Atmospheric boundary layer over steep surface waves

Turbulent air-sea interactions coupled with the surface wave dynamics remain a challenging problem. The needs to include this kind of interaction into the coupled environmental, weather and climate models motivate the development of a simplified approximation of the complex and strongly nonlinear interaction processes. This study proposes a quasi-linear model of wind-wave coupling. It formulates the approach and derives the model equations. The model is verified through a set of laboratory (direct measurements of an airflow by the particle image velocimetry (PIV) technique) and numerical (a direct numerical simulation (DNS) technique) experiments. The experiments support the central model assumption that the flow velocity field averaged over an ensemble of turbulent fluctuations is smooth and does not demonstrate flow separation from the crests of the waves. The proposed quasi-linear model correctly recovers the measured characteristics of the turbulent boundary layer over the waved water surface.     

Fragmentation of the “bag-breakup” type as a mechanism of the generation of sea spray at strong and hurricane winds

Experiments on generation mechanisms of sea water spray under hurricane wind were performed using shadow illumination high-speed video recording from several camera positions. Classification of fragmentation mechanism leading to generation of spray was conducted. Statistics of the events causing generation of spray was studied. A phenomenological model was developed based on the methods of statistical physics to describe the “bag-breakup” generation mechanism. The function of spray generation was conducted on this basis, which describes well the experimental data obtained under hurricane winds in the natural and laboratory conditions.     

The coalescence of two rotating sunspots during the emergence of an active region

The rare phenomenon of the coalescence of two rotating sunspots of the same magnetic polarity during the emergence of the active region NOAA 11117 is investigated using data from the SDO space observatory. The coalescing spots rotated in opposite directions. The leading spot which formed from this process rotated counterclockwise with an angular velocity of 4°/h. A possible explanation is presented, based on a model of the emerging, twisted magnetic Ω flux tube that interacts with convective flows as it crosses the convective zone.