Thermodynamic fluxes in the undisturbed tropical marine boundary layer

The flux-profile relationships of thermodynamic variables and several chemical species were studied using an instrumented aircraft in the undisturbed tropical convective boundary layer (CBL). The Pacific Atmospheric Sulfur Experiment (PASE) is a comprehensive study of sulfur oxidation over the central equatorial Pacific Ocean over open waters near Christmas Island (2°N, 157°W) that took place during August 2007. During this time the Intertropical Convergence Zone (ITCZ) was north of the study area, meaning that the aircraft was flown in southeasterly trade winds and relatively pollution-free southern hemispheric air. The study area was intentionally positioned in clean, Southern Hemisphere air and close to the equatorial cold tongue (SSTs around 27°C) to capture the relatively high dimethyl sulfide (DMS) flux out of the water. DMS is important due to its ability to seed clouds from cloud condensation nuclei (CCN) through photochemical reactions (DMS → SO2 → SO4 2- → CCN).;Accurate and high-speed airborne measurements were made of temperature, pressure, wind velocity, water vapor, ozone, and sulfur species. Measurements were taken on horizontal flight legs at multiple vertical levels from 30 meters to the top of the boundary layer, typically 500--600 meters. These quantities were measured at sufficient resolution to determine vertical fluxes through eddy correlation. The vertical variation of fluxes provides valuable information about the physics and chemistry of the CBL, as well as the air/sea exchange of heat, momentum and gases. The vertical wind and thermodynamic profiles were calculated using both the mean values from the horizontal flight legs, and ascent/descent profiles made by the aircraft. The diurnal variation of the fluxes was estimated by separating fluxes into time of day bins. The thermodynamic variables did not show diurnal variation, while the chemical species (ozone, dimethyl sulfide, and sulfur dioxide) did show significant diurnal variation, consistent with photochemistry. The flux profiles were also extrapolated downward to the surface, allowing comparison with bulk parameter estimates of air/sea exchange at the standard 10 meter height. Values of the wind speeds ranged from about 6-9 m/s at 10 meters above sea level. The level of agreement of the extrapolated fluxes versus the bulk parameters is investigated. The weakly unstable state of the atmosphere (zi/L values averaging -5.3) is also investigated with respect to the bulk parameters. The virtual potential temperature budgets were in balance for early morning research flights (RF) 6 and 13, but had positive residuals between 1 and 2.5°C per day for all other daylight flights. A daytime radiative convergence is discussed to close these budgets. Water vapor budgets were not in balance and the residuals are random, however tendencies in the water vapor fluxes are shown to be small compared to the ambient water vapor concentrations.