1. Revisiting HF Ground Wave Propagation Losses Over the Ocean: A Comparison of Long‐Term Observations and Models.
- Author
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Kirincich, Anthony and Emery, Brian
- Subjects
THEORY of wave motion ,OCEAN currents ,GRAVITY waves ,OCEAN waves ,SEA level ,OCEAN - Abstract
Understanding variations in the received power levels for land‐based high frequency radar (HFR) systems is critical to advancing radar‐based estimates of winds and waves. We use a long‐term record of one‐way HFR power observations to explore the key factors controlling propagation losses over the ocean. Observed propagation loss was quantified using an 8‐month record of radio frequency power from a shore‐based transmitter, received at two locations: an offshore tower and a nearby island. Observations were compared to environmental factors such as wind speed and air temperature as well as models of path loss incorporating smooth and rough surface impedances and varying atmospheric properties. Significant differences in the observations at the two sites existed. One‐way path loss variations at the tower, a wavelength above mean sea level, were closely related to atmospheric forcing, while variations at the distant island site were dominated by wind‐driven surface gravity wave variability. Seasonal variability in ocean conductivity had no significant effect on over‐ocean path losses. Simplistic analytical models of path loss were found to have more skill than either ground wave propagation models or more complex numerical models of field strength in matching the observations, due in part to under‐observation of the atmosphere but also the differences in rough surface impedance between models of ocean waves. Plain Language Summary: Understanding variations in the returning signal power for coastal ocean current radar systems is critical to enabling radar‐based estimates of winds and waves. We use a long‐term record of direct measurements of received power to examine the variability of the energy losses along the ocean path over time due to changes in the local environment. Received power levels were observed over a period of up to 8 months at both an offshore tower and a nearby island site, with ranges of 7 and 45 km from the transmitter. Changes in the power, due to path losses, varied over timescales of days at both sites, but for different reasons. Variations at the tower were related to air‐sea temperature differences and humidity, but variations at the distant island site were more closely related to changes in ocean wave conditions. Neither were well predicted by advanced numerical models of path loss. The predicted effect of the surface gravity waves on path loss was highly dependent on the estimate of the wavefield used, with simpler representations proving to be more useful. Key Points: One way path loss variations of high frequency ground waves over the ocean are dominated by wind‐driven surface gravity wave variabilitySeasonal variability in ocean conductivity has no detectable effect on over‐ocean path lossesSimplistic analytical models of path loss have more skill than complex numerical models, due in part to under‐observation of the atmosphere [ABSTRACT FROM AUTHOR]
- Published
- 2023
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