Fluxes in Advective and Transition Boundary Layers

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These investigators will use measurements that were made by an instrumented aircraft and by surface towers in a recent project at North Park, Colorado to study the ways in which variability in the land surface affects transport of heat, moisture, and momentum in the atmospheric boundary layer overlying that surface. Most studies of the boundary layer have examined daytime conditions when surface heating leads to well-developed turbulence and predictable transport, and most studies have been conducted over uniform terrain. This study will instead focus on the transition from nighttime stable conditions to daytime convective conditions and will use measurements from a region where there is significant surface heterogeneity resulting from interspersed patches of grass and brush. Heterogeneous surface conditions such as these are very common, so the study will provide valuable new information on realistic boundary layers. The investigators note that heterogeneity influences the surface fluxes and the development of the boundary layer in ways that are not simply deduced from the properties of boundary layers over uniform terrain or even terrain with discontinuous transitions. Some specific hypotheses are offered to guide the analyses of this data set, including that the impact of surface heterogeneity is expected to be greatest under stable conditions when the eddy size is smallest and that transient modes such as pulsating drainage flows or mesoscale features in the wind field frequently alter the surface flux patterns and net time-average flux. The research will establish a basis for improved representation of transport processes in boundary layers over inhomogeneous terrain and so should influence future models of weather and climate. The investigators plan several steps that will facilitate use of their results. A web site will be used to provide access to data and results, and a workshop will be held to help inform potential users including water management and Forest Service personnel. The project will also help develop capabilities for similar studies in the future through the support and education of a graduate student. These steps should help publicize the results from their research among modelers and other potential users.