Call for papers
Motivation
Aims of the Workshop
Steering Committee
As part of the IGBP iLEAPS Program’s 1st iLEAPS Science Conference in Boulder 21-26 January 2006, a workshop on the measurement of surface exchange by eddy flux techniques in ‘Difficult Conditions’ will be held. The motivation for this workshop is explained in detail below.
The workshop will be closely targeted on the topics set out in the draft timetable and each session will be led by an invited review. This will be followed by two 20 minute presentations chosen to illustrate or complement the review. We are now calling for workers in the field to submit abstracts for these short presentations. The title of the abstract should include a clear indication of the session for which the abstract is intended. We plan to publish papers based on the short contributions and invited reviews in a special edition of a major journal.
The older sites in the Fluxnet have been gathering data continuously for over a decade and many can boast at least five-year records of carbon and energy exchange. In several countries the question of moving the flux towers from a research to an operational status to provide data for ambitious multi-platform data assimilation schemes is under active discussion. However, reliable flux measurements at nighttime or at sites subject to advective flows remains a serious obstacle to routine 24 hour operation.
It is still common to find that the surface energy balance during the day is not closed at flux sites in tall forests. As a general rule, the sum of eddy fluxes of sensible and latent heat falls short of the sum of net radiation and storage measurements by 15-30%. Explanations based on undercounting the contribution of low frequency eddies, on instrumental aliasing and on local advection have been advanced but there is no consensus at present. This is disturbing as the errors indicated by the mismatch are serious and systematic.
At night, flux corrections based on u* thresholds are widely applied to replace aerodynamic estimates when turbulence is weak and the sum of eddy fluxes and storage terms fails to give convincing measures of CO2 respiration. Aerodynamic measurements are then replaced by calibrated biological models. While this is a perfectly valid procedure, it degrades the independence of the aerodynamic approach, which we would prefer to use as an independent constraint on biological models, biomass assays and concentration inversions.
Similarly, rapid flushing of the canopy CO2 at dawn or during intermittent turbulent events in stable conditions is usually spatially localized and may lead to systematic bias in estimates of regional exchange from a single tower. Many sites, chosen for reasons other than micrometeorological convenience, suffer from systematic advection in some or all wind directions. Operational approaches to correct for advection are not yet available. Recent research at several centres is showing that failure to close the Carbon budget at night, when u* is small, is related to advective drainage flows so that the mechanisms of stably stratified canopy flows and advection are closely linked.
Some of these topics were raised at the US DoE sponsored Fluxnet Techniques Workshop in Corvallis, Oregon three years ago1 but there the focus was primarily on ideal sites. Since that time several groups have embarked on experimental (both field and wind tunnel) and theoretical approaches to these problems. It is now timely to assess the state of the art, to exchange techniques and information and to plan a cooperative research program for the future.
1 See, ‘Handbook of Micrometeorology’, Lee, X., Massman, W. and Law, B. (Eds) Kluwer 2004, 250pp.
Surface exchange measurements in programs like Fluxnet are motivated by important questions in biology, ecology and biogeochemical cycling. Hence we must accept that measurement sites will not conform to the ideal conditions of homogeneity and stationarity that allow simple application of the eddy flux approach. We define ‘difficult’ conditions, therefore as circumstances that cause systematic departures from horizontally homogeneous, stationary flow conditions. An incomplete list includes surface heterogeneity, topography, intermittent atmospheric phenomena like waves and nocturnal jets and strong diabatic forcing.
This workshop, therefore aims to:
It will address these aims by:
Our current understanding of what constitutes ‘difficult conditions’ suggests the following topic areas:
And within these areas we will discus:
John Finnigan CSIRO Australia
Ray Leuning CSIRO Australia
Dave Schimel NCAR
Gaby Katul Duke University, NC
Marc Aubinet FSAGX, Belgium
Alessandro Cescatti CEALP, Italy