Revised Estimates of Ocean Surface Drag in Strong Winds

Air-sea drag governs the momentum transfer between the atmosphere and the ocean, and remains largely unknown in hurricane winds. We revisit the momentum budget and eddy-covariance methods to estimate the surface drag coefficient in the laboratory. Our drag estimates agree with field measurements in low-to-moderate winds, and previous laboratory measurements in hurricane-force winds. The drag coefficient saturates at $2.6 \times 10^{-3}$ and $U_{10} \approx 25\ m\ s^{-1}$, in agreement with previous laboratory results by Takagaki et al. (2012). During our analysis, we discovered an error in the original source code used by Donelan et al. (2004). We present the corrected data and describe the correction procedure. Although the correction to the data does not change the key finding of drag saturation in strong winds, its magnitude and wind speed threshold are significantly changed. Our findings emphasize the need for an updated and unified drag parameterization based on field and laboratory data.Comment: 13 pages, 5 figure

Tunable Narrow Band Difference Frequency THz Wave Generation in DAST via Dual Seed PPLN OPG

We report a widely tunable narrowband terahertz (THz) source via difference frequency generation (DFG). A narrowband THz source uses the output of dual seeded periodically poled lithium niobate (PPLN) optical parametric generators (OPG) combined in the nonlinear crystal 4-dimthylamino-N-methyl-4-stilbazolium-tosylate (DAST). We demonstrate a seamlessly tunable THZ output that tunes from 1.5 THz to 27 THz with a minimum bandwidth of 3.1 GHz. The effects of dispersive phase matching, two-photon absorption, and polarization were examined and compared to a power emission model that consisted of the current accepted parameters of DAST

Is the State of the Air-Sea Interface a Factor in Rapid Intensification and Rapid Decline of Tropical Cyclones?

Tropical storm intensity prediction remains a challenge in tropical meteorology. Some tropical storms undergo dramatic rapid intensification and rapid decline. Hurricane researchers have considered particular ambient environmental conditions including the ocean thermal and salinity structure and internal vortex dynamics (e.g., eyewall replacement cycle, hot towers) as factors creating favorable conditions for rapid intensification. At this point, however, it is not exactly known to what extent the state of the sea surface controls tropical cyclone dynamics. Theoretical considerations, laboratory experiments, and numerical simulations suggest that the air-sea interface under tropical cyclones is subject to the Kelvin-Helmholtz type instability. Ejection of large quantities of spray particles due to this instability can produce a two-phase environment, which can attenuate gravity-capillary waves and alter the air-sea coupling. The unified parameterization of waveform and two-phase drag based on the physics of the air-sea interface shows the increase of the aerodynamic drag coefficient with wind speed up to hurricane force ( m s−1). Remarkably, there is a local minimum—“an aerodynamic drag well”—at around m s−1. The negative slope of the dependence on wind-speed between approximately 35 and 60 m s−1favors rapid storm intensification. In contrast, the positive slope of wind-speed dependence above 60 m s−1 is favorable for a rapid storm decline of the most powerful storms. In fact, the storms that intensify to Category 5 usually rapidly weaken afterward

The Air-Sea Interface and Surface Stress Under Tropical Cyclones

Tropical cyclone track prediction is steadily improving, while storm intensity prediction has seen little progress in the last quarter century. Important physics are not yet well understood and implemented in tropical cyclone forecast models. Missing and unresolved physics, especially at the air-sea interface, are among the factors limiting storm predictions. In a laboratory experiment and coordinated numerical simulation, conducted in this work, the microstructure of the air-water interface under hurricane force wind resembled Kelvin-Helmholtz shear instability between fluids with a large density difference. Supported by these observations, we bring forth the concept that the resulting two-phase environment suppresses short gravity-capillary waves and alters the aerodynamic properties of the sea surface. The unified wave-form and two-phase parameterization model shows the well-known increase of the drag coefficient (Cd) with wind speed, up to ~30 ms−1. Around 60 ms−1, the new parameterization predicts a local peak of Ck/Cd, under constant enthalpy exchange coefficient Ck. This peak may explain rapid intensification of some storms to major tropical cyclones and the previously reported local peak of lifetime maximum intensity (bimodal distribution) in the best-track records. The bimodal distribution of maximum lifetime intensity, however, can also be explained by environmental parameters of tropical cyclones alone

Sea Spray Generation Function in Major Tropical Cyclones

Sea spray is a factor in thermodynamics, intensity, and intensification of tropical cyclones. However, the sea spray generation function under major tropical cyclone conditions is still virtually unknown and the scatter of data between different field experiments is significant. In this work we have conducted a computational fluid dynamics experiment using the approach that has been partially verified with data from the air-sea interaction facility SUSTAIN. In the computational model, the sea spray generation function has been studied using the Volume of Fluid (VOF) method. This method is enhanced with a Volume of Fluid to Discrete Phase transition model (VOF to DPM). Due to dynamic remeshing, VOF to DPM resolves spray particles ranging in size from tens of micrometers to a few millimeters (spume). The water particles that satisfy the condition of asphericity are converted into Lagrangian particles involved in a two-way interaction with the airflow. The size distribution of non-spherical spray particles is represented by the equivalent radii calculated from the particle mass. The sea spray generation function has been calculated for category 1, 3, and 5 tropical cyclones. A comparison with the data available from literature for a category 1 tropical cyclone shows that our sea spray generation function is close to those found by Zhao et al. (2006) and Troitskaya et al. (2018) for the radius range of spume. Our sea spray generation function results in the spray-induced stress exceeding the interfacial wind stress at approximately 60 m/s wind speed. Connection of spray-induced enthalpy flux to the sea spray generation function is more complicated due to the suspension and evaporation of small-size particles in the turbulent boundary layer (Richter’s and Peng 2019 effect of negative feedback)

Wave energy level and geographic setting correlate with Florida beach water quality

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in Marine Pollution Bulletin 104 (2016): 54-60, doi:10.1016/j.marpolbul.2016.02.011.Many recreational beaches suffer from elevated levels of microorganisms, resulting in beach advisories and closures due to lack of compliance with Environmental Protection Agency guidelines. We conducted the first statewide beach water quality assessment by analyzing decadal records of fecal indicator bacteria (enterococci and fecal coliform) levels at 262 Florida beaches. The objectives were to depict synoptic patterns of beach water quality exceedance along the entire Florida shoreline and to evaluate their relationships with wave condition and geographic location. Percent exceedances based on enterococci and fecal coliform were negatively correlated with both long-term mean wave energy and beach slope. Also, Gulf of Mexico beaches exceeded the thresholds significantly more than Atlantic Ocean ones, perhaps partially due to the lower wave energy. A possible linkage between wave energy level and water quality is beach sand, a pervasive nonpoint source that tends to harbor more bacteria in the low-wave-energy environment.This work is funded by the NSF-NIEHS Oceans and Human Health Program (NIEHS # P50 ES12736 and NSF #OCE0432368/0911373/1127813)

Modification of Turbulence at the Air-Sea Interface Due to the Presence of Surfactants and Implications for Gas Exchange. Part I: Laboratory Experiment

The air-sea gas transfer of gases like CO2 is substantiallydetermined bythe properties of the aqueous diffusion sublayer and free-surface turbulent boundarylayer. Little is known about the effect of surfactants on turbulence in the near-surface layer of the ocean. In order to investigate the effect of surfactants on turbulent exchanges below the air-sea interface, we have conducted a series of laboratoryexperiments at the UM RSMAS Air-Sea Interaction Saltwater Tank (ASIST) facility. Results from these experiments demonstrate that the surfactant monolayer suppresses turbulence and reduces drag below the water surface and increases the surface drift velocity. This effect is important for parameterization of the interfacial component of gas exchange under low wind speed conditions. From the theoretical standpoint, the mechanism of the turbulence reduction can be explained bythe modification of the “streaks” in the buffer zone near the interface byvisco-elastic properties of the water surface when surfactants are present. These findings are consistent with results from high-resolution non-hydrostatic numerical simulations presented in a companion paper.https://nsuworks.nova.edu/occ_facbooks/1051/thumbnail.jp

Wind Speed Dependence of Single-Site Wave-Height Retrievals from High-Frequency Radars

Wave-height observations derived from single-site high-frequency (HF) radar backscattered Doppler spectra are generally recognized to be less accurate than overlapping radar techniques but can provide significantly larger sampling regions. The larger available wave-sampling region may have important implications for observing system design. Comparison of HF radar–derived wave heights with acoustic Doppler profiler and buoy data revealed that the scale separation between the Bragg scattering waves and the peak energy-containing waves may contribute to errors in the single-site estimates in light-to-moderate winds. A wave-height correction factor was developed that explicitly considers this scale separation and eliminates the trend of increasing errors with increasing wind speed

Potential Effect of Bio-Surfactants on Sea Spray Generation in Tropical Cyclone Conditions

Despite significant improvement in computational and observational capabilities, predicting intensity and intensification of major tropical cyclones remains a challenge. In 2017 Hurricane Maria intensified to a Category 5 storm within 24 h, devastating Puerto Rico. In 2019 Hurricane Dorian, predicted to remain tropical storm, unexpectedly intensified into a Category 5 storm and destroyed the Bahamas. The official forecast and computer models were unable to predict rapid intensification of these storms. One possible reason for this is that key physics, including microscale processes at the air-sea interface, are poorly understood and parameterized in existing forecast models. Here we show that surfactants significantly affect the generation of sea spray, which provides some of the fuel for tropical cyclones and their intensification, but also provides some of the drag that limits intensity and intensification. Using a numerical model verified with a laboratory experiment, which predicts spray radii distribution starting from a 100 μm radius, we show that surfactants increase spray generation by 20–34%. We anticipate that bio-surfactants affect heat, energy, and momentum exchange through altered size distribution and concentration of sea spray, with consequences for tropical cyclone intensification or decline, particularly in areas of algal blooms and near coral reefs, as well as in areas affected by oil spills and dispersants