The Chemistry And Dynamics Of Marine Stratocumulus PDF Download

Energy exchange between the tropical oceans and the atmosphere plays an important role in the climate of the planet. By far the most abundant form of this transfer occurs in regions of shallow (generally non-precipitating) convection that takes place underneath the gentle lid of the trade wind inversion. Understanding the atmospheric dynamics and exchange of chemical species between the ocean and atmosphere in this region is a critical step on the path to accurate modeling of the earth's climate. Fourteen research flights were conducted with the National Center for Atmospheric Research (NCAR) C-130 near Christmas Island (2°N, 157°W) during the summer of 2007 as part of the Pacific Atmospheric Sulfur Experiment (PASE). Vertical profiles from near the surface to above the trade wind inversion (observed at a mean altitude of 1350m) were used to characterize the vertical structure of the dynamic and chemical scalars within the marine boundary layer. The marine atmosphere was divided into four levels, a surface layer (~30m) overlain by a turbulent mixed layer which extends to ~550m, overlain by a less turbulent buffer layer (BuL), extending to ~1350m, overlain by the free troposphere (FT). The SL and the mixed layer together constitute the marine boundary layer (MBL). Separating the MBL and the BuL is a weak inversion with a much stronger inversion separating the BuL from the FT. This work focuses on dimethyl sulfide (DMS), ozone (O3) and the boundary layer dynamics of the region. In the MBL, DMS and O3 both exhibited the well-known diurnal cycle of buildup at night followed by daytime destruction. DMS ranged from 50-95 pptv in the daytime to 90-110 pptv at night and O3 from 16-18 ppb during the daytime to 17-21 ppb at night. Contributions from horizontal advection are included using a multivariate regression of the observed mixing ratio as a function of time and space within the MBL to estimate the mean gradients and trends. With this technique we can use the residual term in the budget as an estimate of overall photochemical oxidation. Error analysis of the various terms in the DMS budget indicate that chemical losses acting on time scales of up to 110 hours can be inferred with this technique. On average, photochemistry accounted for ~ 7.4 ppt hr−1 loss rate for the seven daytime flights, with an estimated error of 0.6 ppt hr−1. The loss rate due to expected OH oxidation is sufficient to explain the net DMS destruction without invoking the action of additional oxidants (e.g., reactive halogens). The observed ocean flux of DMS averaged 3.1 (± 1.5) [mu]mol m−2d−1, and generally decreased throughout the sunlit hours. Averaged over the mission, horizontal advection was negligible in the DMS budget but was significant in the budgets of individual flights. The ozone budget included the same dynamical terms as the DMS budget but also included loss to photolysis, OH and HO2. Photolysis is the dominant chemical sink (~ 0.29 ppb/hour). Horizontal advection and vertical flux divergence contribute similar amounts to the budget (0.08 ppb hr−1, 0.06 ppb hr−1). The advective source is consistent with the picture from the Total Ozone Mapping Spectrometer (TOMS) indicating higher levels of ozone upwind from the PASE region. The entrainment flux from the FT to the BuL was estimated at 0.07 ppb m s−1. A budget of turbulent kinetic energy (TKE) exhibited evenly distributed shear production throughout the MBL along with an 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 confirms 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. Surface latent heat fluxes were measured by eddy covariance and were on average found to be 30% less the standard NOAA bulk model. The Bowen ratio averaged 0.05 with very little flight to flight variability (± 0.03). The observed east-southeasterly winds averaged 8 ms−1 (at 10 meters) in this region feeding into the ITCZ located at approximately 10 degrees N. On most flights a low level jet was observed either within or just above the BuL. During the four week mission, SST over the entire region decreased by 1.5C as a tropical instability wave brought colder water to the equatorial mid Pacific with winds surface winds increasing by 0.5 m s−1 during the experiment. The shape of the cospectra between vertical wind speed and potential temperature exhibited the traditional Kaimal form; however, water vapor and DMS cospectra exhibited less power at the highest frequencies with their cospectral peaks shifted toward larger scales.