In inland water such as lakes, reservoirs, and ponds, the gas exchange of slightly soluble gases such as carbon dioxide, dimethyl sulfide, methane, or oxygen across a clean and nearly flat air-water interface is routinely described using a water-side mean gas transfer velocity , where overline indicates time or ensemble averaging. The conventional micro-eddy surface renewal model predicts , where is the molecular Schmidt number, is the water kinematic viscosity, and is the waterside mean turbulent kinetic energy dissipation rate at or near the interface. While between 0.39 and 0.46 has been reported across some data sets, others report large scatter or variability around this value range.
Gabriel G. Katul,
Duke University
It is shown here that this scatter can be partly explained by high temporal variability in instantaneous around its mean value, a mechanism that was not previously considered. As the coefficient of variation in increases, must be adjusted by a multiplier that was derived from a log-normal model for the probability density function of . Reported variations in in the macro-scale Reynolds number can also be partly attributed to intermittency effects in . Such intermittency is characterized by the long-range (i.e. power-law decay) spatial auto-correlation function of . That varies with a macro-scale Reynolds number does not necessarily violate the micro-eddy model. Instead, it points to a coordination between the macro- and micro-scales arising from the transfer of energy across scales in the energy cascade. This coordination suggests that the macro-scale eddies energize and deliver the micro-scale eddies to the air-water interface, but the actual mass exchange process is dominated by frequent collisions of micro-eddies with the interface.
Gabriel G. Katul received his BE in 1988 at the American University of Beirut (Beirut, Lebanon), his MS in 1990 at Oregon State University (Corvallis, OR) and his doctoral degree in 1993 at the University of California in Davis (Davis, CA). He holds the George Pearsall distinguished professorship at the Department of Civil and Environmental Engineering at Duke University (Durham, NC). He was a visiting fellow at University of Virginia (USA) in 1997, the Commonwealth Science and Industrial Research Organization (Australia) in 2002, the University of Helsinki (Finland) in 2009, the FulBright-Italy Distinguished Fellow at Politecnico di Torino (Italy) in 2010, the École polytechnique fédérale de Lausanne (Switzerland) in 2013, Nagoya University (Japan) in 2014, the University of Helsinki (Finland) visiting fellow in 2017, the Karlsruher Institute for Technology (Germany) in 2017, Princeton University (USA) in 2020, and CzechGlobe (Brno – Czech Republic) in 2023. He received several honorary awards, including the inspirational teaching award by the students of the School of the Environment at Duke University (in 1994 and 1996), an honorary certificate by La Seccion de Agrofisica de la Sociedad Cubana de Fisica in Habana (in 1998), the Macelwane medal and became thereafter a fellow of the American Geophysical Union (AGU, in 2002), the editor’s citation for excellence in refereeing from the AGU (in 2008), the Hydrologic Science Award from the AGU (in 2012), the John Dalton medal from the European Geosciences Union (in 2018), the Outstanding Achievements in Biometeorology Award from the American Meteorological Society (AMS, in 2021), was elected as a fellow of the AMS (in 2024), and the Hydrologic Science Medal from the AMS (in 2025). Katul was elected to the US National Academy of Engineering (in 2023) for his contributions in eco-hydrology and environmental fluid mechanics. He served as the Secretary General for the Hydrologic Science Section at the AGU (2006-2008). His research focuses on micro-meteorology and near-surface hydrology with emphasis on heat, momentum, carbon dioxide, water vapor, ozone, particulate matter (including aerosols, pollen, and seeds) and water transport in the soil-plant-atmosphere system as well as their implications to a plethora of hydrological, ecological, atmospheric and climate change related problems. He is currently serving as one of the editors-in-chief of a new journal published by the Academic Research Alliance (ARC) titled “ARC-Geophysical Research” that seeks to reduce monetary considerations in academic publishing.