and impact of the various processes that determine PBL structure. I will illustrate this by showing examples from current research efforts in theory, modeling, and observations, and try to project future progress in these areas.