Date of Award
Doctor of Philosophy (PhD)
Aerospace Engineering Sciences
William J. Emery
Robert R. Leben
Jeffrey P. Thayer
Conventional satellite altimetry provides very satisfactory results in open ocean conditions. Yet in coastal zones altimetry faces various problems, including less reliable standard corrections, and altimeter waveform (return echo) degradation arising from the rapid changes in the sea state or land contamination in the altimeter footprint. Different retracking (technique to retrieve geophysical information from the waveforms) methods have been developed, but the optimal method may turn out to be a combination of several retrackers, and may depend on the sea state. The coastal High-Frequency Radar (HFR) ocean surface currents are evaluated to test if they can be exploited to validate the coastal altimeter Sea Surface Height (SSH) measurements.
A method to retrieve the geostrophic velocities from the HFR sea surface currents is established in the offshore region where the altimeter data are trusted. At the large mesoscales this method provides HFR SSH that are in very good agreement with altimetry gridded products with correlations larger than 0.8. Along a Jason-1 or Jason-2 altimeter track the agreement depends essentially on the wind history, and a limitation is the smooth field produced by the method. Nevertheless, more than half the cases match with a mean root-mean-square difference of 2.5 cm (6.7 cm/s) in sea level anomaly (across -track geostrophic velocity anomaly).
Even without land contamination in the altimeter footprint, the waveforms may be degraded by non-homogeneous ocean dynamics, and during rain or low wind events. The HFR surface currents provide information on the sea surface height, so that various altimetry retrackers are tested under those conditions. Also referencing to the HFR SSH, several coastal retrackers can be evaluated in the near-shore regions.
Even though these studies demonstrate the value of HFR as a tool to correct coastal satellite altimetry measurements over a section of an altimeter track, which is an improvement compared to sparse in-situ measurements, caution must be taken when generalizing the methodology since the inferred geostrophic HFR datasets may still be contaminated by ageostrophic components, especially during high wind events.
Roesler, Carolyn J., "Evaluating the Use of High-Frequency Radar Coastal Currents to Correct Satellite Altimetry" (2014). Aerospace Engineering Sciences Graduate Theses & Dissertations. 5.