Characteristics of marine boundary layers during two Lagrangian measurement periods 1. General conditions and mean characteristics

Two sets of Lagrangian measurements were made during the southern Aerosol C haracterization Experiment (ACE1) South of Tasmania, Australia, in December 1995. This paper intends to provide an overview of the general conditions encountered during the two intensive observational periods. The measurement s by the NCAR C-130 provide the main data set for this study. We also use t he sea surface temperature obtained from the R/V Discoverer and the Europea n Centre for Medium-Range Weather Forecasts (ECMWF) analyses field for the large-scale divergence field. Emphases of the paper are on the atmospheric and oceanic environment and the: boundary layer mean structure during the s ix flights in the two Lagrangian measurement periods. The large scale featu res, such as variations of sea surface temperature, synoptic conditions, an d large-scale velocity fields, are discussed. These large-scale environment s had significant influences on boundary layer turbulence and the inversion structure. The boundary layer mean structure and its evolution along the L agrangian trajectory are also studied using two-dimensional cross-section p lots of vertical and horizontal (along the flight track) variation of poten tial temperature, water vapor, wind components, and ozone concentration. Th e most prominent feature of the boundary layer is the two-layered structure observed throughout Lagrangian B and during the last flight of Lagrangian A. The two layers have detectable differences in potential temperature, wat er vapor, and, to a lesser extent, ozone concentration. These differences m ake it necessary to study the exchange between the two layers. Low-level cl oud structure and cloud microphysics are also discussed. We emphasize, thou gh, that the results on cloud fractions should be used with caution due to the variable nature of the cloud bands observed during ACE1.