Schedule May 21, 2018
Temperature fronts in stable boundary layers
Peter Sullivan (NCAR / UCAR)

Unstable conditions in the lower layer of the atmosphere (that which interacts with the surface the boundary layer), often characterized by chaotic airflow that rises due to surface heating, have been extensively studied in recent years. However, the dynamics in stable boundary layers (ones with a cool surface under warm air) - also quite complex - are not as well understood. Stable conditions are common, occurring at nighttime when warm air moves over cooler water, or when hurricanes leave behind a cool ocean surface after extracting its heat. This develops atmospheric stratification (layering of the atmosphere by density) that inhibits vertical Lmovement. Understanding the dynamics of stable boundary layers has the potential to improve weather and climate forecasts, design constraints for wind turbines, and air quality measures. The current study uses a large eddy simulation (LES) - an efficient turbulence model - to study the characteristics of these conditions, and the fluxes (movement of heat, gas, and momentum) that arise from them. Three different degrees of stratification under moderate, spatially uniform wind speeds are simulated. Surprisingly, the results show that in all cases, horizontal temperature fronts (boundaries separating two different temperatures) develop and move downstream despite uniform initial conditions, appearing due to turbulent (chaotic) fluctuations in the flow. They form at angles from the horizontal surface that are dependent on the degree of background stratification. Upon further investigation of the flow fields, it is found that the fronts arise due to coherent vortices (consistent swirling structures) which enable horizontal and vertical flux of momentum, heat, and gas despite the stratification that inhibits it. These results are confirmed by observations of similar conditions taken from a weather tower - confirming that these structures occur in real life, and that they can be identified.

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