Turbulent Boundary Layer Dynamics and its Relationship to Shallow Cumulus Convection over the Central Equatorial Pacific

Eleven research flights of the National Center for Atmospheric Research (NCAR) C-130 aircraft are analyzed in the vicinity of Christmas Island (2°N, 157°W) during August/September 2007 as part of the Pacific Atmospheric Sulfur Experiment (PASE). In addition to sulfur gases DMS and SO₂, and other chemical scalars (O₃ and CCN), standard meteorological variables such as temperature, moisture, and winds were measured in-situ at high rate. The nocturnal and daytime flights of PASE were focused on turbulent flux profiles below the trade wind inversion (observed at a mean altitude of 1350m asl). Vertical profiles and long, level legs from near the surface to the lower free troposphere were used to characterize the mean vertical structure of the marine boundary layer (MBL, typically located up to near cloud base at ~550m asl) and the buffer layer (BuL, the intermittently turbulent and occasionally cloudy layer above.) A budget of turbulent kinetic energy (TKE) exhibited evenly distributed shear production throughout the MBL along with the expected linear profile of buoyancy production. Two loci of approximately equal parts shear production, transport, and buoyancy production sustain TKE in the BuL at levels of ~70% that within the MBL. A mean cloud fraction profile from the experiment evinces a bimodal distribution of trade wind cumuli with a major peak at the top and a secondary peak in the lower third of the BuL, consistent with the picture of shallow convection supplying the bulk of the TKE to this layer, but not uniformly in the vertical. On eight of the eleven flights a low level jet was observed either within or just above the BuL, and contributes significantly to turbulent activity in the BuL. Moreover, observations of the mean profiles of various reactive scalars provide a unique constraint on average BuL mixing time scales. During the four week mission, sea surface temperatures over the region decreased by 1.5K as the advancing cold tongue brought cooler water and higher winds to the equatorial mid Pacific. This large scale phenomenon is shown to be principally responsible for a subsiding trade inversion and the increasing trends observed in TKE and cloud cover over the region during late boreal summer.