Development and Testing of Instrumentation for Ship-Based UAV Measurements
We have developed instrumentation packages for unmanned aerial vehicles (UAVs) to measure ocean surface processes along with momentum fluxes and latent, sensible, and radiative heat fluxes in the marine atmospheric boundary layer (MABL). The packages have been flown over land on BAE Manta C1s and over water on Boeing-Insitu ScanEagles. The low altitude required for accurate surface flux measurements (< 30 m) is below the typical safety limit of manned research aircraft; however, with advances in laser altimeters, small-aircraft flight control, and real-time kinematic differential GPS, low-altitude flight is now within the capability of small UAV platforms. Fast-response turbulence, hygrometer, and temperature probes permit turbulent flux measurements, and short- and long-wave radiometers allow the determination of net radiation, surface temperature, and albedo. Onboard laser altimetry and high-resolution visible and infrared video permit observations of surface waves and fine-scale (O(10) cm) ocean surface temperature structure.
Flight tests of payloads aboard ScanEagle UAVs were conducted in April 2012 at the Naval Surface Warfare Center Dahlgren Division (Dahlgren, VA), where measurements of water vapor, heat, and momentum fluxes were made from low-altitude (31-m) UAV flights over water (Potomac River). ScanEagles are capable of ship-based launch and recovery, which can extend the reach of research vessels and enable scientific measurements out to ranges of O(10-100) km and altitudes up to 5 km. UAV-based atmospheric and surface observations can complement observations of surface and subsurface phenomena made from a research vessel and avoid the well-known problems of vessel interference in MABL measurements. We present a description of the instrumentation, summarize results from flight tests, and discuss potential applications of these UAVs for ship-based MABL studies.
High-Resolution Airborne Waveform Lidar for Oceanographic Research
It is now accepted that to better understand the coupling between the atmosphere and the ocean, surface-wave processes must be taken into account. Traditional airborne lidar systems and in situ instrumentation have limited directional and frequency responses and do not have the resolution required to fully test modern theories of directional wave spectra. Directional observations at lower and higher wavenumbers, the latter being close to the end of the gravity-wave range, are especially limited, but are important as they need to be resolved in current wind-wave models.
Over the past two years, we have integrated a novel, portable, high-resolution airborne topographic lidar with video and hyperspectral imaging systems. The scanning waveform lidar is coupled to a highly accurate GPS/inertial measurement unit permitting airborne measurements of the sea surface elevation and whitecap coverage with swath widths of up to 800m under the aircraft track over water, and horizontal spatial resolution as low as 0.2m. We describe system performance, and present preliminary results from recent measurements, where we obtained wave directional spectra down to wavelengths of 0.8m.
A Portable Airborne Scanning LiDAR System for Ocean and Coastal Applications
A portable compact airborne scanning LiDAR (Light Detection And Ranging) system based on the RIEGL LMS-Q240i has been developed and its functionality demonstrated for oceanographic and coastal measurements. Differential GPS (DGPS) and an Inertial Navigation System are synchronized with the LiDAR, resulting in vertical rms errors of less than 9 cm. Surveys with this airborne system are compared with ground-based DGPS surveys of fixed targets. Measurements of the Southern California coastline and nearshore surface wave fields from seventeen research flights between August 2007 and December 2008 are analyzed and discussed. The October 2007 landslide on Mt. Soledad in La Jolla, California was documented by two of the flights. The topography, lagoon, reef, and surrounding wave field of Lady Elliot Island in Australiaís Great Barrier Reef were measured in April 2008 with the airborne scanning LiDAR system on eight research flights. Applications of the system, including coastal topographic surveys, wave measurements, ship wake studies, and reef research, are presented.
Measurements of EM Bias: Wave Slope and Altitude Effects
Measurements of em bias at Ku band (14 GHz) were made from a platform in Bass Strait during the austral winter of 1992. Two Ku-band Doppler scatterometers were installed on the Snapper platform at 15 and 25m above MSL in 57m of water 30 km off the coast of Victoria, Australia. Approximately two months of data were collected and included microwave backscatter, microwaveDoppler velocity, along with supporting wind and wave measurements. The normalized em bias b = e/Hs, where e is the em bias and Hs is the significant wave height, is usually correlated with the wind speed and significant wave height; however, this leads to significant anomalies at at low wind speeds where b typically has a finite value at zero wind speed, presumably due to the effects of residual waves and swell generated at remote locations. These measurements were carried out to examine em bias in strong wind and wave conditions at a site exposed to the Southern Ocean, to investigate improved parameterizations of the bias, and to study the effect of small altitude differences on em bias measurement.
Waves breaking on a beach generate what is one of nature's most soothing sounds. It may be better to leave its description to the poets, but in recent years acoustical oceanographers have taken an interest as applications in shallow water expand to include the need for a better understanding of the acoustics in the surf zone. In the fall of 1996 the Marine Physical Laboratory at SIO conducted the Adaptive Beach Monitoring (ABM) experiment at Red Beach, Camp Pendleton, California. The ABM experiment was designed to use active and passive acoustics to monitor the surf zone and beach. As part of the ABM experiment, the authors measured the ambient sound across the surf zone along with supporting measurements of surface waves, currents and local meteorological conditions over a period of three weeks in November 1996. These measurements were supplemented by additional data from La Jolla Shores Beach at SIO during the fall of 1997. We present these measurements of ambient noise in the surf zone and relate them to local wind-generated waves, swell and the tides.
Bubble Entrainment by Breaking Waves and their Effects on the Inherent Optical Properties of the Upp
Breaking waves at the oceanís surface inject bubbles and turbulence into the water column. During periods of rough weather, the scales of wave breaking tend to increase with increasing sea states, resulting in mixing of the surface waters and the turbulent transport of bubbles to depth. Bubbles can significantly change the optical properties of water depending on their concentrations and size distribution, introducing potentially significant errors in retrieval of remotely-sensed hyperspectral data products. The development of acoustic bubble measurement techniques now allows for oceanic bubble size distributions to be resolved across a wide range of radii with high temporal resolution. Field measurements of bubbles are presented and their effect on the inherent optical properties are estimated using Mie scattering calculations.
Laboratory Measurements of Langmiur Circulationsand Turbulence
This poster describes the results of the laboratory experiments of the initial stages of the surface flow when a body of water is exposed to an increasing wind stress starting from rest. The laboratory measurements show that when exposed to an increasing wind starting from rest, surface current and wave generation is accompanied by a variety of phenomena that occur over comparable space and time scales. Of particular interest is the generation of small scale, streamwise vortices, or Langmuir circulations; the clear influence of the circulations on the structure of the growing wave field, and the subsequent transition to turbulence of the surface flow. Following recent work by Melville, Shear & Veron (1998) and Veron & Melville (1999), we show that the waves that are initially generated by the wind are then strongly modulated by the Langmuir circulations that follow. Direct measurements of the modulated wave variables are qualitatively consistent with geometrical optics and wave action conservation, but quantitative comparison remains elusive. The onset of the Langmuir circulations leads to a significant increase in the heat transfer across the surface. The implications of the measurements for air-sea fluxes, especially heat and gas transfer, and sea-surface temperature, are discussed.
Laboratory and Field Measurements of Turbulence under Breaking Waves
This poster presents laboratory and field testing of a pulse-to-pulse coherent acoustic Doppler profiler for the measurement of turbulence in the ocean. In the laboratory, velocities and wavenumber spectra collected from Doppler and Digital Particle Image Velocimeter (DPIV) measurements compare very well. Turbulent velocities are obtained by identifying and filtering out deep water gravity waves in Fourier space and inverting the result. Spectra of the velocity profiles then reveal the presence of an inertial subrange in the turbulence generated by unsteady breaking waves. In the field, comparisons of the profiler velocity records with a single point current measurement are satisfactory. Again wavenumber spectra are directly measured and exhibit an approximate -5/3 slope. It is concluded that the instrument is capable of directly resolving the wavenumber spectral levels in the inertial subrange under breaking waves, and therefore is capable of measuring dissipation and other turbulence parameters in the upper mixed layer/surface-wave zone.
Field Measurements of Bubbles Injected by Breaking Waves
Acoustical techniques for in situ measurements of bubbles allows for robust field measurements under a wide range of environmental conditions. Bubbles will significantly modify the acoustical properties of water by scattering, dispersing, and attenuating incident sound waves. A recently developed technique which uses broadband acoustic pulse propagation (frequency band of 4 kHz - 100 kHz) across a fixed pathlength allows for the direct measurement of the sound speed and attenuation at ping rates of a few Hz. The resulting acoustic data can be inverte d using either a resonant approximation method (Wildt 1946, Clay and Medwin 1979) or a finite-element method which corrects for off-resonant contributions to the attenuation (Commander & McDonald 1991). The ability to rapidly measure the acoustical properties of the bubbly medium allows the investigator to resolve the bubble size distributions with high temporal resolution.