Diploma Thesis
Bestimmung turbulenter Flüsse auf einer Waldlichtung mit der Modified-Bowen-Ratio-Methode
Georg Jocher (03/2009-10/2009)
Support: Thomas Foken, Johannes Lüers
The EGER project (ExchanGE processes in mountainous REgions) took place in the years 2007 and 2008 at the measurement site of the BayCEER (Bayreuth Center of Ecology and Environmental research; region Waldstein/Weidenbrunnen), basically consisting of two intensive observation periods (IOP1 and IOP2) in september/october 2007 and june/july 2008. The main intention of this project was to investigate different micrometeorological, chemical and biological processes in the array of soil, vegetation and atmosphere and to learn about their interactions. In the course of this project, during IOP1 a modified-bowen-ratio measurement complex was placed on a clearing (diameter about 100 m) in the 23 m high spruce stand, who dominates at this measurement site. During IOP2, this measurement complex on the clearing was completed by a radiation measurement complex and the determination of different soil parameters. The investigations showed that the bowen ratios on the clearing are < 1, above the spruce stand in a height of 36 m >1. However, the gradients of the temperature and humidity on the clearing were very small, because the distance between the two measurement points of temperature and humidity in two different heights was too small. So it was only possible in the midday hours to determine the bowen ratio with an error less than 20%. Another big problem was the determination of the dry temperature in the upper measurement height because of immense variations of this scalar, probably caused by a radiation error. However, the determination of the turbulent fluxes during the summer measurement period IOP2 was more exactly and the fluxes showed larger values, caused by the bigger turbulence during this time. The small bowen ratios on the clearing, which were during IOP2 comparable with the bowen ratios in the stand, induced events of free convection ( ), because you can see the clearing as a source of humidity, and the density of humid air is smaller than the density of almost dry air. Free convection events appeared during IOP1 and during IOP2, most times based on the clearing. In these cases, free convection was also to detect above the stand in 36 m height, and the clearing was coupled with the atmosphere above the stand. In a few cases free convection appeared only above the stand caused by very large incoming radiation already in the morning. The observation of the integral turbulence characteristics of the vertical wind showed as a tendency, that above the stand the influence of stability is very small, in the most cases you can find there near neutral conditions. In contrast the clearing follows the change of stability relative clearly. An interesting aspect of the investigations on the clearing is, that obviously the closure of the energy balance depends on the wind direction. Basically, the sum of sensible and latent heat flux was bigger than the available energy on the clearing during IOP2, a similarity with the circumstances in the stand. But the discrepancy increased, if the wind was coming from south or south-west directions. The reason for this is probably an additional energy transport to the clearing, caused by secondary circulations during a wind of southern directions. Responsible for this is a deforested area in the south of the clearing and the “Kyrill” forest aisle.