Gusts within plant canopies are extreme value processes

Plant canopy turbulence is characterised by energetic high-velocity downdrafts (sweeps) and updrafts (ejections) that punctuate otherwise quiescent flow and thereby make the dominant contribution to turbulent transport. It is suggested that this turbulence provides a natural setting for the emergence of extreme value processes. The suggestion is supported by simulation data obtained from Lagrangian probability density function models. Predicted wind speed frequency distributions within the lower portions of plant canopies are consistent with observations and are very well represented by Gumbel (extreme value) distributions. This is interpreted as a signature of strong (maximal) intermittent gusts making the dominant contribution to the turbulence. The findings complement the apparent ubiquity of generalised Gumbel distributions of global quantities in boundary-layer turbulence, equilibrium phase transitions and non-equilibrium models with self-organised criticality. In these examples, however, there are no known direct relationships between the global quantities and extreme value processes. The new findings are interesting from a practical perspective because correct parametrisation of the wind speed frequency distribution is required for the accurate quantification of seed abscission and the removal of pollens and passively released pathogenic spores from plant surfaces.