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Structure and morphology of the convective boundary layer in mountainous terrain De Wekker, Stephanus Franz Joseph

Abstract

Mountainous terrain can significantly modify convective boundary layer (CBL) structure and morphology compared to the situation over flat terrain and consequently can affect the transport and mixing of air pollutants. It is uncertain i f and how concepts of the CBL that have been developed in flat terrain are applicable in mountainous terrain. This dissertation attempts to address this issue by investigating the morphology and structure of the CBL in a deep, narrow mountain valley, near a mountain base, and over a mountain range. An integrative approach was taken by making use of three data sets, a numerical modeling system, and a Lagrangian particle dispersion model. The data set obtained in a deep, narrow valley was used to evaluate the performance of a mesoscale modeling system in complex terrain. CBL structure was captured surprisingly well given that surface layer and turbulence parameterization schemes are strictly valid only for flat and homogeneous terrain. In the deep, narrow valley, a distinction could be made between a 'conventional CBL' with characteristics similar to those over flat terrain and a 'valley CBL' which extends beyond the height of the conventional CBL and is not well-mixed in its upper part. Upvalley flows prevailed throughout the valley CBL but the horizontal wind structure was inhomogeneous, with larger wind speeds on the eastern than on the western side of the valley. Aerosols were found up to the height of the valley CBL. Near a mountain base, enhanced heating aloft was related to a reduced boundarylayer growth, resulting in a depression of the CBL height. Vertical and horizontal advection of warm air associated with the thermally driven circulation along the mountain slope play a role in the enhanced heating aloft. Over a mountain range, it was found that aerosol layer heights exceed CBL heights without the formation of separate elevated aerosol layers as found in previous studies. Particles are carried above the CBL up to the 'mountain CBL' height by venting mechanisms associated with the presence of thermally driven mesoscale flows. The 'valley CBL' and 'mountain CBL' heights are more important parameters for air-pollution studies than the conventional CBL height since they indicate the height up to which air pollutants can be transported and/or mixed.

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