The Lidars in Flat Terrain (LIFT) Experiment

The atmospheric boundary layer (ABL) is “that part of the troposphere that is directly influenced by the presence of the earth’s surface, and responds to surface forcings with a timescale of about an hour or less” (Stull 1988). Mixing in the ABL can be driven by surface heating (free convection) and wind shear (forced convection). A capping inversion usually limits the depth of the daytime convective boundary layer (CBL) to approximately 1–3 km. The ABL connects the earth’s surface with the overlying atmosphere. Turbulent motions within it control fluxes of heat, moisture, trace gases, pollution, and momentum. Study of the ABL is important for reasons ranging from improving short-range weather forecasts to understanding global climate change. Also, we spend our lives in this part of the atmosphere. The first observations of the ABL were obtained primarily using towers, balloons, and kites (e.g., Lewis 1997). Instrumented aircraft and radar wind profilers have been used to reach higher, providing in situ and remotely sensed observations throughout the entire boundary layer. Examples of boundary layer measurements and instrumentation may be found in Kaimal et al. (1976), Lenschow (1970), Young (1988), and Angevine et al. (1994). These various measurement systems have different advantages and limitations and when used together are complementary. The Lidars in Flat Terrain (LIFT) experiment used three modern lidars and two types of radars to study the CBL and its morning and evening transitions. LIFT was a companion experiment to the Flatland96 experiment described by Angevine et al. (1998). Flatland96 used three 915-MHz wind profilers, one rawinsonde system, and three enhanced surface flux measurement stations (Flux-PAM) and focused on the study of the boundary layer top and entrainment zone. Flatland96 and LIFT took place close to the Flatland Atmospheric Observatory near Urbana, The Lidars in Flat Terrain (LIFT) Experiment