TEMPORAL EVOLUTION AND SPATIAL VARIATION OF THE BOUNDARY-LAYER OVER COMPLEX TERRAIN

The temporal and spatial evolution of the convective boundary-layer ha s a strong effect on air pollutant dispersion, especially under inhomo geneous conditions with varying orography and/or land use. During the TRACT experiment the evolution of boundary-layer height in space and t ime was measured by radiosondes and aircraft during three special obse rvation periods (SOPs). The observations show that (a) The boundary-la yer height follows the height of the underlying orography. The correla tion between boundary-layer height and orography holds best in the mor ning hours, but decreases slightly during the afternoon. This result c onfirms with previous observations on a smaller scale, which show that the PBL top confirms to the underlying orography until the boundary-l ayer depth exceeds the characteristic scale of the obstacle. (b) Coinc ident with the spatial Variation of the boundary-layer height, a stron g spatial variation of air pollutants can be observed. (c) On two of t he three SOPs surface fog-covered parts of the TRACT area (the upper R hine valley) until noon. Due to the reduced surface heating the bounda ry-layer height in the foggy areas was much lower than in the clear ar eas. (C) 1998 Elsevier Science Ltd. All rights reserved.