A historical perspective on the Webb, Pearman and Leuning density correction theory

The density correction theory of Webb et al. (1980, Quart J Roy Meteorol Soc, 106, 85, hereafter WPL) is an essential component of modern micrometeorology. In this paper provides a historical perspective on the theory and review the research on the density effects since the publication of the WPL paper. (A fuller discussion on this topic can be found in Lee and Massman, 2011, Bound-Layer Meteorol, 139: 37-59.) The assumption of zero surface source of dry air is a fundamental novelty that gives the WPL theory its enduring vitality. Considerations of mass conservation show that, in a non-steady state, the WPL mean vertical velocity and the thermal expansion velocity are two distinctly different quantities of the flow. Furthermore, the integrated flux will suffer a systematic bias if the expansion velocity is omitted or if the storage term is computed from time changes in the CO2 density. A discussion is provided on recent efforts to address several important practical issues omitted by the original theory, including pressure correction, unintentional alternation of the sampled air, and error propagation. These refinement efforts are motivated by the need for an unbiased assessment of the annual carbon budget in terrestrial ecosystems in the global eddy flux network (FluxNet).