Microwave-acoustic water level sensor comparisons: Sensor response to change in oceanographie and meteorological parameters

The Center for Operational Oceanographic Products and Services (CO-OPS) of the National Ocean Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, conducts thorough evaluations of new environmental measurement sensors through its Ocean Systems Test & Evaluation Program. New water level sensors that employ microwave (MW) radar technology are now being evaluated and compared with other devices including the Aquatrak acoustic (AC) sensor that presently serves as the operational standard for the National Water Level Observation Network (NWLON). While no part of a MW radar gage has contact with the water and radar transmission in the GHz range is insensitive to temperature and humidity change, the beam measuring distance to the water surface is allowed to transmit in open air with no confining hardware such as a wave guide or stilling well. Further study is therefore needed to fully understand the effect of a dynamic air-water interface on MW radar measurements. Four MW radar sensors (Sutron RLR-0002, Miros SM-094, Design Analysis H3611 and Ohmart/Vega Vegapuls 62) were mounted on the U.S. Army Corps of Engineers Field Research Facility (FRF) pier at Duck, NC where oceanographic and meteorological data were used to evaluate MW radar sensor performance under open ocean conditions. Water level 6-minute series from each MW sensor were compared with 6-minute series recorded by the NWLON AC sensor at Duck during September-October, 2008. MW-AC zero-mean difference series contained power (variance) at certain frequencies and cross-power (covariance) at these same frequencies with parameters such as wave height, wave steepness, longshore current speed, and air/water temperature ratio. MW-AC differences for all MW sensors showed noticeable change when Hmo wave heights exceeded 2 meters and marked change when Hmo wave heights exceeded 3 meters during storm events. Wave steepness increased abruptly at the onset of these events followed by a prolonged roll-off toward lower values. MW-AC differences for the Sutron and Mros sensors responded first negatively then positively to steepness episodes while the Design Analysis and Vegapuls sensors responded positively but with a noticeable lag during larger events. Coherence analyses for MW-AC difference versus wave height and wave steepness underscore similarities between the Sutron and Miros sensors in one group and between the Design Analysis and Vegapuls sensors in another group, each characterized by a set of distinctive frequencies. The same analyses involving longshore current speed and air/water temperature ratio hint at weaker associations at lunar diurnal and semidiurnal periods for the former and solar diurnal and semidiurnal periods for the latter.