Surface Turbulent Fluxes

Abstract

Micrometeorological methods to measure surface fluxes have several key properties which make them useful in global change research. The methods do not disturb the Vegetation, they can operate for extended periods, and they provide a spatial average of fluxes over typically a few hundred to a few thousand m². Developments in both hardware and theory have moved the subject forward considerably over the past 40 years. We have moved from a position where micrometeorology was used in small research-led experiments, usually of quite short duration and in fair weather, to a point where fluxes are measured continuously and routinely at a number of sites around the globe as part of a flux network. In addition, surface flux measurements are no longer seen just as an interest in themselves but rather as a component of much wider research. Surface fluxes are required to validate process-based or empirical models operating at smaller and greater scales and they provide essential groundtruth observations for remote sensing. The changes in instrumentation and methodology are easily demonstrated by considering the series of large-scale experiments supported by BAHC and others. In the early HAPEX-MOBILHY experiment (1986) for instance, the dominant surface flux methodology was the aerodynamic gradient technique (with the exception of a Hydra eddy covar iance Station over forest, see Gash et al. 1989); by the time of HAPEX-Sahel (1992), eddy covariance Systems were as important as gradient and Bowen ratio techniques and by the time of the BOREAS (Boreal Ecosystem-Atmosphere Study) experiments in the mid-1990s, surface fluxes were almost entirely measured by eddy covariance.