Boundary layer meteorology and carbon cycle science

Carbon cycle research has expanded vigorously in the past decade. A great deal of this work has focused on terrestrial ecosystems given the evidence for a large and highly variable sink of carbon in these systems. The carbon cycle science community has absorbed many tools developed by the micrometeorologists, most notably eddy-covariance flux measurements. Current work is focused on using atmospheric budgets to derive ecosystem-atmosphere exchange of carbon dioxide at much larger scales than can be addressed via long-term eddy-covariance measurements. This research direction has raised several new problems in boundary layer meteorology. This talk will review opportunities for the micrometeorological community to advance the field of carbon cycle science while also addressing fundamental problems in boundary layer meteorology.

Traditional atmospheric budget calculations require precise, abundant and continuous boundary layer CO2 mixing ratio measurements. One way to obtain such measurements is by carefully calibrating the CO2 mixing ratio measurements on eddy-covariance flux towers. These measurements, however, represent surface layer, not boundary layer mean, mixing ratios. Sub-sampling surface layer data for convective conditions eliminates most of this problem, but there is still a small offset between surface layer and ABL mean mixing ratios that can be significant given the precision required for CO2 budget studies. Micrometeorological theory suggests a means to correct for this offset, but complications exist, and some members of the carbon cycle community remain skeptical of this approach.

The physiology of ecosystem-atmosphere CO2 fluxes, however, is a combination of two fundamental processes, respiration and photosynthesis. These processes are difficult to distinguish during the day, which limits our ability to develop a mechanistic understanding of the fluxes derived using daytime ABL budgets. The result is that nighttime ABL budgets are an emerging focus of the carbon cycle science community. Characterizing the depth of mixing in the nighttime boundary layer is much more problematic than the analogous daytime problem. Progress characterizing nighttime mixing depths, perhaps aided by an emerging North American network of instrumented tall towers, would be of great value to carbon cycle science.

An alternative approach to studying ABL budgets has also emerged. This approach relies upon characterizing the rate of exchange between the ABL and the overlying free troposphere. It is analogous in many ways to estimating fluxes via a surface layer flux-gradient relationship. The approach is very simple compared to the construction of complex mesoscale ABL budgets. The limits of this approach compared to a complete budget approach, however, remain uncertain.

Finally, the carbon cycle science community remains handicapped by uncertainty in the accuracy of the eddy-covariance approach, particularly at night. Nocturnal systems are rarely one-dimensional, yet this is assumed for tower-based flux measurements. To compensate, nocturnal measurements under very stable conditions are often neglected, but the basis for these decisions to screen suspect data remains hypothetical. Research on this topic is needed to improve confidence in long-term applications of eddy covariance that are an integral part of the global carbon cycle observing network.