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The National Oceanic and Atmospheric Administration (NOAA) is transitioning the primary water level sensor at the majority of tide stations in the National Water Level Observation Network (NWLON) from an acoustic ranging system to a microwave radar system. Field comparison of the acoustic and microwave systems finds statistically equivalent performance when temperature gradients between the acoustic sensor and water surface are small and when significant wave height is less than roughly 0.5 m. When significant wave height is greater than approximately 0.5–1 m, the acoustic system consistently reports lower water levels. An analysis of 2 months of acoustic and microwave water level data at Duck, North Carolina, finds that the majority of differences between the two sensors can be attributed to systemic errors in the acoustic system and that the microwave system captures water level variability with higher fidelity than the acoustic system. NWLON real-time data products include the water level standard deviation, a statistic that can serve as a proxy for significant wave height. This study identifies 29 coastal water level stations that are candidates for monitoring wave height based on water level standard deviation, potentially adding a significant source of data for the sparsely sampled coastal wave fields around the United States, and finds that the microwave sensor is better suited than the acoustic system for wave height estimates.

Abstract. An empirical fetch-limited ocean wave spectrum has been combined with an acoustic ray-based model to predict the acoustic signal time-angle fluctuations induced by sea surface roughness. Rough sea surface realizations are generated and used as sea surface boundaries with the acoustic model. To validate this model, results are compared against experimental data collected in a fetch limited region. These data includes simultaneous wind speed and acoustic propagation (1-18 kHz) measurements in a fetch limited coastal region. Modeled time-angle fluctuations compare well with field data at lower wind speeds (< 10 m/s).

Abstract. Seiches are normal modes of water bodies responding to geophysical forcings with potential to significantly impact ecology and maritime operations. Analysis of high-frequency (1 Hz) water level data in Monterey, California, identifies harbor modes between 10 and 120 s that are attributed to specific geographic features. It is found that modal amplitude modulation arises from cross-modal interaction and that offshore wave energy is a primary driver of these modes. Synchronous coupling between modes is observed to significantly impact dynamic water levels. At lower frequencies with periods between 15 and 60 min, modes are independent of offshore wave energy, yet are continuously present. This is unexpected since seiches normally dissipate after cessation of the driving force, indicating an unknown forcing. Spectral and kinematic estimates of these low-frequency oscillations support the idea that a persistent anticyclonic mesoscale gyre adjacent to the bay is a potential mode driver, while discounting other sources.

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Abstract-At each measurement height, a range of different surface wave conditions were generated in the tank, including regular controlled wavelength waves as well as irregular waves, simulating real ocean conditions.Results indicate that in some cases, continuously generated uniform wavelength waves caused offsets in measured water level for all sensors, and these offsets depend on the ratio between the width of the sensor footprint on the water surface and dominant wavelength of surface waves present. The impact of surface waves on measured water level varied across different sensors, due to different filtering and range tracking algorithms employed. Results will be used to gain a better understanding of sensors' processing capabilities and to ensure that each sensor's parameters are optimally configured for additional future field tests. A detailed overview of the setup and execution of this unique laboratory test will be presented along with analysis results summarizing the observed wave induced offsets. Recommendations on filtering methods for removing high frequency surface wave induced noise from long term MWWL measurements will also be discussed.

A microwave radar water level sensor, the Design Analysis Waterlog H-3611 has recently entered service at tide stations operated by the National Oceanic and Atmospheric Administration (NOAA), National Ocean Service (NOS), Center for Operational Oceanographic Products and Services (CO-OPS) as part of the National Water Level Observation Network (NWLON). The microwave water level sensor combines high accuracy with low sensitivity to variations in air temperature and humidity but differs from other water level sensors in utilizing an unconfined radar beam aimed vertically downward to the water surface. Many potential benefits of using microwave radar sensors for short-term flood advisories and long-term sea level monitoring have been identified by several organizations throughout the ocean observing community. The most notable advantage of radar sensors is their ability to measure water level remotely with no parts directly in contact with the water column. Water level measurement stations that employ remote radar sensor technology will avoid many problems typical of long-term subsurface ocean sensors including biological fouling and corrosion. Remote sensing also results in a significant reduction in system hardware components and overall installation and maintenance requirements. Results from a series of laboratory and field tests conducted by CO-OPS over the last few years have led to operational use of Waterlog radar units in certain specific applications. It is acknowledged, however, that most test data collected and analyzed to date for the purpose of assessing the sensor's capabilities have focused on enclosed coastal regions with limited fetch and a low-wave environment (average significant wave height nominally less than 1 m). Although these test results are relevant to many CO-OPS applications of interest, including the majority of NWLON stations located in low-wave environments, uncertainty remained after the initial test phase and led to a new outlook toward sensor performance in open ocean environments that experience significant wave heights frequently in excess of 1 m. Additional testing under these conditions has presented a greater challenge as both the test and reference NWLON sensors are likely to encounter limitations in the presence of large waves. Analytical tools including spectral analysis of sensor output signals and an evaluation of sample statistics were needed to better understand the nature of these limitations. In order to address the remaining uncertainty in sensor performance capability, a multi-sensor test deployment was recently conducted at an open-ocean test site, the U.S. Army Corps of Engineers Field Research Facility at Duck, NC (Duck FRF). The resulting multi-sensor data set has allowed sensor measurement error to be estimated for the first time from measurement residuals about an ensemble average series. Comparing ensemble-based error estimates with corresponding Duck FRF nearshore wave data averaged over selected measurement periods shows that sensor error increases, as expe...