Abstract
Understanding turbulent fluxes of momentum, mass, and energy across the air-sea boundary are fundamental to our ability to model and parameterize a number of multidimensional geophysical processes, such as wind-wave generation, oceanic circulation, and air-sea gas transfer. The physical nature of the near surface boundary layer remains less known, especially under high winds due to the development of an intermediate substrate layer of large spray droplets known as spume, between the atmosphere and ocean surface. Presence of these spume droplet effects the aerodynamic resistance of the prevailing winds over the surface and thus the behavior of surface drag coefficient. The size-dependent vertical distribution of spume particles in high wind conditions is necessary to understand their effect on air-sea fluxes of heat and momentum. Given spume’s role in mediating air-sea exchange at the base of tropical cyclones or other storm events, the predominant focus of present literature studies on spray dynamics has been within the marine environment. In contrast, spume production in non-seawater bodies have not been extensively studied and potential differences between sea and freshwater are neglected. Thus any significant differences between sea and freshwater remain unquantified. Direct measurements of the physical processes happening at this interface remains scarce till date due to difficulty in making robust measurements in the field. Laboratories on the other hand remains the primary means for directly observing spray processes near the surface, and offers promising aspects for improving our understanding by learning these processes in a controlled environment. There is no standardization on the water type used for these experiments and any potential effects water masses have on the spume generation process is unknown. This adds uncertainty in our ability to make physically realistic spume generation functions that are ultimately applied to the geophysical domain. To address this gap, we have conducted a series of laboratory experiment at the Air-Sea Interaction tank facility (ASIST) of the University of Miami, directly comparing spume concentrations, and surface drag coefficient behavior above fresh and real seawater for 10-m equivalent wind speeds up to 54 m/s. Direct measurements of the near-surface processes were made and directly related to local sources of variance. Droplets in the air above the intensely breaking wind-waves were optically observed and their distribution as functions of wind speed, height, and droplet radius was compared between the two water types. Drag coefficient was calculated using the eddy covariance method on the three-dimensional wind data observed using a sonic anemometer. Our results show significant differences in the spume generation as well as in the surface drag coefficient behavior for the two water types. Substantially higher concentrations of seawater spume were observed as compared to freshwater across all particle sizes and wind speeds. The seawater particles’ vertical distribution was concentrated near the surface, whereas the freshwater droplets were more uniformly distributed. Statistical analysis of these findings suggest significant differences in the size- and height-dependent distribution response to increased wind forcing between fresh and seawater. Drag coefficient values for seawater were found less than that of freshwater at all wind speeds suggesting modulation of momentum fluxes in the near surface layer due to the presence of spray droplets. These findings were generally unexpected and point to an unanticipated role of physiochemical processes in the spume generation mechanism which may impact spray-mediated flux parameterization over water bodies of different salinities. This body of work represents a multi-faceted approach to understanding physical air-sea interactions in varied regimes and using a wide array of investigatory methods.