In our review of the United States PBL research from 1991 to 1994, we see increasing interest in studying complex PBL regimes that interact with clouds and precipitation, or that occur over nonhomogeneous surfaces. This is driven by an interest in solving the complex problems of global change or weather forcasting, and is enabled by improved computers and instrumentation. Our understanding of these complicated regimes is still primitive, however, and development of parameterization schemes for these regimes is slow. The main obstacle is that obtaining statistics for these PBL regimes is difficult, and the likelihood of finding a simple scaling law to generalize the statistics is small, because external forcings (e.g., precipitation, cumulonimbus downdrafts) are too varied. Furthermore, these PBLs are likely to undergo rapid temporal or spatial changes, and these natural variabilities make it difficult to perform time or spatial averaging (e.g., Smith et al., 1992; Ledvina et al. 1993; Mahrt et al. 1994). Given the desire to investigate more complex flows, it is not surprising that LES and other models, as controlled numerical experiments, are increasingly being used to study boundary-layer physics.
In the future, work on more complex PBLs will continue. Field experiments conducted from 1991-1994, in particular TOGA COARE, the Storm-Scale Operational Research and Meteorology (STORM) Fronts Experiment Systems Test (FEST), and the Boreal Ecosystem Atmosphere Study (BOREAS), will continue to exploit new technology to document, in unprecedented detail, PBLs in the equatorial Pacific, the North American winter, and the sub-Arctic summer. Increasingly sophisticated models will combine PBL physics with different types of forcing. These efforts could stimulate the application of four-dimensional data assimulation techniques to PBL problems. Turbulent motions may not be amenable to such techniques, but well-organized coherent structures or well-organized forcing phenomena (e.g., an evolving squall line) could be. Also, new methods of analyzing turbulence data are being proposed, including wavelet analysis (e.g., Mahrt 1991; Farge 1992) and fractal dimension evaluation (e.g., Sykes and Gabruk 1994).
There is increasing emphasis on the development of PBL-transport and PBL-cloud parameterizations for GCMs, especially for coupled-climate-system studies. LES has played an unprecedented role in testing parameterization schemes. The first results are soon expected from a world wide effort, the PBL Modeling Evaluation and Development Project at NCAR, whose task is to categorize existing GCM PBL parameterization schemes into archetypes, and examine their accuracy in parameterizing PBLs against a comprehensive LES database. Cloud-resolving cumulus ensemble modeling (e.g., Weissbluth and Cotton 1993; Krueger and Bergeron 1994) has also shown potential use for calibrating, testing, and developing PBL cloud schemes. Finally, direct numerical simulation technique is beginning to emerge as tools for studying boundary-layer processes (e.g., Coleman et al. 1994).
Acknowledgments. We thank Anders Andren, Keith Ayotte, Steve Krueger, Don Lenschow, Larry Mahrt, Peter Sullivan, Qing Wang, and Jeff Weil for their comments.