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.

In Part I of this study, we used high-speed video to identify “bag breakup” fragmentation as the dominant mechanism by which spume droplets are generated at gale-force and hurricane wind speeds. We also constructed a spray generation function (SGF) for the bag-breakup mechanism. The distinctive feature of this new SGF is the presence of giant (~1000 μm) droplets, which may significantly intensify the exchange between the atmosphere and the ocean. In this paper, Part II, we estimate the contribution of the bag-breakup mechanism to the momentum and enthalpy fluxes, which are known to strongly affect the development and maintenance of hurricanes. We consider three contributions to the spray-mediated aerodynamic drag: 1) “bags” as obstacles before fragmentation, 2) acceleration of droplets by the wind in the course of their production, and 3) stable stratification of the marine atmospheric boundary layer due to levitating droplets. Taking into account all of these contributions indicates a peaking dependence of the aerodynamic drag coefficient on the wind speed, which confirms the results of field and laboratory measurements. The contribution of the spray-mediated flux to the ocean-to-atmosphere moist enthalpy is also estimated using the concept of “reentrant spray,” and the equation for the enthalpy flux from a single droplet to the atmosphere is derived from microphysical equations. Our estimates show that a noticeable increase in the enthalpy exchange coefficient at winds exceeding 30–35 m s⁻¹ is due to the enhancement of the exchange processes caused by the presence of giant droplets originating from bag-breakup fragmentation.

This work is devoted to a theoretical study of the hydrodynamic instability of the water–air interface. The development of this instability may result in “bag-breakup” fragmentation, which is one of the main sources of droplets in hurricane winds. A hypothesis according to which the initial water surface elevations subjected to fragmentation are formed by the hydrodynamic instability of disturbances of the wind drift current in the water is proposed. The weakly nonlinear stage of instability in the form of a resonant three-wave interaction is considered. It is found that the nonlinear resonant interaction of a triad of wind drift perturbations, one wave of which is directed along the flow and two other waves are directed at an angle to the flow, leads to an explosive increase in amplitudes. As part of the piecewise continuous model of the drift current profile, the characteristic time and spatial scales of disturbances are found and it is shown that their characteristic dependences on the air friction velocity agree with previously obtained experimental data.

Runoff is one of the most important components of the hydrological cycle, and having complete series of runoff data is essential for any hydrological modelling process. This study aims to estimate the runoff at Al-Jawadiyah hydrometric station using artificial neural networks. This study used only the runoff data at Al-Jawadiyah station in addition to the runoff values measured at Al-Amiri station on the Syrian-Lebanese border. Many experiments were conducted and a very large number of artificial neural networks were trained with changing the number of hidden layers, the number of neurons and the training algorithms until the best network was reached according to the regression criteria and the root mean of the error squares between the measured values and the predicted values, and the network (2:12:1) was adopted in the process of filling the gaps in the runoff time series at Al-Jawadiyah station during the study period. This study recommends working on preparing complete series of hydrological and climatic measurements that form a basis for preparing an accurate hydrological model for the study area.

Выполнено исследование влияния ранее обнаруженного явления генерации брызг за счёт дробления типа “парашют” на обмен импульсом в атмосферном пограничном слое над морской поверхностью при ураганных ветрах на основе результатов лабораторного эксперимента. Показано, что аэродинамическое сопротивление определяется вкладом трёх факторов: во-первых, сопротивление, оказываемое куполами “парашютов” как препятствиями; во-вторых, ускорение воздушным потоком капель, образующихся при дроблении. Третий фактор связан со стратификацией приводного слоя атмосферы за счёт присутствия левитирующих капель. Комбинация всех трёх факторов приводит к немонотонной зависимости коэффициента аэродинамического сопротивления от скорости ветра, подтверждающей результаты натурных и лабораторных измерений.