Multiscale Dynamics of the Tropical Marine Boundary Layer and its Effect on Cloudiness Over the Equatorial Pacific

Aircraft observations over the central equatorial Pacific are analyzed to elucidate the relationships among turbulent kinetic energy (TKE), sea surface temperature (SST), wind speed, turbulence mixing, and cloudiness over scales ranging from the atmospheric boundary-layer depth to the mesoscale ripples of the oceanic cold tongue that inundated the region over the course of the month long field campaign. Fourteen research flights were conducted with the National Center for Atmospheric Research (NCAR) C-130 near Christmas Island (2°N, 157°W) during August-September 2007 as part of the Pacific Atmospheric Sulfur Experiment (PASE). In addition to the sulfur gases DMS and SO₂, and ozone, standard meteorological variables were measured at high rate in a systematic manner within a fixed region. Vertical profiles from near the surface to above the trade wind inversion (observed at a mean altitude of 1350 m) are used to characterize the mean vertical structure and energetics of the marine boundary layer (MBL, typically located up to cloud base at ~550m amsl), and the buffer layer (BuL, the intermittently turbulent layer lying between the MBL and trade inversion.) A strong anticorrelation is found between the surface wind speed and SST indicative of air-sea interaction on extra-MBL scales.

The TKE budget reveals an overall parity in MBL turbulence generation between the expected linear profile of buoyancy and shear production that is distributed nearly evenly throughout the mixed layer above its usual maximum in the surface layer. Similarly, equal parts shear and buoyancy production sustain TKE in the BuL at levels approximately 70% the MBL TKE. On most flights a low level jet was observed either within or just above the BuL providing the elevated layer with a significant source of TKE. Estimates of the MBL scalar eddy diffusivity obtained from observations of the mean DMS gradients and fluxes are presented and their non-dimensionalized form exhibits an excellent correlation with |z_i/L|. Observed profiles of reactive scalars (O₃ and DMS) with known chemical lifetimes are used to derive a mixing time scale within the BuL of approximately 30 hours. Finally, because the observed cloud fraction appears to be controlled by the strength of cold air advection and the free tropospheric stability, we suggest that wind shear may play an important role in modulating trade cumulus abundance in the tropics.