Evolution of the Marine Boundary Layer During Lagrangian Measurements of ACE-1

Qing Wang, Naval Postgraduate School, Monterey, CA 93943

The NCAR C-130 made three research flights during the Lagrangian B intensive observational period of ACE-1 to measure boundary layer turbulence, thermodynamics, and aerosol properties while following a tagged air column. The evolution of the marine boundary layer and the effects of boundary layer processes on the evolution of marine aerosols are studied using this dataset.

The atmospheric boundary layer observed during Lagrangian B was characterized by a two-layered structure with substantial differences in potential tmperature, moisture, ozone, and aersol concentrations between the two layers. On average, the upper boundary layer was about 3 K warmer, and 2 g/kg drier than the lower boundary layer. In addition, the lower boundary layer was generally better mixed vertically compared to the upper boundary layer. The differences among sounding profiles from the three flights also suggest that mixing occured between the two layers during the two day time period.

Large spatial variations in sea surface temperature were found during the Lagrangian B period. The air temperature was warmer than the sea surface temperature while the air-sea temperature difference increased as the air column moved southward, resulting in increasingly stable thermal stratification in the lower boundary layer. In spite of the stable stratification, turbulent mixing was significant in the lower boundary layer as a result of strong wind shear near the surface. The shear generated turbulent kinetic energy was thus the major mechanism that generate entrainment mixing at the top of the surface-based layer. The entrainment velocity was estimated using the flux method. This parameter was used to quantify the exchange of moisture and aerosols between the upper and the lower boundary layers.