Leonid Ivanov

Measurements of ice parameters in the Beaufort Sea using the Nortek AWAC acoustic Doppler current profiler

Measurements of sea ice presence, ice keel draft, and ice thickness are important for climate studies and engineering design and maintenance of coastal and offshore structures. Building on the success of upward looking sonar systems for ice measurements, Nortek recently introduced new firmware and measurement methods to make observations of sea ice with the Nortek AWAC acoustic Doppler current profiler. The AWAC employs a dedicated, narrow, upward looking vertical transducer to acoustically measure waves (summer) and ice (winter). One of the first commercial applications of this new measurement capability was a project in 2008 to measure ice parameters in support of the design of an offshore structure and its ice armoring system in the Beaufort Sea, Alaska. An analysis of instrumental and environmental error is described. The AWAC ice measurements are compared to a co-located ASL ice profiler and show a mean difference of less than 0.05 m. Ice formation began in mid-October when air temperatures were routinely below -10° C. The thickness increased almost linearly until it peaked in mid-May at about 1.8 m. This represents an increase in ice thickness of approximately 0.01 m/day. The near-surface current velocity from the Nortek AWAC was used to estimate the horizontal length scales and the direction of movement of mobile ice blocks.

Classification of strong current events based on Gulf of Mexico BOEM NTL dataset

Strong currents are a matter of great concern for the maritime industries in the Gulf of Mexico. Oil industry operators working in the gulf are required to measure ocean current profiles and transmit these data in near-real-time. The data are collected primarily using TRDI ADCPs mounted on the structures or nearby. The individual data ensembles are sent to NDBC where they are processed, quality checked, archived and made available to the public through the NDBC website. In 2011, Woods Hole Group Inc. (WHG) was contracted by DeepStar Technology Development for Deepwater Research to create a database of deep ocean currents for the Gulf of Mexico. One of the objectives of the project was to identify and make plots of strong current (“energetic”) events. WHG performed a thorough inspection of the data, including a review of the temporal and spatial continuity of the data, cross-check between neighboring stations and between current data and satellite altimetry charts. Based on this extensive dataset, a total of 560 strong current events were identified and classified according to the candidate mechanisms that cause these current intensifications.

Modeling the performance of a winged sub-surface float for acoustic profiling and wave measurements

Near-surface submerged floats have been used to support upward-looking acoustic Doppler profilers to collect current and/or directional wave measurements. However, mooring drawdown and tilt in strong currents pose a design challenge to keep the instruments sufficiently level and close to the surface to collect good quality data. We describe a concept for a relatively small winged float utilizing current-generated lift to minimize drawdown in very high currents, while also maintaining a stable attitude for the sensor. The winged float design consists of a streamlined body with double (box) wings, having modest static buoyancy and tail fins to control the bodys attitude while maintaining a near-constant angle of attack of its wings. We model the performance of the winged float by incorporating iteratively the dynamic lift and drag of the wings as well as the static buoyancy and drag of the body itself. Standard aerodynamic formulas are used to estimate wing lift and drag under varying current conditions. Model results indicate that a float equipped with 2 wings, each having a 0.75m chord length and 1.5m span (2.25m2 total wing area) outperforms a 65 inch syntactic float when the current profile is characterized by a strong near-surface current and has a comparable performance to a 65 inch float when the current profile has a sub-surface maximum. We conclude that a winged float, combined with a properly designed mooring, can provide a suitable platform for long-term wave and near-surface current profile measurements in strong current regimes without the need for a surface buoy or massive buoyancy modules.

Current and wave measurements off the coast of New Jersey during the second most severe storm of the past 28 years

Woods Hole Group, Inc. (WHG), configured, assembled and deployed a Mooring Systems Inc. Trawl Resistant Bottom Mount (TRBM) platform equipped with current, wave, and water quality instrumentation at a site called NJORD, located approximately 17 miles southeast of Ocean City, NJ. On March 6-7, 2013, the area offshore New Jersey experienced severe storm conditions due to an extratropical cyclone passing approximately 100 miles south of the measurement site. NOAAs National Data Buoy Center has been collecting wave data at a nearby location since 1985. According to this record, the storm of March 6-7, 2013, in terms of wave height (Hm0=7.8m), ranks as the second most severe storm in nearly 30 years. The maximum significant wave height (Hm0 = 8.1m) was recorded by the NOAA Buoy on November 13, 2009. This most severe storm event was also associated with an extratropical cyclone that passed south of the site as it translated towards east-northeast. That fact makes the data collected at the NJORD site during typical severe storm conditions valuable. The measurements collected by WHG during the storm are of interest from an acoustic measurement perspective because significant wave height was a large fraction of the water depth, producing actively breaking waves in 28m of water. The performance of the bottom-mounted AWAC in this high energy environment is evaluated.

Current measurements from a deep real-time metocean mooring: lessons learned on real-time data QA/QC

An automated Quality Assurance and Quality Control (QA/QC) procedure implemented by Woods Hole Group, Inc. (WHG) to improve the quality of ADCP data collected on a deep Real-Time Metocean Mooring (RTMM) is described. The RTMM is equipped with three TRDI ADCPs and one Nortek single-point current meter (SPCM). A full water column current profile is collected every 20 minutes and the data are sent by satellite to a shore-based computer and web server, which creates graphical displays in near-real-time and serves them over the Internet to users web browsers. WHGs automated QA/QC procedure was developed based on analysis of a first batch of ADCP data collected by the RTMM and is system-specific. The analysis of the data and of data quality indicators revealed major causes that lead to the occurrence of spurious and/or noisy current data. It also suggested approaches to improving the quality of the data provided to the client in near-real-time. The analysis revealed certain limitations of the QA/QC algorithm implemented by NDBC and recommended by QARTOD. From our experience with the RTMM data, the QA/QC of ADCP data should not be limited to automated review of quality parameters measured by the ADCP on individual ensemble-averaged profiles. A better result is achieved by adjusting ADCP setup and customizing data screening thresholds for a specific deployment. The thresholds should be customized based on the analysis of the system-specific data.

Characteristics of the Atmospheric Boundary Layer in Nantucket Sound

Characteristics of the Atmospheric Boundary Layer (ABL) in Nantucket Sound are described using meteorological and oceanographic observations collected on an offshore scientific met-ocean data acquisition tower. The data monitoring system includes two independent meteorological systems, each composed of wind speed and direction sensors installed at three levels on opposite sides of the 60-m steel monopole tower. The study was motivated by the necessity to accurately predict the vertical offshore wind profile for the design of a wind farm in Nantucket Sound, Massachusetts. The analysis of the collected data has revealed certain site-specific peculiarities in the ABL structure. In particular, ABL properties and the wind profile are notably affected by the proximity of the site to the coast

Observations of downward and upward propagating near-inertial waves in the western Gulf Of Mexico

Institute Mexicano Del Petroleo in cooperation with Woods Hole Group from Massachusetts deployed a deepwater mooring in January 2005 off Nautla, Mexico. The mooring was deployed over the continental slope at approximately 1500 m depth. The instrumentation consisted of an RD Instruments, Inc 300 kHz Workhorse Sentinel ADCP, two 75 kHz Workhorse Long Ranger ADCPs, and two Nortek, As Aquadopp single point current meters. CTD surveys of the area were conducted at the time of mooring deployment and recovery to assess vertical stratification of the water column. The deployment produced excellent quality ADCP data covering the upper 950 m of the water column with high temporal and vertical resolution. These data provide unusually clear evidence of the two sets of energetic near-inertial oscillations: waves propagating in the upward direction almost throughout the entire water column up to the upper portion of the pycnocline, and waves propagating downward from their generation region at the base of the mixed layer. Details of the vertical distribution of near-inertial wave properties are presented.