The stable boundary layer and the ‘accumulation layer’

During the intensive field campaign (IFC) of the Hudson Valley Ambient Meteorology Study (HVAMS, September-October 2003) a number of instrument platforms were used to study the stable boundary layer. We examine sodar records and profiles made using two types of tethered balloon sondes (TAOS, NCAR, the primary source, and the ASRC CO2 sonde), We find evidence of the stable boundary layer at 150-250 m level consistent with standard work. However, accumulation of scalars occurs in a much shallower, 20-60 m layer that we refer to as the accumulation layer. To determine how the accumulation layer forms, we examine the history of nocturnal turbulent mixing during the HVAMS IFC as measured at a 20 m flux tower installed near the tethered balloon site. Profiles of the wind vector, potential temperature, specific humidity and CO2 concentration made during ‘close approaches’ by the University of Wyoming King Air at rural airfields at first light illustrate how these layers evolve after the surface layer becomes convective.