Vertical dispersion in the nocturnal, stable surface layer above a forest canopy

Abstract

A field study was conducted to compare vertical plume dispersion in the nocturnal stable surface layer (NSSL) using a light detection and ranging (lidar) system to the predictions of three models. A total of twenty 5-min fogs were released from the top of a 34 m tower over a 20 m forest. The plumes were scanned for the 5-min release period and on through the ensuing 10-min drift periods. Micrometeorological measurements were taken at the same time from the tower to characterize the state of the atmosphere and to calculate the model inputs. The runs were conducted on four successive August early morning periods between 12:30 AM and 3:00 AM in the NSSL. Three stability parameters were compared and it was found that the buoyancy length provided a more consistent measure of stability than the scaled local Obukhov length or the bulk Richardson number. This was because a lack of turbulence strength was present during these periods and the use of a mean temperature gradient term in the formulation of the buoyancy length. The lidar measurements of vertical dispersion were compared to model estimates. It was shown that a general dispersion model that includes plume buoyancy effects displayed less bias and was more reliable than models based on Lagrangian particle motion and similarity theory. Also, the ergodic condition, which assumes equal time and space averages, was met in the NSSL over the forest canopy.