With the increasing emphasis on coupled climate systems, planetary-boundary-layer (PBL) research has also gained increasing attention. Atmospheric and oceanic PBLs serve as the interface between all of the system components: atmosphere, ocean, land, and biosphere. Boundary-layer clouds also play important roles in climate, e.g., trade-wind cumulus in the hydrological cycle and subtropical marine stratus in the Earth's radiation budget. Therefore, proper PBL parameterization schemes are needed to accurately link all of the climate system components and to represent cloud formation and dissipation.
Before we review recent progress (1991--1994) in atmospheric PBL research, we first provide a brief historical review of the key developments from earlier years. As we will see, many of the recent studies are natural extensions of the earlier research. (Because of the vast number of articles, however, we cite only few references that were published before 1991.) The earliest PBL research focused on the surface layer. Even though Monin-Obukhov (M-O) surface similarity theory, which relates the vertical gradients of mean wind and temperature to the surface fluxes through universal stability functions, was developed in 1954, it was not until 1971 that stability functions were established from field measurements. Around 1970, gust-probe instrumented aircraft, convective tank experiments, and large-eddy simulations (LESs, where large-eddy fluctuations are explicitly calculated in the numerical integration and the net effect of small-scale fluctuations, which lie typically within the inertial subrange, is parameterized based on the inertial-range spectrum) extended our understanding of convective PBLs to the outer region of the layer. These laboratory and numerical studies not only gave us the first look at the mixed-layer structure, but also quantified the differences between convective and neutral PBLs. They also brought our attention to the effects of entrainment at the top of the PBL. The search for horizontally-homogeneous, easily-sampled boundary layers also led naturally to aircraft measurements of the PBL over tropical and subtropical oceans. In addition to providing statistics valuable for comparison with LES and PBL parameterization schemes, these field programs provided insights into the mesoscale organization of low clouds and their relationship to PBL structure.
PBL modeling in the 70's emphasized the second-order turbulence closure approach, in which the second-moment statistics are predicted based on their prognostic equations. On the other hand, some modelers preferred a much simpler approach for the convective PBL, mixed-layer (bulk) modeling, which assumes that turbulent mixing is so effective that all conserved mean fields are immediately well mixed within the bulk of the layer.
In the 1980's, research on the subtropical marine stratus cloud regime progressed rapidly, in recognition of its important role in climate, and also because the stratus cloud regime is horizontally homogeneous, which is easier to study than the cumulus cloud regime. The effects of turbulent transport, drizzle, solar absorption, cloud-top entrainment instability, and cloud-top longwave cooling in maintaining and dissipating the stratus cloud regime were examined mainly through field measurements. Second-order closure and mixed-layer modeling approaches were extended for stratus-topped PBL applications.
Field experiments during the 1980's continued to enlarge their focus beyond the horizontally-homogeneous cloudless convective PBL. In addition to the increased emphasis on the stratus-topped PBL, field programs were designed to elucidate the effects of terrain and inhomogeneities in surface characteristics. Many of the 1991-1994 papers reviewed in this article use data from these experiments. Meanwhile, increasing supercomputer power increased the reliability of LES as a supplementary or even alternative source of data for understanding PBL turbulence and developing PBL parameterizations. The trend of exploring more complicated PBL flow regimes continues into the 1990's.
New textbooks on the PBL subject that appeared between 1991--1994 include Garratt [1992] and Kaimal and Finnigan [1994].
Our overview begins with the progress in our fundamental understanding of the clear PBL regime (section 2) and cloudy PBL regime (section 3). Progress in development of PBL parameterization schemes for large-scale meteorological models is discussed in section 4. Recent development in remote sensing is given in section 5. Future outlook appears in section 6. Our overview focuses on two subjects: understanding PBL dynamics and developing PBL parameterizations. Boundary-layer-cloud microphysics, surface processes, turbulence inside the forest canopy, land-sea breezes, air-sea interaction, and the Arctic boundary layer are covered by other review articles in this issue. Lenschow [1994] recently completed a review on advances in measurement techniques, which did not cover remote sensing development. Thus in section 5, measurement techniques, we restrict our review to remote sensing studies only. The references selected in this review are restricted to publications in American journals, but include also a few American-authored papers published in foreign journals.