An Estimation of the Lagrangian Length Scale within a Plant Canopy.

Studies of greenhouse gas fluxes of various plant communities have advanced the understanding of bulk interactions between the atmosphere and ecosystems. Micrometeorological instrumentation is currently unable to resolve scalar sources and sinks within plant canopies. Analytical Lagrangian analyses capable of predicting concentration profiles from known source distributions provide the opportunity to calculate source/sink distributions through inverted forms of these equations. Source profiles of carbon dioxide, ammonia, heat, and water vapour within canopies calculated using inverse Lagrangian models produce logical results. However, uncertainties concerning estimates of the essentially immeasurable Lagrangian length scale (L_L), a key input, impede the practicality of this method. Small errors in suggested parameterizations of L_L can lead to irrational predictions of source distributions. The present study seeks to evaluate L_L within an agricultural canopy by using field measurements to constrain the Warland-Thurtell (2000) Lagrangian equation. Measurements of net CO₂ flux, soil CO₂ flux, and in-canopy profiles of CO₂ concentrations taken in a cornfield in 2008 provided the information required to solve for L_L. Comparing these optimized values of L_L to those predicted by models, along with previous information on the structure of in-canopy turbulence, will improve the overall understanding of Lagrangian length scales, and help to improve source distribution predictions.