Developing Improved Models of the Stable Boundary Layer Incorporating the Residual Layer Region

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

This research consists of three separate, but closely coupled objectives: The first objective is to analyze high-resolution vertical profiles of temperature, wind, and turbulence dissipation rate using the tethered lifting system (TLS) in conjunction with concurrent 55-m tower data and related data sets during CASES-99, a large cooperative experiment to observe boundary layer processes. The work will establish the capability of the TLS to extend some of the tower observations through the stable boundary layer (SBL) and well into the so-called residual layer (RL). To provide one important example, TLS profiles of the turbulence dissipation rate will be compared with tower-obtained vertical velocity variance profiles. If this comparison is successful under most nighttime conditions, it will enable a determination of the mixing height (MH) of the SBL, even though the MH lies well above tower heights. Comparable studies using other tower/TLS measurements will be conducted. The second objective is to examine fine-scale wind speed, wind direction, temperature, and turbulence structure in the residual layer during over forty hours of observations obtained over five separate CASES-99 nights. The SBL structure during these periods ranged from traditional, through thin-traditional, and contained to at least one night when the SBL could be classified as upside down. It will be demonstrated that the RL does not fit the current view that it is a region of statically stable temperature profiles and minimal turbulence activity, and that it is merely a passive remnant of the previous day's convective boundary layer. Rather, it will be demonstrated that the RL is a very dynamic, complex region with significant wind shears, temperature gradients, turbulence structure, and, on occasion, pronounced gravity wave activity. More significantly, it will be shown that the vertical profiles of small-scale turbulence often exhibit extreme intensity variations (more than two orders of magnitude) over very small altitude ranges, and on time scales of a few tens of minutes. These enhanced turbulence regions, even after local midnight, can, on occasion, be comparable to nighttime levels at the surface, and could have a significant impact on vertical transport between the RL and SBL. The third and ultimate objective is to incorporate the above concepts into current models of both the SBL and the RL in order to improve understanding of both upper level SBL dynamics and SBL/RL interactions. In this work, the focus will be on modeling the morning transition and how it is impacted by processes in the RL during the preceding night. Although it would be difficult to directly include some aspects of this research into single column models, the results should prove invaluable as a means of comparing model-derived quantities with measured quantities to be able to understand the importance of and sensitivity to the studied processes. Intellectually, the above activities will provide critical new information for the theoretical/modeling communities on complex, dynamical processes throughout the SBL and well into the RL. On a broader scale, the anticipated results will provide valuable new insight into long-distant pollution transport in the RL and illuminate the as-yet unexplained, sporadic coupling between the two regions. Results from these efforts will provide a useful framework for designing future field campaigns. Finally, the post-doctoral candidate will learn to work in both modeling and observational research arenas, which is very important for future success in this field.