Wade R. McGillis

Environmental turbulent mixing controls on air-water gas exchange in marine and aquatic systems

[1] Air-water gas transfer influences CO 2 and other climatically important trace gas fluxes on regional and global scales, yet the magnitude of the transfer is not well known. Widely used models of gas exchange rates are based on empirical relationships linked to wind speed, even though physical processes other than wind are known to play important roles. Here the first field investigations are described supporting a new mechanistic model based on surface water turbulence that predicts gas exchange for a range of aquatic and marine processes. Findings indicate that the gas transfer rate varies linearly with the turbulent dissipation rate to the 1/4 power in a range of systems with different types of forcing - in the coastal ocean, in a macro-tidal river estuary, in a large tidal freshwater river, and in a model (i.e., artificial) ocean. These results have important implications for understanding carbon cycling.

The Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds

The Office of Naval Researchs Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Marthas Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over sca...

Hydrological performance of extensive green roofs in New York City: observations and multi-year modeling of three full-scale systems

Green roofs can be an attractive strategy for adding perviousness in dense urban environments where rooftops are a high fraction of the impervious land area. As a result, green roofs are being increasingly implemented as part of urban stormwater management plans in cities around the world. In this study, three full-scale green roofs in New York City (NYC) were monitored, representing the three extensive green roof types most commonly constructed: (1) a vegetated mat system installed on a Columbia University residential building, referred to as W118; (2) a built-in-place system installed on the United States Postal Service (USPS) Morgan general mail facility; and (3) a modular tray system installed on the ConEdison (ConEd) Learning Center. Continuous rainfall and runoff data were collected from each green roof between June 2011 and June 2012, resulting in 243 storm events suitable for analysis ranging from 0.25 to 180 mm in depth. Over the monitoring period the W118, USPS, and ConEd roofs retained 36%, 47%, and 61% of the total rainfall respectively. Rainfall attenuation of individual storm events ranged from 3 to 100% for W118, 9 to 100% for USPS, and 20 to 100% for ConEd, where, generally, as total rainfall increased the per cent of rainfall attenuation decreased. Seasonal retention behavior also displayed event size dependence. For events of 10‐40 mm rainfall depth, median retention was highest in the summer and lowest in the winter, whereas median retention for events of 0‐10 mm and 40C mm rainfall depth did not conform to this expectation. Given the significant influence of event size on attenuation, the total per cent retention during a given monitoring period might not be indicative of annual rooftop retention if the distribution of observed event sizes varies from characteristic annual rainfall. To account for this, the 12 months of monitoring data were used to develop a characteristic runoff equation (CRE), relating runoff depth and event size, for each green roof. When applied to Central Park, NYC precipitation records from 1971 to 2010, the CRE models estimated total rainfall retention over the 40 year period to be 45%, 53%, and 58% for the W118, USPS, and ConEd green roofs respectively. Differences between the observed and modeled rainfall retention for W118 and USPS were primarily due to an abnormally high frequency of large events, 50 mm of rainfall or more, during the monitoring period compared to historic precipitation patterns. The multi-year retention rates are a more reliable estimate of annual rainfall capture and highlight the importance of long-term evaluations when reporting green roof performance.

Gas diffusion through columnar laboratory sea ice: implications for mixed‐layer ventilation of CO2 in the seasonal ice zone

Gas diffusion through the porous microstructure of sea ice represents a pathway for ocean.atmosphere exchange and for transport of biogenic gases produced within sea ice. We report on the experimental determination of the bulk gas diffusion coefficients, D, for oxygen (O2) and sulphur hexafluoride (SF6) through columnar sea ice under constant ice thickness conditions for ice surface temperatures between -4 and -12°C. Profiles of SF6 through the ice indicate decreasing gas concentration from the ice/water interface to the ice/air interface, with evidence for solubility partitioning between gas-filled and liquid-filled pore spaces. On average, DSF6 was 1.3 × 10-4 cm2 s-1 (±40%) and DO2 was 3.9 × 10.5 cm2 s-1 (±41%). The preferential partitioning of SF6 to the gas phase, which is the dominant diffusion pathway produced the greater rate of SF6 diffusion. Comparing these estimates of D with an existing estimate of the air.sea gas transfer through leads indicates that ventilation of the mixed layer by diffusion through sea ice may be negligible, compared to air.sea gas exchange through fractures in the ice pack, even when the fraction of open water is less than 1%.

Rain-induced turbulence and air-sea gas transfer

=4 for a range of rain rates with broad drop size distributions. The hydrodynamic measurements elucidate the mechanisms responsible for the rain-enhanced k results using SF6 tracer evasion and active controlled flux technique. High-resolution k and turbulence results highlight the causal relationship between rainfall, turbulence, stratification, and air-sea gas exchange. Profiles of e beneath the air-sea interface during rainfall, measured for the first time during a gas exchange experiment, yielded discrete values as high as 10 �2 Wk g �1 . Stratification modifies and traps the turbulence near the surface, affecting the enhancement of the transfer velocity and also diminishing the vertical mixing of mass transported to the air-water interface. Although the kinetic energy flux is an integral measure of the turbulent input to the system during rain events, e is the most robust response to all the modifications and transformations to the turbulent state that follows. The Craig-Banner turbulence model, modified for rain instead of breaking wave turbulence, successfully predicts the near-surface dissipation profile at the onset of the rain event before stratification plays a dominant role. This result is important for predictive modeling of k as it allows inferring the surface value of e fundamental to gas transfer.

Detecting the Unexpected: A Research Framework for Ocean Acidification

The threat that ocean acidification (OA) poses to marine ecosystems is now recognized and U.S. funding agencies have designated specific funding for the study of OA. We present a research framework for studying OA that describes it as a biogeochemical event that impacts individual species and ecosystems in potentially unexpected ways. We draw upon specific lessons learned about ecosystem responses from research on acid rain, carbon dioxide enrichment in terrestrial plant communities, and nitrogen deposition. We further characterize the links between carbon chemistry changes and effects on individuals and ecosystems, and enumerate key hypotheses for testing. Finally, we quantify how U.S. research funding has been distributed among these linkages, concluding that there is an urgent need for research programs designed to anticipate how the effects of OA will reverberate throughout assemblages of species.

Observations of flow repeatability and secondary circulation in an oscillating grid-stirred tank

An oscillating grid-stirred tank was studied for flow repeatability and the existence of secondary circulations. For the particular tank studied, results indicate that mean flow values may vary by up to 25% and turbulent fluctuations may vary by up to 15% from one run to another. This result was observed to exhibit a potential grid geometry dependence. More importantly, there is evidence of significant flow field sensitivity to initial conditions. Particle image velocimetry results were used to reveal secondary mean flows in the grid-stirred tank. Because these characteristics are believed to be intrinsic to grid-stirred tanks, studies using such tanks must recognize and consider these effects.

Productivity of a coral reef using boundary layer and enclosure methods

to 13.7 mmol O2 m −2 h −1 . Productivity measurements from the enclosure method ranged from −11.0 to 12.9 mmol O2 m −2 h −1 . During the study, the mean hourly difference between the methods was 0.65 mmol O2 m −2 h −1 (r 2 =0 .92), resulting in well‐reconciled estimates of net community production between the boundary layer (−33.1 mmol m −2 d −1 ) and enclosure (−46.3 mmol m −2 d −1 ) techniques. The results of these independent approaches corroborate quantified rates of metabolism at Cayo Enrique Reef. Close agreement between methods demonstrates that boundary layer measurements can provide near real‐time assessments of coral reef health. Citation: McGillis, W. R., C. Langdon, B. Loose, K. K. Yates, and J. Corredor (2011), Productivity of a coral reef using boundary layer and enclosure methods, Geophys. Res. Lett., 38, L03611, doi:10.1029/2010GL046179.

Direct measurements of CO2 flux in the Greenland Sea

[1] During summer 2006 eddy correlation CO2 fluxes were measured in the Greenland Sea using a novel system set‐up with two shrouded LICOR‐7500 detectors. One detector was used exclusively to determine, and allow the removal of, the bias on CO2 fluxes due to sensor motion. A recently published correction method for the CO2‐H2O cross‐correlation was applied to the data set. We show that even with shrouded sensors the data require significant correction due to this cross‐correlation. This correction adjusts the average CO2 flux by an order of magnitude from −6.7 × 10 −2 mol m −2 day −1 to −0.61 × 10 −2 mol m −2 day −1 , making the corrected fluxes comparable to those calculated using established parameterizations for transfer velocity. Citation: Lauvset, S. K., W. R. McGillis, L. Bariteau, C. W. Fairall, T. Johannessen, A. Olsen, and C. J. Zappa (2011), Direct measurements of CO2 flux in the Greenland Sea, Geophys. Res. Lett., 38, L12603, doi:10.1029/2011GL047722.

Bio-optical observations of the 2004 Labrador Sea phytoplankton bloom

[1] A unique time series of moored bio‐optical measurements documented the 2004 spring‐summer bloom in the southern Labrador Sea. In situ and satellite chlorophyll data show that chlorophyll levels in the 2004 bloom were at the upper end of those typically observed in this region. Satellite chlorophyll and profiling float temperature/salinity data show that the main bloom, which typically peaks in June/July, is often preceded by ephemeral mixed layer shoaling and a lesser, short‐lived bloom in May; this was the case in 2004. The particulate backscatter to beam attenuation ratio (bbp[470 nm]/Cp[660 nm]) showed peaks in the relative abundance of small particles at bloom initiation and during the decline of the bloom, while larger particles dominated during the bloom. Chlorophyll/Cp and bbp/chlorophyll were correlated with carbon export and dominated by changes in the pigment per cell associated with lower light levels due to enhanced attenuation of solar radiation during the bloom. An NPZ (nutrients, phytoplankton, zooplankton) model captured the phytoplankton bloom and an early July peak in zooplankton. Moored acoustic Doppler current profiler (ADCP) data showed an additional mid‐June peak in zooplankton biomass which was attributed to egg‐laying copepods. The data reported here represent one of the few moored time series of Cp, bbp and chlorophyll extending over several months in an open ocean region. Interpretation of data sets such as this will become increasingly important as these deployments become more commonplace via ocean observing systems. Moreover, these data contribute to the understanding of biological‐physical coupling in a biogeochemically important, yet poorly studied region.