Beyond the “Textbook ABL”: Numerical Simulations and Experimental Implications of Baroclinicity and Unsteadiness

Abstract:

Understanding and predicting the flow of air, and how it transports heat and trace gases, in the atmospheric boundary layer are increasingly becoming critical to a wide range of applications including wind and solar energy, urban design, agriculture, and assessment of climate change impacts and adaptations. These applications all require a level of sophistication and detail in our ability to probe and model the ABL and its interaction with the earth surface that manifestly exceeds our current capabilities. Previous work largely focused on the “textbook ABL”, which is barotropic, in (quasi) steady-state, and interacts with a horizontal and homogeneous earth surface; it is evident that the “real-world ABL”, even over flat terrain, rarely meets these simplifying conditions. In this talk we overview two complicating features that have been largely overlooked thus far despite their ubiquity: baroclinicity and unsteadiness. Large-eddy simulations of ABL flow with a time-varying (unsteady) or height-varying (baroclinic) pressure forcings are analyzed to understand how they modulate the bulk structure (mean fields) and turbulence (higher order moments). Our results indicate that for the unsteady ABL, the dynamics are primarily controlled by the relative magnitudes of three times scales: the inertial time scale (~ 12 hours in mid latitude), the turbulent time scale (~ 0.5 hours), and the forcing variability time scale (varies depending on meso and synoptic scale dynamics). For the baroclinic simulations, the strength and more importantly the direction of the baroclinicity can result in profiles that are vastly different from the classic barotropic case, with for example peaks in the turbulent kinetic energy that are in the middle of the layer. Both features also results in first and second order moments that, if interpreted to results from a steady barotropic case, can be highly misleading when experimental results are analyzed.