Bestimmung der Mischungsschichthöhe aus über einen Zeitraum von 16 Monaten durchgeführten SODAR-Messungen mittels automatischer Auswertung

Abstract

The dispersion of emitted and formed air pollutants depends decisively on the meteorological conditions in the atmospheric boundary-layer. This includes the mixing-layer height (MLH) that defines the volume within which an air pollutant can be diluted, and the vertical wind profile that determines the direction and speed of the dispersion. The MLH cannot be measured by in-situ methods for longer time periods. It has therefore to be derived from ground-based remote sensing methods. An acoustic remote sensing instrument, a long-range SODAR, has been operated for nearly two years in the framwork of a research project (VALIUM, a AFO2000-project financed by the German Federal Ministry of Education and Research (BMBF)) on the grounds of an industrial plant in the town of Hannover. The maximum range of this remote sensing device is 1200 m, the vertical resolution has been chosen between 12.5 and 25 m, the temporal resolution was 30 min. The device returns profiles of the acoustic backscatter intensity, of the three wind components, and of the variance of the vertical wind component. The MLH can be derived from the backscatter intensity and the variance of the vertical wind component. This report contains results for the period October 2001 until March 2003. The MLH is retrieved by scanning the vertical profiles from below using two criteria. The first criterion that diagnoses a strong decrease in the backscatter intensity with height is designed to find the top of a turbulent layer. During daytime this turbulent layer is very often identical to the convective boundary layer. The second criterion that diagnoses maximum values of the backscatter intensity profiles is formulated in order to find surface and lifted temperature inversions. Only those maxima are accepted by the criterion where the variance of the vertical velocity component remains below a given threshold. This excludes backscatter maxima from enhanced turbulence intensity. The lower value returned from both criteria is taken as the MLH. This report presents mainly statistical evaluations of the MLH data set, among which are monthly frequency distributions of MLH, monthly frequency distributions of the daily MLH maximum, and mean daily courses of MLH for selected months. All these results show a distinct yearly course. The interannual variability of the results is demonstrated at one example that compares February 2002 and February 2003.