Nocturnal pollutant transport and LLJ structure during SOS-99 in Nashville

The importance of low-level-jet (LLJ) development at night in the nocturnal redistribution of photochemical pollutants produced during the previous daytime was a significant finding of the 1995 SOS Nashville campaign (Banta et al., 1998: JGR, 103, 22519-22544). The jet is produced by ageostrophic, inertial accelerations of the lower ABL flow as the flow is isolated from surface friction via surface cooling. The LLJ represents a significant departure from the daytime flow regime, so it can carry pollutants in unexpected directions, based on daytime wind patterns. To further investigate the LLJ's properties, dynamics, and effects on pollutant transport and diffusion, ETL's mini-MOPA Doppler lidar and OPAL surface-based ozone lidar were deployed to Nashville during SOS-99 along with the arrays of profilers, surface stations, aircraft, and other measurement platforms. The lidar documented important spatial variations in the direction, speed, and evolution of the LLJ in the 50-150 m AGL height interval, and perhaps even more significantly, differences in the structure of the flow below the jet–-from nearly laminar to wavy to fully turbulent–-with corresponding effects on the vertical transport from (and to) the surface. The effects of these processes on nocturnal layering and vertical distributions of pollutants, aerosol, and momentum were effectively demonstrated by data from both lidar systems.