A number of theoretical and numerical studies have dealt with evening transition, but available field data on the topic is meagre. It has been argued that the collapse of turbulence (onset of buoyancy effects, characterized by the sign reversal of heat fluxes) in the atmospheric boundary layer (ABL) upon starting of radiative cooling at the ground level initiates at the top of the ABL. The process implies that the switching of heat flux near the surface occurs at last given that large eddies are affected by the overlying stratification first. Laboratory experiments show that collapse of turbulence is followed by flow horizontal layering that causes dissipation of turbulent kinetic energy fast because of strong shear between the layers. These concepts have not been investigated using field data, and the present field study was aimed at delving into them as well as further exploring the role of thermodynamics properties, including that of moisture in the formation of different ABL layers.

An intensive 15-day field experiment was conducted in heterogeneous flat terrain site (Whitefield) with scattered trees, located just outside the Notre Dame University Campus (IN), including days with high pressure, relatively dry (40%), and high (100% condensing) humidity 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 around sunset.

Preliminary 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. Investigations of buoyancy and moisture fluxes and their co-spectra at different heights are required to understand how different dynamical processes influence various scales. This paper will discuss the results in the framework of a broad picture of physical mechanisms associated with flow transition in the atmosphere, highlighting features in convective-to-stable and convective-to-neutral regimes.