Investigating Turbulent Exchange of Scalars within the Stable Boundary Layer over an Urban and a Semi-Urban Research Site.

This observational study investigates the physical mechanisms of turbulent exchange within the stable boundary layer (SBL). The research was conduct at the Howard University Interdisciplinary Research Building (HU-IRB) in Washington, DC, and Howard University, Beltsville Campus (HUBC), Maryland. HUBC is located approximately 12 miles northeast of HU-IRB on 110 acres in suburban Maryland. The campus itself is in a rural setting and contains minimal development with not more than 5 percent of the land area occupied by building structures, making it an ideal environment for studying a range of surface-atmospheric interaction processes. A comprehensive set of instruments have been deployed including spectral and broadband radiometers, upper air soundings systems, 31 m flux and meteorological tower, gas analyzers, particle samplers, micro-wave radiometer, ceilometers, and a wind profiler. Recently, at HU-IRB, several instruments were installed, such as a wind Lidar, met data, and a ceilometer. This study focuses the nocturnal exchange within the SBL during the Lower-Level Jet (LLJ). The LLJ is one of the most prevalent features of the nighttime wind flow regime close to the surface in this area. Its importance is well known and documented, ranging from air quality, aeronautic, climate, and weather. Generally, under LLJ nights, the wind is calm at the surface, and it only can be measured by high-resolution instruments such as sonic anemometers. Also, under clear sky the SBL is basically formed by radiative cooling. This provokes a strong temperature inversion at the lowest levels of the atmosphere. During those clear and calm nights with the LLJ presence, the calm and very stable condition is often interrupted by intermittent wind burst occurrences, denominated as coherent structures (CS). These CS bring air from the residual layer to the surface. These CS bring to the surface warmer, drier, and most likely ozone rich air from aloft. The objectives of this study are: 1) detect CS and quantify their strength and their impact on scalars at the tower level, 2) determine the importance of wind and thermodynamical conditions for the occurrence of CS. Specifically, to investigate the effect of the shape wind profile, such as the presence of an inflection point, and the importance of the Kelvin-Helmholtz instability. The current instrumentation at HUBC is able to detect the occurrence of CS.