Temporal evolution and spatial variation of the boundary layer over complex terrain

The temporal and spatial evolution of the convective boundary-layer has a strong effect on air pollutant dispersion, especially under inhomogeneous conditions with varying orography and/or land use. During the TRACT experiment the evolution of boundary-layer height in space and time was measured by radiosondes and aircraft during three special observation periods (SOPs). The observations show that (a) The boundary-layer height follows the height of the underlying orography. The correlation between boundary-layer height and orography holds best in the morning hours, but decreases slightly during the afternoon. This result confirms with previous observations on a smaller scale, which show that the PBL top confirms to the underlying orography until the boundary-layer depth exceeds the characteristic scale of the obstacle. (b) Coincident with the spatial variation of the boundary-layer height, a strong spatial variation of air pollutants can be observed. (c) On two of the three SOPs surface fog-covered parts of the TRACT area (the upper Rhine valley) until noon. Due to the reduced surface heating the boundary-layer height in the foggy areas was much lower than in the clear areas.