Understanding the impact of surface waves on Microwave Water Level measurements

The National Oceanic and Atmospheric Administration (NOAA) Center for Operational Oceanographic Products (CO-OPS) has been conducting a series of tests of several different types Microwave Water Level (MWWL) sensors in order to gain an understanding of sensor functions and performance capabilities and to assess the suitability for incorporation of these sensors into the NOAA National Water Level Observation Network (NWLON). On March 5-6, 2008, one particular laboratory test of four different microwave sensors was conducted with the following objectives: 1) determine the impact of surface gravity waves on the accuracy of measured water level and 2) collect a data set that can be used to develop techniques for removing high frequency surface wave induced noise from long term microwave water level records. The two day test was conducted at the Naval Surface Warfare Center (NSWC) Maneuvering and Sea Keeping Basin (MASK) in Carderock, Maryland. This facility includes a 110 m long, 73 m wide, 6.1 m deep indoor tank, with the capability to generate controlled, multi-directional surface waves. During the test, the four sensors measured water level in the tank from above, at four different sensor-to-water ranges, 3, 5, 7, and 9 meters. 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.