The “Whitefield” Flow Transition Experiment

The study of land–atmosphere interaction is widely recognized as a crucial component of regional-, continental-, and

global-scale numerical models. Predictions from these large-scale models are sensitive to small-scale surface layer processes such as heat and moisture fluxes at the air–soil–vegetation interface as well as boundary layer treatments. For example the soil moisture boundary condition has considerable influence on weather forecasts. Heterogeneous soil moisture conditions can occur on many scales both naturally or/and through human modification and both types of heterogeneity can introduce

dramatic variability in boundary layer surface forcing. In addition, transient processes such as transition from convective to stable stratification typically occurring in the early evening are poorly understood and yet not well captured within large scale models. A number of theoretical and numerical studies have dealt with evening transition, but available field data on the topic is meagre. This transient phenomenon is characterized by the collapse of turbulence (onset of buoyancy effects, characterized by the sign reversal of heat fluxes) in the atmospheric boundary layer (ABL). Laboratory experiments show that collapse of turbulence is followed by flow horizontal layering that causes dissipation of turbulent kinetic energy to be fast because of strong shear developing between the layers. These concepts have not been investigated using field data.

Motivated by the above, the Whitefield flow transition experiment was designed to explore these concepts as well as further investigating the role of thermodynamics properties, including that of moisture in the formation of different ABL layers.

It consisted of an intensive 15-day field campaign which took place in heterogeneous flat terrain site (Whitefield) with scattered trees, located just outside the Notre Dame University Campus (IN), covering mostly the days with high pressure and both relatively dry and moist conditions. A complete suite of instruments was deployed, including a fully instrumented 15 m tower with 3 levels of turbulence measurements, a Doppler lidar, a sodar/rass and a ceilometer. Turbulence measurements were also taken at high frequency (2 kHz) using a hot film COMBO sensor calibrated in situ using a Campbell sonic anemometer (20 Hz). The measurements were complemented with frequent tethered balloon flights up to 50 m and thermal images taken using an infrared camera. It was observed that during clear but moist evenings the transition was prolonged to a few hours after sunset, but the transition was sharp on drier nights and occurred about an hour before sunset.

Results suggest that moisture levels are a crucial factor in determining the period over which transition occurs. The presence of a thick moist layer near the surface appears to delay the onset of stable stratification, which in some cases never occurs leading to neutral conditions. Interesting dynamics take place when air moisture reach saturation which encourages us to further study these processes.