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Abstract
Affordable high quality charge-coupled device (CCD) video cameras and image processing software are powerful tools for bubble measurements. Because of the wide variation between bubble populations, different bubble measurement systems (BMSs) are required depending upon the application. Two BMSs are described: a mini-BMS designed to observe the background bubble population from breaking waves, and a large-BMS designed to noninvasively determine the time-resolved bubble distribution inside dense bubble plumes and near the interface, as are details of the analysis techniques. Using the two systems in conjunction with each other allowed size distributions over the range 15–5000-μm radius to be obtained.
The BMSs were designed for application to breaking-wave bubble plumes in the field or laboratory. Distributions measured by both BMSs in aerator-generated plumes agreed very well for the overlapping size range. Also presented are observations of bubble plumes produced by breaking waves in a large wind-wave flume, and calibration experiments showing the effect on measured bubble size due to blur induced by slow shutter speeds.
Abstract
Affordable high quality charge-coupled device (CCD) video cameras and image processing software are powerful tools for bubble measurements. Because of the wide variation between bubble populations, different bubble measurement systems (BMSs) are required depending upon the application. Two BMSs are described: a mini-BMS designed to observe the background bubble population from breaking waves, and a large-BMS designed to noninvasively determine the time-resolved bubble distribution inside dense bubble plumes and near the interface, as are details of the analysis techniques. Using the two systems in conjunction with each other allowed size distributions over the range 15–5000-μm radius to be obtained.
The BMSs were designed for application to breaking-wave bubble plumes in the field or laboratory. Distributions measured by both BMSs in aerator-generated plumes agreed very well for the overlapping size range. Also presented are observations of bubble plumes produced by breaking waves in a large wind-wave flume, and calibration experiments showing the effect on measured bubble size due to blur induced by slow shutter speeds.
In the surface layer over the ocean the Monin–Obukhov similarity theory is often applied to construct vertical profiles of pressure, temperature, humidity, and wind speed. In this context, the rough boundary layer is derived from empirical relations where ocean wave characteristics are neglected. For seas where wind speed is less than ~ 10 m s−1 there is excellent agreement for both meteorological and microwave propagation theory and measurements. However, recent evidence indicates that even small waves perturb these profiles. It is, therefore, hypothesized that mechanical forcing by sea waves is responsible for modifying scalar profiles in the lowest portion of the surface layer, thereby reducing the effects of evaporation ducting on microwave signal propagation. This hypothesis, that a rough sea surface modifies the evaporation duct, was the primary motivation for the Rough Evaporation Duct (RED) experiment.
RED was conducted off of the Hawaiian Island of Oahu from late August to mid-September 2001. The Scripps Institution of Oceanography Research Platform Floating Instrument Platform, moored about 10 km off the northeast coast of Oahu, hosted the primary meteorological sensor suites and the transmitters for both the microwave and the infrared propagation links. Two land sites were instrumented—one with microwave receivers and the other with an infrared receiver—two buoys were deployed, a small boat was instrumented, and two aircraft flew various tracks to sense both sea and atmospheric conditions.
Through meteorological and propagation measurements, RED achieved a number of its objectives. First, although we did not experience the desired conditions of simultaneous high seas, high winds, and large surface gradients of temperature and humidity necessary to significantly affect the evaporation duct, observations verify that waves do modify the scalars within the air–sea surface layer. Second, an intriguing and controversial result is the lack of agreement of the scalar profile constants with those typically observed over land. Finally, as expected for the conditions encountered during RED (trade wind, moderate seas, unstable), we show that the Monin–Obukhov similarity theory, combined with high-quality meteorological measurements, can be used by propagation models to accurately predict microwave signal levels.
In the surface layer over the ocean the Monin–Obukhov similarity theory is often applied to construct vertical profiles of pressure, temperature, humidity, and wind speed. In this context, the rough boundary layer is derived from empirical relations where ocean wave characteristics are neglected. For seas where wind speed is less than ~ 10 m s−1 there is excellent agreement for both meteorological and microwave propagation theory and measurements. However, recent evidence indicates that even small waves perturb these profiles. It is, therefore, hypothesized that mechanical forcing by sea waves is responsible for modifying scalar profiles in the lowest portion of the surface layer, thereby reducing the effects of evaporation ducting on microwave signal propagation. This hypothesis, that a rough sea surface modifies the evaporation duct, was the primary motivation for the Rough Evaporation Duct (RED) experiment.
RED was conducted off of the Hawaiian Island of Oahu from late August to mid-September 2001. The Scripps Institution of Oceanography Research Platform Floating Instrument Platform, moored about 10 km off the northeast coast of Oahu, hosted the primary meteorological sensor suites and the transmitters for both the microwave and the infrared propagation links. Two land sites were instrumented—one with microwave receivers and the other with an infrared receiver—two buoys were deployed, a small boat was instrumented, and two aircraft flew various tracks to sense both sea and atmospheric conditions.
Through meteorological and propagation measurements, RED achieved a number of its objectives. First, although we did not experience the desired conditions of simultaneous high seas, high winds, and large surface gradients of temperature and humidity necessary to significantly affect the evaporation duct, observations verify that waves do modify the scalars within the air–sea surface layer. Second, an intriguing and controversial result is the lack of agreement of the scalar profile constants with those typically observed over land. Finally, as expected for the conditions encountered during RED (trade wind, moderate seas, unstable), we show that the Monin–Obukhov similarity theory, combined with high-quality meteorological measurements, can be used by propagation models to accurately predict microwave signal levels.
As part of the U.K. contribution to the international Surface Ocean-Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosolsize spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.
As part of the U.K. contribution to the international Surface Ocean-Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosolsize spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.
Abstract
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