A detailed characterization of the wintertime boundary layer using tethered balloons

A detailed examination of the structure and evolution of the wintertime boundary layer was conducted from 3 January – 14 February 2004 near Lancaster, PA in support of the research objectives of the Mid-Atlantic/Northeast – Visibility Union. Two tethered balloons were used to deploy meteorological sensors, condensation particle counters, laser-diode scatterometers, and filter samplers to altitudes of 750 m AGL, while a suite of ground-based instruments measured trace gas and particle concentrations and meteorological parameters. January 2004 was characterized by a very active synoptic pattern that frequently brought Arctic air into the mid-Atlantic region and resulted in this being the 10th coldest January on record. Tethered balloon measurements were primarily limited to times when progressive anticyclones moved over the site, bringing clear skies, strong nocturnal radiational cooling, and wind speeds not in excess of12 ms-1. Daytime conditions were marked by the rapid development of the nearly adiabatic mixed layer of uniform winds extending to a depth of 500 – 700 m AGL and capped by a subsidence inversion. The nighttime periods were considerably more interesting with complex stratification embedded in and above the inversion, with significant variability in wind speed and direction, water vapor mixing ratio, particle concentration, and scattering coefficient observed across layers that were often only tens of meters thick. Depending on the atmospheric condition around sunset and the rate of development of the nocturnal inversion, high concentrations of particles were found trapped near the surface and/or in shallow stable layers within the inversion. The tethered balloons were deployed to capture this detail by first performing a vertical profile using the 12 m3 blimp to examine the boundary layer structure. Once potential layers of interest were identified in the profile, a second balloon was parked at that altitude to conduct long-duration (10-12 hour) time series of meteorological variables and particle concentrations. The measurements obtained using the single-site tethered balloons are being integrated into a regional context by incorporating surface and aloft observations from the NWS network, as well as regional profiler data and WRF and Eta model output. Preliminary results suggest that high concentrations of particles (12,000 – 50,000 cm-3) are trapped in wintertime stable layers, and are subsequently mixed to the surface the following day. Moreover, black carbon appears to contribute a significant fraction to the total particle count in winter. Finally, tethered balloons provide detailed profiles of meteorological variables (T, p, z, q, and vector wind) with vertical resolution of 0.3 m that show considerable structure and variability in the wintertime boundary layer. These data can be used to validate boundary layer parameterization schemes used in numerical models.