AN ASSESSMENT OF SATELLITE ALTIMETRY IN PROXIMITY OF THE MEDITERRANEAN COASTLINE

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1 AN ASSESSMENT OF SATELLITE ALTIMETRY IN PROXIMITY OF THE MEDITERRANEAN COASTLINE Fenoglio-Marc L. (1), Vignudelli S. (2), Humbert A. (1), Cipollini P. (3), Fehlau M. (1), Becker M. (1) (1) Institut für Physikalische Geodäsie, Technische Universität Darmstadt, Germany (2) Consiglio Nazionale delle Ricerche, Istituto di Biofisica, Area Ricerca CNR San Cataldo, Pisa, Italy (3) National Oceanography Centre, Southampton, European Way, SO14 3ZH Southampton, United Kingdom ABSTRACT Altimeter data from various satellite missions are investigated in the coastal zone to determine the minimum distance from the land at which they remain usable. The data used here are the standard Level 2 products and corresponding re-tracked data sets, when available. An attempt is made to determine to what extent the land and ocean characteristics might affect the altimeter data in coastal regions. The different datasets are inter-compared and the satellite-derived sea level variability is compared with that measured from tide gauges at selected sites. 1. INTRODUCTION Satellite radar altimeters measure the sea surface height as an average value over the altimeter footprint. The height accuracy is essentially determined by knowledge of the satellite orbit, the altimetric range, the environmental range corrections and the characteristics of the sea surface. While over open ocean waters there is a long history of research into quantifying uncertainty in altimeter data, in coastal regions the quality of the measurements has been less investigated so far [1, 2]. The situation in these areas is more problematic, due to the inadequacy of some corrections in shallow waters and to the land contamination in the footprints very close to the coast. Satellite altimetry has an observational record of almost 15 years from a series of missions (ERS-1 and - 2, TOPEX/Poseidon, Geosat Follow-On, Envisat, Jason-1). This huge volume of data needs to be reanalyzed and possibly exploited in the coastal region, in order to provide valuable new information about many hitherto under-sampled sections of the world s coasts. In the last few years new tracker s for the raw altimeter data have been developed to increase the number of usable footprints [3, 4]. Some re-tracked products already exist [5] whose suitability to investigate the coastal regions needs to be assessed. Selection criteria specific to coastal regions and the best suitable environmental corrections need to be investigated as well [6]. In this paper we re-analyse the widely distributed 1 Hz data and some recently available higher rate re-tracked data in proximity of selected coastlines in the Mediterranean Sea. Our main purpose is to establish to what extent altimeters are applicable close to the coast. Results from the different missions are inter-compared and then tested with corroborative in situ measurements, such as sea level variability measured from tide gauges at selected sites. 2. DATA SETS USED The region chosen as test zone for this work is the northernmost portion of the Western Mediterranean Sea, which extends between the Italian regions of Liguria and Tuscany (north and east) and the French island of Corsica (south) (Figure 1). The time period of the investigation is that of the available re-tracked product, specifically from July 2000 to August We use altimeter data from TOPEX/Poseidon, ERS-2 and ERS-1 missions. The corrections and the selection criteria used are shown in Tables 1 and 2, respectively. The GEBCO bathymetry data are used to compute depth and distance to the coast. The Level 2 Geophysical Data Records (GDR) of the Topex/Poseidon and ERS-2 altimeter missions are extracted from the Radar Altimeter Database System (RADS) [7]. They are 1 Hz data. The Retracked Geophysical Data Records (R-GDR) from the Topex/Poseidon mission are provided by NASA-JPL Physical Oceanography Distributed Active Archive Center (PODAAC, P. Callahan, release 2.1). The temporal coverage is from July 28, 2000 to August 11, 2002, corresponding to cycles 290 to 364 (without 362). The last 21 cycles cover the TOPEX-Jason collinear period. Two types of retracking data are available, corresponding to two different algorithms (least squares and maximum a posteriori). Both 1 Hz and 10 Hz data are available. Proc. Envisat Symposium 2007, Montreux, Switzerland April 2007 (ESA SP-636, July 2007)

2 Retracked data from the ERS-1 geodetic mission (ERS-1 GM) are kindly provided by De Montfort University and the Danish National Space Center (P. Berry and O. Andersen). A 2-parameter retracked product is available, which is based on eleven different retrackers. The data are available with a frequency of 18 Hz. Range and geophysical corrections were already applied [8,9]. Sea level is routinely measured at selected sites in the study region by acoustic tide gauges operated by the National Mareographic Service of the Agency for Environmental Protection and Technical Services (APAT). Data are available as quality controlled hourly values of sea surface elevation, the reference being a benchmark connected to the national datum. We select the tide gauges located in Imperia, Genova, Livorno, Porto Torres and Macinaggio, whose data are available in the time interval chosen. reference the CLS01 mean sea surface. We select the data that satisfy the criteria listed in Table 2. Table 1. Environmental and Instrumental corrections applied to Topex GDR and R-GDR and to ERS-2 GDR T/P GDR ERS-2 GDR T/P R-GDR wet Radiometer Radiometer Radiometer tropospheric dry ECMWF ECMWF ECMWF tropospheric Response to pressure ionospheric Dualfrequency IRI95 Dualfrequency Ocean tide Solid earth FES2004 FES2004 FES2004 tide Load tide FES2004 FES2004 FES2004 Pole tide Sea state bias Bm3/bm4 B3/bm4 Bm3/bm4 Table 2. Selection criteria applied to Topex GDR and R-GDR and to ERS-2 GDR Figure 1. Study area showing some T/P ground tracks and the position of ERS tracks. 3. APPLIED CORRECTIONS As we compare sea level heights from altimetry with tide gauge observations, we do not correct the altimeter measurements for the ocean tide, the pole tide and the barometer effect [10]. We apply the standard corrections to the altimeter measurements as summarised in Table 1. These include corrections for environmental (path delay in troposphere and ionosphere, response to pressure), geophysical (solid earth tide and load tide) and sea state bias corrections. Sea level height anomalies are computed using as Filters TP/ERS-2 RADS_GDR RGDR N obs 8.5 < val < 10.5 >= 10 Sigma range 0 < val < < v < 0.15 Dry tropo -2.4< v < < v< -1.9 Radio wet tropo -0.6 < v < < v < Iono dual < v < < v < 0.04 freq. Solid tide -1 < v < 1-1< v < 1 Load tide -0.5 < v < < v < 0.5 Sea state -1. < v < bias SWH 0 < v < 8. 0 < v <11.0 Sigma SWH 0 < v < < v < 0.9 Backscatter 6 < v < 27 db 7dB < v <30dB sigma0 Wind speed 0 < v < 30 m/s 0 < v < 30m/s Off nadir -1d20 < v < 1d AGC_RMS_K --- < 100 AGC_Pts_Avg ---- > 10 Flag ocean Checked GeoBad_1 bit1 Flags Iono Checked Iono_Bad Flag land radiometer Checked / no checked Geo_Bad_1 bit2 Flag no rain Checked Geo_Bad_2 bit0 Flag no ice Checked Geo_Bad_1 bit3 Flag TMR_Bad Checked TMR_Bad

3 The differences between the handling of Topex and ERS-2 from the RADS database are the choice of the type of ionosphere correction applied (no dual frequency available in ERS-2) (Table 1) and the check of the radiometer flag in Topex, but not in ERS-2 (Table 2). The reason for this last difference is the elimination of a huge amount of ERS-2 data, not only near to the coast, when the check of the radiometer instrumental flag is included in selection criteria for ERS-2. The handling of Topex GDR and RGDR data should in principle be the same, however differences can arise from the unavailability of some of the corrections as directly applicable by the user (e.g. the sea state bias) or from checking of flags that have slightly different names in the RADS database. 4. ALONG-TRACK COMPARISON Figure 2 displays the sea level anomaly for GDR, R- GDR (from both retrackers) versus the latitude for cycle 328 and pass 44 (see position in Figure 1). The bathymetry is shown as a blue line and points are colour-coded depending on the distance to the coast. The closest distance to the coast near Genova for the selected cycle and track is 17.2 km with retracked data, whereas it is 27.3 km for GDR. There is a bias of a few cm between the GDR and the R-GDR sea level anomalies. Figure 2. Sea level anomaly relative to the CLS01 mean sea surface along Topex cycle 328, pass 44 in the Ligurian Sea from GDR and RGDR retrackers 1 and 2. The bathymetry is given by the blue continuous line Fig. 3 displays the sea level anomaly versus the latitude for ERS-2 cycle 55 and pass 801 near Genova. The bathymetry is shown as a blue line. The closest distance to the coast for the selected cycle and pass is 5 km. Figure 3. Sea level anomaly relative to the CLS01 mean sea surface along ERS-2 cycle 55, pass 801 in the Ligurian Sea (from the GDR). The bathymetry is given by the blue continuous line Figure 4. Sea level anomaly relative to the CLS01 mean sea surface from the retracked ERS-1 GM data at 18Hz (dots) and at 1 Hz (after smoothing with a 19-point Hanning window and resampling) along pass N in the Ligurian Sea. The bathymetry is given by the blue continuous line. The left panel of Fig. 4 displays the tracks of the retracked data from the ERS-1 geodetic mission. Track is highlighted and shown in the right panel. It is located in the same region as tracks in Fig. 2 and 3.

4 Retracker No. 9 performs normal Brown waveform retracking and occurs most often. Values corresponding to retracker No. 8, which does a fat patch waveform retracking, are shown as triangles. Their color gives the distance to the coast. The 1 Hz data, in grey, are obtained by filtering the 18 Hz data. The closest usable point to coast is at a distance of 1.6 km. The along-track sea level anomaly has a behaviour comparable to the along-track sea level anomaly shown in Figs. 2 and 3 for the Topex and ERS-2 passes near to Genova. Sea level anomalies from the Topex retracked data are significantly smaller than those from the Topex GDRs. This is seen in Figure 2 and in Fig. 7. The distances to the coast of the nearest points depends on the different morphological coastal conditions in the areas. There is a slight evidence of larger differences between R-GDR and GDR towards larger distances from coast. 5. COMPARISON AT NORMAL POINTS We compute time series at normal points (NPs) along the tracks. We define the normal points on the basis of the equator passing time of the track; we do not account for cross-track and along-track departure of the point from its mean position over the considered interval. The six NPs nearest to the coast are investigated analysing their following properties: number of time points, distance to coast, depth, standard deviation of the differences between altimeter and tide gauge sea level (Table 3). Figure 6. Scatterplots of sea level heights near Genova from Topex GDR and RGDR data. Figure 5. Sea level anomaly time-series at a normal point near Genova (pass 44, normal point 862) from Topex GDR (green), RGDR1 (red) and RGDR2 (violet). The blue and black curves are interpolated heights from the tide gauge and their differences w.r.t. RGDR1, respectively Using the retracked dataset more time samples are retained at each normal point and we approach more to the coast. The correlation of the altimeter and tide gauge time series is lower. Only values within 3 σ from the mean are considered. Figure 7. Scatterplot of sea level heights near Porto Torres from Topex GDR and RGDR data. The average difference between the retrackers 1 and 2 is small. In pass 044 of cycle 328 the difference between the retrackers varies from to 0.062m. The largest variations in this pass over all cycles go from to 2.031m. The differences between the retrackers are represented by a gaussian-like distribution with peak near to zero.

5 Table 3. Statistics at Normal points near Genova (pass 44) from GDR, RGDR1 and RGDR2: number of times, correlation and standard deviation (mm). NP Depth Dcoast Nt Cor Std Nt Cor Std Nt Cor Std (km) GDR GDR GDR RGDR1 RGDR1 RGDR1 RGDR1 RGDR1 RGDR CONCLUSIONS The along-track comparison of retracked 18 Hz ERS-1 data of the geodetic mission and of the GDR and RGDR missions show a similar behaviour of the sea level height anomaly in the same area. The analysis of sea level height variability from the Topex exact repeat mission shows that starting from 25 kilometres from the coast we could extract time-series at the normal points with more than 50% of the available data from the 1 Hz GDR data. Using the same criteria for the retracked Topex data, we obtain more data at distances smaller than 25 Kilometres from the coast and a comparable number of data at bigger distances. The use of the retracked data is therefore promising for coastal investigations. At present, for our example of tide gauge stations, the correlation between the retracked and the tide gauge data is lower than that of the GDR data. This holds independently from the number of points in the timeseries. This may be due to points not yet eliminated by the selection criteria or to other reasons presently under investigation. Acknowledgements. We kindly acknowledge ESA, CNES, JPL and the Montfort University for the altimetry data, APAT for the tide gauge data. 7. REFERENCES 1. Vignudelli S., Snaith H. M., Lyard F., Cipollini P., F. Venuti, Birol F., Bouffard J., Roblou L.: Satellite radar altimetry from open ocean to coasts: challenges and perspectives, United States Society of Photo-Optical Instrumentation Engineers (SPIE), Vol. 6406, 64060L 1-12, doi: / , Vignudelli S., Cipollini P., Roblou L., Lyard F., Gasparini G. P., Manzella G. M. R., and Astraldi M., Improved satellite altimetry in coastal systems: Case study of the Corsica Channel (Mediterranean Sea), Geophys. Res. Lett., 32, L07608, doi:1029/2005gl22602, Deng X., W. Featherstone, C. Hwang and P.A.M. Berry, Estimation of contamination of ERS- 2 and Poseidon satellite radar altimetry close to the coasts of Australia, Marine Geodesy, 25: Hwang C., J. Guo, Y. Liu and X. Deng, Coastal gravity anomaly from retracked Geosat/GM altimetry: improvement, limitation and role of airborne gravity data, submitted to J. of Geodesy Callahan P.S. and E. Rodriguez, Retracking of Jason-1 data, Mar. Geod. 27: Carrère, L., and F. Lyard (2003), Modeling the barotropic response of the global ocean to atmospheric wind and pressure forcing - comparisons with observations, Geophys. Res. Lett., 30(6), 1275, doi: /2002gl Naejie M., E. Doornbos, L. Mathers, R. Scharroo, E. Schrama and P. Visser (2002), Radar Altimeter Database System: Exploitation and Extension, Final Rep. NUSP , Space Res. Organ. Neth., Utrecht, Netherlands 8. Andersen, O. B., S. Dreyer, P. Knudsen, P. A. M. Berry, E. L. Mathers, R. Trimmer, S. Kenyon, Deriving 2hz ERSs-1 Geodetic Mission Altimetry for gravity and marine geoid purposes, ESA-special publication SP572, Andersen, O. B. and P. Knudsen, The role of Satellite Altimetry in Gravity Field ing in Coastal Areas, Phys. Chem. Earth, 25 (1), 17-24, Fenoglio-Marc L., Groten E. and Dietz C., Vertical Land Motion in the Mediterranean Sea from altimetry and tide gauge stations, Marine Geodesy 27 (3-4),pp