Measuring the Transition From the Convectively Unstable Boundary Layer to the Stable Nocturnal Boundary Layer

While it is widely known that much of the Great Plains exhibits a characteristic transition from a convectively unstable daytime boundary layer to the stable nocturnal boundary layer near sunset, there remains much to be learned about this process from observations. As part of the Plains-Elevated Convection at Night (PECAN) field campaign, extensive measurements were made at Greensburg, Kansas, one of the PECAN's fixed-location measurement sites, from June 1 to July 15 2015. Measurements were made from a 16-m flux tower, a 6-m tripod flux tower, a smaller 3-m scalar and radiation tripod tower, a sonic detection and ranging (SODAR) device, and a tethered balloon system. The 18-m flux tower was outfitted with four levels of 3-D sonic anemometer and fast hygrometers to measure turbulence and scalar perturbations, six levels of mean wind measurements, and twelve levels of temperature and humidity measurements. The 6-m flux tower made similar turbulence measurements at the same two lowest levels as the 18-m tower at a nearby location. Since the 18-m tower was situated on a 1.3 m × 1.3 m concrete foundation, both near-surface temperature and humidity were sampled from a small 3-m tripod tower at six levels below 2.5 m with three levels of concurrent soil temperature and two levels of moisture measurements. The SODAR measurements provided a vertical profile of mean wind and turbulence continuously up to approximately 250 m above the surface. While the tower and SODAR measurements were continuous, numerous tethered balloon based measurements spanned multiple days throughout the experiment period with the intention of targeting the transition period between the daytime convective boundary layer and the stable nocturnal boundary layer. In this presentation, primary focus will be placed on analyzing this transition period using all available measurements. In particular, the boundary layer evolution observed from the tethered balloon measurements will be examined in conjunction with the evolution of the surface fluxes seen from the towers. The transient nature of the associated turbulence fields will be discussed. Additionally, a thorough examination of the stable surface layer flux-profile relationship using this suite of measurements will be made and subsequently compared with empirical data collected from other historically relevant experiments such as the 1968 Kansas experiment.