A Laboratory Investigation of Spume Generation in High Winds for Fresh and Seawater

Data are archived at the University of Miami repository under the name Spray Concentration Measurements from ASIST for Freshwater and Seawater.The article of record as published may be found at https://doi.org/10.1029/ 2019JD030928Given spume's role in mediating air‐sea exchange at the base of tropical cyclones or other storm events, the focus of studies on spray dynamics has been within the marine environment. In contrast, spume production in nonseawater bodies has been underexplored and potential differences between sea and freshwater are neglected. The laboratory remains the primary means for directly observing spray processes near the surface because of the challenges to making robust field measurements. There is no standardization on the water type used for these experiments, and the effect this has on the 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. We have conducted a laboratory experiment that aims to address this simple, yet overlooked, question of whether water type impacts the spume droplet concentration entrained in the air flow above actively breaking waves. We compared directly imaged concentrations for fresh and seawater droplets produced in 10‐m equivalent winds from 36–54 m/s. 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. Our statistical analysis of these findings suggests significant differences in the size‐ and height‐dependent distributions response to increased wind forcing between fresh and seawater. These unexpected findings suggest an unanticipated role of the source water physiochemical properties on the spume generation mechanism.NSFONRThis work was funded by NSF through Grant 0933943. Additional support was provided through ONR phase‐resolved wave program under Grant N00014141064

Observations of a coastal buoyant jet with HF Doppler radar

Currents on the inner shelf respond to a combination of forcing by intrusions of ocean fronts, eddies and internal waves as well as winds, surface gravity waves and topographic rectifications. To quantify the effect that the interaction of these processes has on inner shelf flows, a shore-based, 25.4 MHz (High Frequency) Ocean Surface Current Radar (OSCR), operated by the Radar Ocean Sensing Laboratory (ROSL) of the University of Miami, was deployed to measure the surface current vector field at high spatial and temporal resolution over the inner and mid shelf. Surface currents were mapped in near-real time over a domain bounded in the south by the US Army Waterways Experiment Station's Field Research Facility along the central mooring array of the Coastal Ocean Processes (CoOP) shelf experiment and extended about 27 km northward to include two additional CoOP transects. The surface current vectors were measured at a spatial resolution of about 1 km every 20 minutes during October 1994. Over the experimental period of about 30 days, quality data were acquired 96% of the time extending to the maximum theoretical range of 44 km. Time series or the surface current vectors were in close agreement with near-surface measurements from moored current meters. The surface current maps revealed numerous surface current features. One such feature was the buoyant coastal jet formed by the Chesapeake Bay outflow. A five day period when the buoyant jet was evident in the OSCR generated surface current maps is described to demonstrate the capabilities of HF radar to infer dynamical properties of coastal flow features from measured velocity fields

Wavelet analysis of surface current vector fields measured by high frequency Doppler radar

Fourier spectral methods have been widely applied to coastal zone current measurements. However in cases such as riverine tides or estuarine outflow currents exhibit non-stationary characteristics which invalidate the basic assumptions of these methods. Wavelet analysis techniques can be used to determine the temporal evolution of current variance over a range of frequency scales and therefore can provide an improved understanding of event-driven dynamics. Morlet continuous-wavelet transforms were applied to multiple vector time-series measurements from a High Frequency (HF) Doppler radar and moored ADCPs near the mouth of Chesapeake Bay in 1996 and 1997 as well as wind measurements at the Chesapeake Light tower. The time-varying clockwise (CW) and counter-clockwise (CCW) wavelet spectra were computed from each vector time-series. The horizontal, vertical and temporal evolution of high energy scales could then be visualized. Significant short-term intensifications of 30-60 hour CW energy in the region of the outfall plume were observed that were highly coherent with local wind forcing

Refraction and shoaling of surface waves by currents and topography as observed by HF radars

Phased-array Doppler radars were deployed immediately south of the mouth of the Chesapeake Bay during the COPE-3 experiment in the fall of 1997. The radars collected the Doppler spectra and extracted surface current vectors for a period of 45 days. Significant wave height and peak period estimates were obtained from the Doppler spectra using the method originally developed by Barrick (1977). The study region was strongly impacted by the outflow from the Chesapeake Bay. The buoyant plume that emanated from the bay during ebb tides was observed in the surface current maps produced by the HF radars. Significant surface current shear existed at the boundary where the buoyant plume overrode denser shelf water, resulting in a convergence frontal zone. The refraction and shoaling of the surface waves was observed as they propagated over these regions. The current-induced shoaling of incident waves was isolated from the effects of local topography. The location of significant wave height growth and dissipation was found to be dependent upon the tidal stage and the wind and wave direction. Regional remote sensing of both waves and currents was necessary to identify these high energy regions which are of considerable interest for studies of mixing at the estuarine front as well as for the safe maritime operations

HF Radar Observation of Wave Directional Spectra in a Strong Current Regime

Dual Wellen HF Radar (WERA) systems have been observing near-surface currents and wave parameters over the Southeast Florida shelf since June 2004 as a part of the Southeast Atlantic Coastal Ocean Observing System (SEACOOS). The region of coverage includes the Florida Straits and the Florida Current (FC) which typically has maximum surface velocities approaching 2 ms"1. The echo-Doppler spectra are also routinely recorded and archived at both stations which allows post-processing to extract surface wave directional spectra using an iterative approach as implemented in Seaview Sensing reg software. Both WERA sites operated continuously during the passage of Hurricane Jeanne over the Florida Straits on 25 Sept 2004. Although it passed ~ 200 km to the north of the measurement domain, the local mean winds exceeded 20 ms -1 and rotated over 270deg. The near-surface currents during the passage of Hurricane Jeanne reflected the influence of the wind as well as the Florida Current. The effect of the wind on the near surface flow was seen in easterly and southerly flow over the shallow shelves near Florida and the Bahamas respectively as well as relatively slow flow in the center of the Florida current. Maximum current velocities were only 130 cms -1 50-60 cms -1 less than typical values. The interaction of these wave fields from differing directions with the high lateral shear of the western edge of the Florida Current was observed every 10 minutes.. The wind-wave component of the spectrum was observed to respond rapidly to the rotating wind-field, but effects of the horizontal shear were observed in the off-wind angle of the wind-wave peak. Tower frequencies were often observed at large angles to the local wind

An EOF analysis of HF Doppler radar current measurements of the Chesapeake Bay buoyant outflow

Surface currents measured by HF Doppler radar as part of a study of the Chesapeake Bay outflow plume are examined using a ‘real-vector’ empirical orthogonal function (EOF) analysis (Kaihatu et al., 1998). Based on about 23 days of nearly continuous data, the analysis shows that the first three EOF modes, judged to be the only significant modes, account for 76% of the variance in the data set. The buoyant outflow occurs primarily in the mean flow field. The first EOF mode is dominated by wind forcing and the second mode by across-shelf semi-diurnal tidal forcing. The third mode exhibits a large-scale horizontal shear and contains a curved region of weak relative flow which appears to delineate the offshore edge of the plume; also, the third-mode response varies over the spring-neap cycle, suggesting a modulation of the outflow plume by a tidal residual eddy. The analysis therefore has provided a useful, exploratory examination of this dataset of surface currents

The spatial-temporal variability of air-sea momentum fluxes observed at a tidal inlet

Coastal waters are an aerodynamically unique environment that has been little explored from an air-sea interaction point of view. Consequently, most studies must assume that open ocean-derived parameterizations of the air-sea momentum flux are representative of the nearshore wind forcing. Observations made at the New River Inlet in North Carolina, during the Riverine and Estuarine Transport experiment (RIVET), were used to evaluate the suitability of wind speed-dependent, wind stress parameterizations in coastal waters. As part of the field campaign, a small, agile research vessel was deployed to make high-resolution wind velocity measurements in and around the tidal inlet. The eddy covariance method was employed to recover direct estimates of the 10 m neutral atmospheric drag coefficient from the three-dimensional winds. Observations of wind stress angle, near-surface currents, and heat flux were used to analyze the cross-shore variability of wind stress steering off the mean wind azimuth. In general, for onshore winds above 5 m/s, the drag coefficient was observed to be two and a half times the predicted open ocean value. Significant wind stress steering is observed within 2 km of the inlet mouth, which is observed to be correlated with the horizontal current shear. Other mechanisms such as the reduction in wave celerity or depth-limited breaking could also play a role. It was determined that outside the influence of these typical coastal processes, the open ocean parameterizations generally represent the wind stress field. The nearshore stress variability has significant implications for observations and simulations of coastal transport, circulation, mixing, and general surf-zone dynamics.Hydraulic EngineeringCivil Engineering and Geoscience