Thermal Micro-Fronts and their implications for the stable boundary layer
Abstract
We investigated the stable boundary layer (SBL) in gentle terrain expecting simple submeso-scale motions during the Shallow Cold Pool experiment. In a cross-valley view, adjacent warm and cold air masses were detected on the valley side slope moving up and down together, which we refer to as thermal micro-fronts (TMFs). TMFs could only be identified by the fiber-optic distributed sensing (FODS) technique that provides spatially continuous temperature measurements. Through the combination of an ultrasonic anemometer network and the FODS transect, TMFs were characterized and their impact on SBL characteristics quantified.
Unexpectedly, the gentle topography induced lee turbulence during dominant wind directions from the north-west and provided shelter for cold-air currents near the valley bottom with winds from west. The TMF boundary is the convergence of those two competing air masses. The convergence creates an updraft and thus induces vigorous turbulence independent of local wind shear. The warm air slides over the cold air enhancing the vertical temperature gradient. Hence, the relationship between buoyancy and turbulence production by shear is not described by similarity theory at this interfacial boundary and the role of topography, even in such gentle terrain, has a larger effect than typically anticipated. The interaction of the two air masses with each other and the local topography was investigated in detail, however the main driver of TMFs remains unclear. Concepts like wind regimes did not determine the occurrence of TMFs. A mix of spatial scales are involved in the formation of TMFs, however, not all scales could be investigated due to the lack of instruments outside the valley. The results from this study demonstrate how the combination of FODS and an ultrasonic network is a powerful tool for investigating the SBL. This new technique should be used to investigate TMFs and their implications for mixing and transport in the SBL.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A13N3145P
- Keywords:
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- 3307 Boundary layer processes;
- ATMOSPHERIC PROCESSES;
- 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 3379 Turbulence;
- ATMOSPHERIC PROCESSES;
- 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCES