Water Level and Wave Height Estimates at NOAA Tide Stations from Acoustic and Microwave Sensors.

Abstract

The National Oceanic and Atmospheric Administration (NOAA) Center for Operational Oceanographic Products and Services (CO-OPS) 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. The primary motivation for this transition is the significant reduction in infrastructure and maintenance costs associated with the microwave sensor, which in ice-free conditions requires no contact with the water surface. The acoustic system requires a protective well that extends from above the highest water level to below the lowest water level and system maintenance requires disassembly, cleaning and dive operations. Installation of a new acoustic system requires nontrivial infrastructure to support the protective well. To assess the relative performance of these two sensor systems, CO-OPS initiated a program to compare performance of the acoustic and microwave systems at operational NWLON stations finding statistically equivalent performance at sites with little or no wave energy. At sites with wave energy (expressed in the standard deviation statistic of the water level estimate) a persistent bias was noted with acoustic water level estimates lower than that of the microwave sensor. This report is the culmination of a study to identify and assess these differences. Water level data from acoustic and microwave sensors covering a period of 19 months at tide stations on both the Atlantic and Pacific coasts are analyzed. Comparison of the acoustic and microwave data reveals that the majority of differences are accounted for by errors in the acoustic system, primarily from undiagnosed temperature gradients and wave-induced water level draw-down. It is also demonstrated that water level resonance inside the acoustic protective well can distort the water level spectral variance, and that the microwave sensor captures water level variability with higher fidelity than the acoustic system when waves are present. The overall results indicate that the microwave sensor is better suited than the acoustic system for water level measurement in locations where temperature differences between the sensor and water are significant or where waves or tidal flows draw down water levels inside the well. We also note that wave height estimates as envisioned by the Integrated Ocean Observing System (IOOS) National Operational Wave Observation Plan (2009) using the NWLON standard deviation statistic are more accurately rendered with the microwave sensor than with the acoustic system. It should be noted that the results of this study do not constitute a general recommendation to replace acoustic sensors with microwave sensors. Just as the acoustic system has limitations from temperature and hydraulic draw-down effects, microwave sensors have limitations such as sidelobe interference, false targets and signal scattering from heavy rain. Such an assessment is a site-specific determination, and should include long term comparisons of sensor data.