New results in the literature have dramatically altered our conceptual framework for the stable nocturnal boundary layer, and also have identified serious inadequacies in our understanding for weak-wind strongly-stratified conditions. Similarity theory, which forms the basis for representation of the turbulent mixing in numerical models, is not supported by recent analyses for weak-wind very-stable conditions. Additional influences not in standard similarity theory include:
a) intermittency on a variety of scales, b) downward transport of shear-generated turbulence from above the surface inversion layer, c) significant horizontal advection over even weak surface heterogeneity with strongly-stratified conditions, d) more efficient transport of momentum compared to transport of heat in some situations and e) coupling with small-scale mesoscale motions, such as gravity waves and meandering motions.
The PI is evaluating the role of these influences in order to generalize commonly-used similarity theories for turbulent fluxes, and for several alternatives to similarity theory as well. These theories are being expanded to include probability distributions to account for the broad range of turbulence characteristics for a given set of external conditions when unpredictable gravity waves and meandering motions also are present. For this work he will use quality-controlled tower flux data from 20 sites that has been accumulated for a wide variety of surface types and climate regimes. The datasets include five micro-networks of flux towers for examination of the spatial structure of the turbulence and mesoscale motions. Computation of fluxes with a new more careful method is essential for this examination of weak turbulence.
Intellectual Merit: The combination of improved analysis techniques and the networks of eddy-correlation measurements yields new insight into the space-time structure of turbulence and mesoscale meandering/gravity waves, and their interaction, for very stable conditions. These analyses improve our understanding of intermittency of turbulence and its influence on the flux-gradient relationship.
Broader Impacts: The work will improve formulations of turbulent mixing and mesoscale transport in atmospheric models, particularly for cases of very weak mixing when risk of frost damage and ground fog formation are greatest. These are the cases where models suffer the largest errors in predicted surface variables. The new formulations of turbulent mixing and mesoscale transport improve modeling of dispersion of atmospheric contaminants in weak-wind nocturnal conditions when buildup of contaminant concentrations are greatest. In addition, the massive quality-controlled datasets are being made available to the research community. Two undergraduate students, and an international graduate student or post-doctoral researcher with separate funding, are working with this veteran researcher on this project.
This project analyzed atmospheric research data from several sites to examine mixing in weak-wind conditions with surface cooling, as occurs in the nocturnal surface layer with clear or partly cloudy skies. These sites include networks of fast response measurements of air temperature and the three wind components. Under these conditions, mixing is very weak and only intermittent, which facilitates strong cooling of the air and can lead to build up of any contaminants released into the atmosphere. Such conditions can also lead to formation of dense fog. The wind direction is variable and unpredictable. This common case is poorly understood because it is complex and has received little previous attention. The research conducted in this project uncovered some of the causes of the wind direction variability and the weak intermittent mixing. Such causes include wave-like motions near the surface, which can propagate long distances and generate local mixing as well as lead to oscillations of wind direction. However, such wave-like motions are not described by existing concepts and theories. Microfronts at the leading edge of density currents (moving pockets of colder air) move horizontally and lead to rapid changes of wind direction and sometimes lead to local mixing and even initiation of wave motions. Drainage of cold air down local slopes also lead to formation of microfronts. Such cold air drainage may flow over even colder air trapped in those valleys with only weak down valley slopes. Winds and mixing are particularly weak in such cold pools. The air near the surface often becomes layered or striated as in the photo of fog. Wave motions and microfronts temporarily increase the weak wind shear, which can generate short-term bursts of mixing and temporarily reduce concentrations of contaminants and the cooling rate. While the research under this project made the first significant advances toward understanding this complex problem, much remains poorly understood.
