Abstract
Accurately estimating air–water gas exchanges requires considering other factors besides wind speed. These are particularly useful for coastal ocean applications, where the sea-state varies at fine spatial and temporal resolutions. We upgrade FuGas 2.5 with improved formulations of the gas transfer velocity parametrized based on friction velocity, kinetic energy dissipation, roughness length, air-flow conditions, drift current and wave field. We then test the algorithm with field survey data collected in the Baltic Sea during spring–summer of 2014 and 2015. Collapsing turbulence was observed when gravity waves were the roughness elements on the sea-surface, travelling at a speed identical to the wind. In such cases, the turbulence driven transfer velocities (from surface renewal and micro-scale wave breaking) could be reduced from ≈20 cm∙h−1 to ≤ 5 cm∙h−1. However, when peak gravity waves were too flat, they were presumably replaced by capillary-gravity waves as roughness elements. Then, a substantial increase in the turbulence and roughness length was observed, despite the low and moderate winds, leading to transfer velocities up to twice as large as those predicted by empirical u10-based formulations.
Highlights
The dynamics of atmosphere–ocean gas exchanges are fundamental to the Earth’s climate, because the ocean acts as both sink and source of greenhouse gases and dimethyl sulfide (DMS) to the atmosphere
The scientific community became increasingly aware that the accurate estimation of the atmosphere–ocean gas transfer velocities and fluxes requires standard universal formulations
Eng. 2020, 8, 435 by Johnson [73], we developed the FuGas (Flux of Gases) numerical scheme [74,75,76] with over 150 available formulations, some competing, others complementary, allowing the users to tune their model from a wide range of driving factors and degrees of complexity
Summary
The dynamics of atmosphere–ocean gas exchanges are fundamental to the Earth’s climate, because the ocean acts as both sink and source of greenhouse gases and dimethyl sulfide (DMS) to the atmosphere. Integrating all the related knowledge is a complex task, when studying the coastal ocean, where the relevant factors have been demonstrated to vary at shorter spatial and temporal scales Illustrating this challenge, the relative proportion between the transfer velocity due to turbulent surface renewal (kwind hereafter) and to wave breaking (kbubble hereafter) under similar wind velocities depends significantly on the age of gravity waves under aerodynamic rough air-flow [22]. We measured the atmospheric and oceanic variables related with turbulence and commonly used in formulations estimating gas transfer velocities. Time sequences with abrupt changes in wind properties were discarded, assuming that the homogeneity of conditions had been severely violated
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