Surface Ocean Dispersion Observations From the Ship-Tethered Aerostat Remote Sensing System

Oil slicks and sheens reside at the air-sea interface, a region of the ocean that is notoriously difficult to measure. Little is known about the velocity field at the sea surface in general, making predictions of oil dispersal difficult. The Ship-Tethered Aerostat Remote Sensing System (STARSS) was developed to measure Lagrangian velocities at the air-sea interface by tracking the transport and dispersion of bamboo dinner plates in the field of view of a high-resolution aerial imaging system. The camera had a field of view of approximately 300 × 200 m and images were obtained every 15 s over periods of up to 3 h. A series of experiments were conducted in the northern Gulf of Mexico in January-February 2016. STARSS was equipped with a GPS and inertial navigation system (INS) that was used to directly georectify the aerial images. A relative rectification technique was developed that translates and rotates the plates to minimize their total movement from one frame to the next. Rectified plate positions were used to quantify scale-dependent dispersion by computing relative dispersion, relative diffusivity, and velocity structure functions. STARSS was part of a nested observational framework, which included deployments of large numbers of GPS-tracked surface drifters from two ships, in situ ocean measurements, X-band radar observations of surface currents, and synoptic maps of sea surface temperature from a manned aircraft. Here we describe the STARSS system and image analysis techniques, and present results from an experiment that was conducted on a density front that was approximately 130 km offshore. These observations are the first of their kind and the methodology presented here can be adopted into existing and planned oceanographic campaigns to improve our understanding of small-scale and high-frequency variability at the air-sea interface and to provide much-needed benchmarks for numerical simulations

Air-Sea Interaction Dynamics Under Hurricane Wind Conditions

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

Sensitivity analysis of convective and PBL parameterization schemes for Luban and Titli tropical cyclones

Tropical cyclones (TCs) are the most distractive natural weather phenomena and cause extensive damage and socioeconomic loss over the North Indian Ocean (NIO) region. Convection and planetary boundary layer (PBL) system play a vital role in the origin and strengthening of the TCs. The various convective and PBL parameterization schemes are available in the statistical model, which integrates these processes. The efficient incorporation of these schemes is vital to enhance the performance of the numerical weather prediction (NWP) model. In the present study, twelve experiments have been designed to carry out the numerical simulations using Advance Research Weather Research and Forecasting (ARW) model. The behavior and performance of the schemes have been evaluated to verify the instantaneous forecast of the TCs. The simulated cyclone track, which is assessed with the Indian Meteorological Department (IMD) best track data, indicates that the vector displacement error and RMSE for the experiment MWBM and YWBM are 0.4 for MLBM and up to 60 mm in Luban. However, it is > 0.6 for YLBM and up to 40 mm for Titli. Based on the results and keeping the cyclone track, intensity, and rainfall, the BMJ convective scheme with the YSU and MYJ PBL has better predicting skills over the NIO region. The KF scheme has better skills in the prediction of TC intensity

Sea Spray Generation in Very High Winds

Abstract Quantifying the amount and rate of sea spray production at the ocean surface is critical to understanding the effect spray has on atmospheric boundary layer processes (e.g., tropical cyclones). Currently, only limited observational data exist that can be used to validate available droplet production models. To help fill this gap, a laboratory experiment was conducted that directly observed the vertical distribution of spume droplets above actively breaking waves. The experiments were carried out in hurricane-force conditions (10-m equivalent wind speed of 36–54 m s−1), and the observed particles ranged in radius r from 80 to nearly 1400 μm. High-resolution profiles (3 mm) were reconstructed from optical imagery taken within the boundary layer, ranging from 2 to 6 times the local significant wave height. Number concentrations were observed to have a radius dependence proportional to r−3 leading to spume production estimates that diverge from typical source models, which tend to exhibit a radius falloff closer to r−8. This was particularly significant for droplets with radii circa 1 mm whose modeled production rates were several orders of magnitude less than the rates expected from the observed concentrations. The vertical dependence of the number concentrations was observed to follow a logarithmic profile, which does not confirm the power-law relationship expected by a conventional spume generation parameterization. These observations bear significant implications for efforts to characterize the role these large droplets play in boundary layer processes under high-wind conditions

Spray Concentration Measurements from ASIST for Freshwater and Seawater

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. The predominant focus of previous studies of spray dynamics has been on the marine environment. Spray dynamics in non-seawater bodies have not been extensively studied, and any significant differences between sea and freshwater remain unquantified. To address this gap, we have conducted the first laboratory experiment directly comparing spume concentrations above fresh and real seawater for 10-m equivalent wind speeds of 36-54 m/s. Droplets in the air above the intensely breaking wind-waves were directly observed and their distribution as functions of wind speed, height, and droplet radius was compared between the two water types. Spume droplets were imaged using a Dantec Dynamics particle image velocimetry (PIV) acquisition system modified to be used in a shadow imaging mode. A camera was positioned outside of the tank and oriented to be looking into a high intensity strobe, also mounted outside of the tank, but directly opposite the camera. For each of the vertical levels (3 total), the wind was allowed to ramp up and time (120 s) was allowed for the tank conditions to become stationary. Then at least 7 consecutive sets of 250 images were acquired for all five wind speeds. 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. Seawater and freshwater height-dependent distributions exhibited different wind-speed dependences. 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 dataset (.mat variable and a .m script) is associated to the above article “A Laboratory Investigation of Spume Generation in High Winds for Fresh and Seawater” by Mehta et al. (2019), submitted in JGR-Atmospheres. Specifically, this is the data used to create figures 2 through 8 in the paper

Observations of Near-Surface Current Shear Help Describe Oceanic Oil and Plastic Transport

Plastics and spilled oil pose a critical threat to marine life and human health. As a result of wind forcing and wave motions, theoretical and laboratory studies predict very strong velocity variation with depth over the upper few centimeters of the water column, an observational blind spot in the real ocean. Here we present the first-ever ocean measurements of the current vector profile defined to within 1 cm of the free surface. In our illustrative example, the current magnitude averaged over the upper 1 cm of the ocean is shown to be nearly four times the average over the upper 10 m, even for mild forcing. Our findings indicate that this shear will rapidly separate pieces of marine debris which vary in size or buoyancy, making consideration of these dynamics essential to an improved understanding of the pathways along which marine plastics and oil are transported

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