This paper describes the results of an experimental and theoretical investigation into the mechanisms by which spume droplets are generated by high winds. The experiments were performed in a high-speed wind-wave flume at friction velocities between 0.8 and 1.5 m s−1 (corresponding to a 10-m wind speed of 18–33 m s−1 under field conditions). High-speed video of the air–water interface revealed that the main types of spray-generating phenomena near the interface are “bag breakup” (similar to fragmentation of droplets and jets in gaseous flows at moderate Weber numbers), breakage of liquid ligaments near the crests of breaking surface waves, and bursting of large submerged bubbles. Statistical analysis of these phenomena showed that at wind friction velocities exceeding 1.1 m s−1 (corresponding to a wind speed of approximately 22.5 m s−1), the main mechanism responsible for the generation of spume droplets is bag breakup fragmentation of small-scale disturbances that arise at the air–water interface under the strong wind. Based on the general principles of statistical physics, it was found that the number of bags arising at the water surface per unit area per unit time was dependent on the friction velocity of the wind. The statistics obtained for the bag breakup events and other data available on spray production through this type of fragmentation were employed to construct a spray generation function (SGF) for the bag breakup mechanism. The resultant bag breakup SGF is in reasonable agreement with empirical SGFs obtained under laboratory and field conditions.

Showing the record strengths and growth-rates, recent hurricanes have highlighted needs for improving forecasts of tropical cyclone intensities most sensitive to models of the air-sea interaction. Especially challenging is the nature of sea-spray supposed to strongly affecting the momentum- and energy- air-sea fluxes at strong winds. Even the spray-generation mechanisms in extreme winds remained undetermined. Basing on high-speed video here we identify it as the bag-breakup mode of fragmentation of liquid in gaseous flows known in a different context. This regime is characterized by inflating and consequent bursting of the short-lived objects, bags, comprising sail-like water films surrounded by massive liquid rims then fragmented to giant droplets with sizes exceeding 500 micrometers. From first principles of statistical physics we develop statistical description of these phenomena and show that at extreme winds the bag-breakup is the dominant spray-production mechanism. These findings provide a new basis for understanding and modeling of the air-sea exchange processes at extreme winds.

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A turbulent airflow with a centerline velocity of 4 m s 21 above 2.5-Hz mechanically generated gravity waves of different amplitudes has been studied in experiments using the particle image velocimetry (PIV) technique. Direct measurements of the instantaneous flow velocity fields above a curvilinear interface demonstrating flow separation are presented. Because the airflow above the wavy water surface is turbulent and nonstationary, the individual vector fields are conditionally averaged sampled on the phase of the water elevation. The flow patterns of the phase-averaged fields are relatively smooth. Because the averaged flow does not show any strongly nonlinear effects, the quasi-linear approximation can be used. The parameters obtained by the flow averaging are compared with the theoretical results obtained within the theoretical quasi-linear model of a turbulent boundary layer above the wavy water surface. The wave-induced pressure disturbances in the airflow are calculated using the retrieved statistical ensemble of wind flow velocities. The energy flux from the wind to waves and the wind-wave interaction parameter are estimated using the obtained wave-induced pressure disturbances. The estimated values of the wind-wave interaction parameter are in a good agreement with the theory.

For species to stay temporally tuned to their environment, they use cues such as the accumulation of degree-days. The relationships between the timing of a phenological event in a population and its environmental cue can be described by a population-level reaction norm. Variation in reaction norms along environmental gradients may either intensify the environmental effects on timing (cogradient variation) or attenuate the effects (countergradient variation). To resolve spatial and seasonal variation in species’ response, we use a unique dataset of 91 taxa and 178 phenological events observed across a network of 472 monitoring sites, spread across the nations of the former Soviet Union. We show that compared to local rates of advancement of phenological events with the advancement of temperature-related cues (i.e., variation within site over years), spatial variation in reaction norms tend to accentuate responses in spring (cogradient variation) and attenuate them in autumn (countergradient variation). As a result, among-population variation in the timing of events is greater in spring and less in autumn than if all populations followed the same reaction norm regardless of location. Despite such signs of local adaptation, overall phenotypic plasticity was not sufficient for phenological events to keep exact pace with their cues—the earlier the year, the more did the timing of the phenological event lag behind the timing of the cue. Overall, these patterns suggest that differences in the spatial versus temporal reaction norms will affect species’ response to climate change in opposite ways in spring and autumn.