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Evaluating the Use of High-Frequency Radar Coastal Currents to Correct Satellite Altimetry

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Evaluating the Use of High-Frequency Radar Coastal Currents to Correct Satellite Altimetry Evaluating the use of High-Frequency Radar coastal currents to correct Satellite Altimetry by Carolyn J. Roesler M.S., ENSPS Strasbourg France, 1991 M.S., University of Boulder Colorado, 2009 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Aerospace Engineering 2014 This thesis entitled: Evaluating the use of High-Frequency Radar coastal currents to correct Satellite Altimetry written by Carolyn J. Roesler has been approved for the Department of Aerospace Engineering Dr. William J. Emery Dr. Robert R. Leben Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. Roesler, Carolyn J. (Ph.D., Aerospace Engineering) Evaluating the use of High-Frequency Radar coastal currents to correct Satellite Altimetry Thesis directed by Dr. William J. Emery Conventional satellite altimetry provides very satisfactory results in open ocean condi- tions. 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 (tech- nique 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. iv 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 improve- ment 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. Dedication To my parents. vi Acknowledgements There are many people I would like to thank for helping me complete this work. I would like to thank all my committee members: Dr. Bill Emery, Dr. Robert Leben, Dr. John Wahr, Dr. Jeffrey Thayer and Dr. Jean Tournadre. My sincerest thanks go to my advisor, Bill Emery, who has always been patient, helpful and supportive. I am grateful to my committee member, Jean Tournadre, for giving me the opportunity to visit Ifremer and for his guidance during this work. I also would like to acknowledge the Coastal Altimetry group for their encouragement. My thanks go to the friendly team members in CCAR for their help and advice, particularly to the people who worked with me in the lab, both past and present, Dan Baldwin, Ian Crocker, Waqas Qazi and Sandra Castro. vii Contents Chapter 1 Introduction 1 1.1 Introduction . .1 1.2 Motivation . .1 1.3 Coastal altimetry background . .4 1.4 Objectives . .7 1.5 Data sets . .8 1.5.1 High-frequency coastal radars . .8 1.5.2 Altimeters . .8 1.6 Dissertation Overview . .8 2 Satellite Altimeter and HFR measurements and Data sets 11 2.1 Satellite Altimetry . 11 2.1.1 Concept . 11 2.1.2 Waveform contamination . 13 2.1.3 Altimetric sea-level height corrections . 16 2.1.4 Datasets . 17 2.2 HFR . 18 2.2.1 Concept . 19 2.2.2 Sea Doppler Spectrum and sea surface current measurements . 20 viii 2.2.3 HFR Systems . 22 2.2.4 CODAR compact design . 24 2.2.5 Maps of Sea surface vector currents . 25 2.2.6 Depth extent of HFR surface currents . 25 2.2.7 Stokes Drift or wave-induced currents . 26 2.2.8 Measurements errors . 27 2.2.9 HFR dataset . 28 3 Dynamics in the California Current System 30 3.1 Introduction . 30 3.2 California Current System . 30 3.3 Spatial and temporal characteristics of the Ocean currents and Wind . 33 3.3.1 Datasets . 33 3.3.2 Ocean Currents: Mean and principal axes of variance . 34 3.3.3 Wind: Mean and Principal axes of variance . 37 3.3.4 Ocean Currents and Wind time series . 38 4 Processing of the HFR currents for the gridded Altimetry product 42 4.1 Introduction . 42 4.2 Estimating geostrophic currents from the HFR surface currents . 42 4.2.1 Ekman Correction . 42 4.2.2 Optimal Interpolation to force geostrophy . 47 4.3 Comparison of Altimetry currents with various HFR processing . 48 4.3.1 3-year time series of Current anomalies averaged over the 100-km wide coastal zone . 48 4.3.2 Spatial statistics using the OI and Ekman Corrected HFR method . 49 4.3.3 Hovm¨ollerdiagram of OI and Ekman Corrected HFR and Altimeter currents . 53 ix 4.4 Processing HFR from currents to Sea Surface Heights . 54 4.4.1 Methodology . 54 4.4.2 Results . 55 5 Methodology along an offshore altimeter track parallel to the coastline 57 5.1 Introduction . 57 5.2 Altimeter Track Location . 57 5.3 Data Processing . 58 5.3.1 Processing of the altimeter data . 58 5.3.2 Processing of the HFR currents . 59 5.4 Results . 62 5.4.1 Along track SLA . 62 5.4.2 Spectral Analysis . 64 5.5 Statistics RMS/ Correlation . 65 5.5.1 Time Averaging choice for the HFR currents . 67 5.6 Influence of the wind history . 68 5.6.1 Wind anomaly and quality of the HFR SLA in terms of RMS . 68 5.6.2 Concerned only with the across-track Ekman current . 69 5.6.3 Wind history: 4 general case studies . 71 5.7 Using an SST image to decide between HFR or Altimetry SLA as a closer estimate . 73 5.8 HFR and Altimetry across track velocity anomaly comparison . 75 5.9 Discussion-Overview . 77 6 Methodology along altimeter tracks crossing the coastline 79 6.1 Introduction . 79 6.2 Motivation . 79 6.3 Methodology . 80 x 6.4 Comparison over the open ocean with standard altimetry product . 81 6.5 Comparison with PISTACH retrackers . 81 6.6 Explaining some disagreement between HFR and altimetry . 82 7 Applications 84 7.1 Introduction . 84 7.2 Open Ocean . 84 7.3 Near shore: case study Jason-2 P206 in the California Bight . 85 8 Conclusions 89 Bibliography 93 Appendix A JGR paper Roesler et al. 2013 99 xi Tables Table 2.1 The 3 color zones refer to the frequency-operating radar coverage; brown: 4 MHz, blue: 12.5 MHz and green: 25 Mhz. The table specifies their extent and spatial resolution. 23 4.1 Statistics for the 3-year time series of zonal (u) and meridional (v) current anomalies averaged over the 100-km wide coastal zone, between altimetry and 4 processed HFR sets. 50 4.2 The 3-year mean correlation between altimetry and the various processed HFR current anomalies. The mean correlation for the zonal (u) and meridional (v) current anomalies is computed for the 54 collocated grid points. 51 4.3 The phase and magnitude of 3-year mean Complex correlation between al- timetry and the various processed HFR current anomalies, computed for the 54 collocated grid points. 53 5.1 RMS and correlation between the HFR/Altimeter SLA for various time-averaging of the HFR currents . 68 xii Figures Figure 1.1 U.S. west coast HFR and altimeter data set coverage. Green corresponds to the HFR 6-km spatial resolution while the red is the 2-km resolution coverage. The blue lines are the ground tracks of the Jason-2 altimeter satellite. The dotted line is P206 for the interleaved Jason-1. .9 1.2 The number of 2-km HFR vectors for the year 2010, are represented by the color background. Not all areas are sampled equally. The blue lines are the ground tracks of the Jason-2 altimeter satellite. The black lines are the ground tracks for the interleaved Jason-1. .9 2.1 Interaction of a pulse of duration τ with a smooth sea surface; the illuminated surface geometry; and the resulting waveform . 13 2.2 Jason-2 Ku Band Echo (in black). Brown waveform (in green) and parameters retrieved by MLE-4 . 13 2.3 (a) 20-Hz Waveforms from Jason-2 Cycle 26 pass 221, February 3 2008; (b) typical Brown Waveform; (c) Consecutive 20-Hz Waveforms, 30 km off shore in the presence of a Sig0-bloom event, over the region indicated by a black box in a) (d) other bloom waveform shapes at 21.18 km and 17.55 km.
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