Significant wave height from ERS altimeter for water wave studies in coastal regions
Wave information in coastal regions is of interest to those involved in wave forecasting applications, to designers of coastal structures as well as to those monitoring beach erosion and sediment transport. In some developing countries settlement on the land-ocean interface is getting increased proportions very rapidly. In others, hazards due to natural disasters like tropical storms and hurricanes, threaten coastal population areas in a nearly regular basis. The needs for wave measurements in shallow waters are obvious. In general, coastal engineering design and operability require of both mean and extreme wave information. Some observations of wave conditions upon which such statistics can be derived exist in coastal waters. However, that is still inadequate. Coastal wave monitoring is practically nonexistent in many countries, specially in our Latin American countries. There are, if any, wave measurements for very short period of time and rather sparse in location. In most cases, measurements would be very localized in response to particular demands associated to beach processes.
The radar altimeter on board the European Satellite ERS-1 acquired a huge amount of significant wave height measurements over the world oceans during the last 5 years or so. The use of similar data bases, mainly from other satellite sensors has already been demonstrated elsewhere for oceanic scale wave statistics (Young, 1994) and global measurements (Chelton et. al., 1981). Part of this type of information, that obtained from coastal regions is what interest us in this work. In spite of its limited amount in time and space coverage we believe it can be considered of great relevance. Significant wave estimates in the continental shelf are then used to contribute to local wave climatology, including wave height distribution.
In particular, we could easily observe the wave field evolution when approaching towards the coast when the waves are generated by intense storms. Probably this is the type of measurements mostly needed, to provide design and safety characteristics for coastal development. The question is on the estimates confidence when dealing with significant wave height from coastal regions. Signal uncertainty when the altimeter flights from land to sea might be a limitation for significant wave height estimation close to shore. Focus on data from coastal waters addressing regional aspects of the wave field is the main goal of this study. Significant wave height for several regions of interest like the southern Gulf of Mexico and Yucatan continental shelf, Gulf of California and Gulf of Tehuantepec, as well as the Argentine continental shelf in South America. Probability distribution for Hs can be provided and usefulness of ERS-1 altimeter data for coastal applications will be proven.
After quality control procedures, data is selected for further analysis to compile significant wave height statistics for the regions of interest. Example of selected data are shown in Figure 1 and 2.
Figure 1: Example of wave and wind data from ERS-1 altimeter, product no. 3670 acquired over the Yucatan continental shelf, in the Gulf of Mexico, starting at 16:38:44 GMT, 18 April, 1992. Lower plot shows the geographical location for this pass at the north of Yucatan peninsula.
Figure 2: Example of wave and wind data from ERS-1 altimeter for product no. 20769 acquired over the southern Gulf of Mexico, starting at 04:25:18 GMT, 10 April, 1992.
Significant wave height for this particular pass (product 3670, Figure 1) is of the order of 1 m, decreasing towards the coastline.
In fact, data shown in Figure 2 represents two blocks of 80 observations each, acquired over 80 seconds approximately. In this case, waves near the coast (close to the southern coastline), are relatively lower. Wind speed shown in both figures somehow correlates with the wave height.
Passes for one cycle (cycle 1 from a 35 days repeat orbit, April 14 to May 19, 1992) used to compile wave statistics for the Southern Gulf of Mexico and Yucatan continental shelf are shown in Figure 3. It can be observed that only data over a limited area is considered. Period of time spans for about one month and spatial coverage is shown to be adequate. However, this does not necessarily mean that this type of coverage include all important vents of any given time and spatial scales.
Figure 3: Passes selected for the southern Gulf of Mexico to compile wave statistics. These correspond to cycle 1 of a 35 day repeat orbit phase. The area covered is limited to coastal region and the continental shelf.
Other regions of interest are the mouth of Gulf of California ( see Figure 4), the Gulf of Tehuantepec (Figure 5), and the continental shelf off Argentina and Uruguay (Figure 6). Area covered by passes from one cycle is shown in those figures.
Figure 4: Passes selected for the mouth of the Gulf of California to compile wave statistics. These correspond to cycle 1 of a 35 day repeat orbit phase. The area also covers the continental shelf of Sinaloa, in mainland Mexico.
Figure 5: Passes selected for the Gulf of Tehuantepec to compile wave statistics. These correspond to cycle 1 of a 35 day repeat orbit.
Figure 6: Passes selected for the continental shelf off Argentina and Uruguay to compile wave statistics. These correspond to cycle 1 of a 35 day repeat orbit phase.
Figure 7: Example of histogram for significant wave height obtained from the ERS-1 altimeter data for the Gulf of Tehuantepec. Data included correspond to cycle 1 of a 35 day repeat orbit phase for some days of April and May, 1992.
An histogram for significant wave height (Hs )can be obtained from the data of each region of interest. Example for the Gulf of Mexico data is shown in figure 7, in particular for some days of April and May, 1992, corresponding to cycle 1 of a 35 day repeat orbit phase. Histogram example for the same cycle and period of time for the continental shelf off Argentina and Uruguay is presented in Figure 8.
Figure 8: Significant wave height histogram obtained from the ERS-1 altimeter data over the continental shelf off Argentina and Uruguay. Data included correspond to cycle 1 of a 35 day repeat orbit phase for some days of April and May, 1992.
When compiling the data from several cycles, 1 and 10 (February and March, 1993) of the same 35 day repeat orbit phase, and those corresponding to March, 1992, another Hs histogram can be constructed Figure 9.
Figure 9: Significant wave height histogram obtained from the ERS-1 altimeter data over the southern Gulf of Mexico and Yucatan continental shelf. Data included correspond to cycle 1 and 10 of a 35 day repeat orbit phase and data from March 1992.
Significant wave height histogram for the Gulf of Tehuantepec is shown in figure 10. Data from only two cycles is used.
Figure 10: Significant wave height histogram obtained from the ERS-1 altimeter data over the Gulf of Tehuantepec. Data included correspond to cycle 1 and 10 of a 35 day repeat orbit phase.
Discussion and plans for further work
A very strict quality control procedure must be followed in order to deal with confident wave height data in coastal regions. Passes over land-ocean transition are particularly susceptible to produce uncertain data. Comparisons with statistics obtained from in situ measurements are desirable. Therefore, it is necessary to promote long term measuring campaigns in coastal stations in the areas of interest. Activities are underway to maintain a long term measuring station near the coast in the north of the Baja California peninsula, within a research project supported by the National Research Council, Mexico (CONACyT).
Description of the wave phenomenon from the spatial point of view is considered to be feasible from further analyzed data. This is an aspect that needs some more attention. On the other hand, spatial coverage for the regions of interest seems to be adequate, considering the purpose of the present work, dealing with the acquisition of significant wave height regional statistics. Furthermore, in particular for the continental shelf in South America, coverage is rather dense, resulting in a good amount of available data for high confidence in statistic estimates.
It seems that compiling statistics from three different cycles, at least for the Gulf of Mexico data, did not make much difference in the shape of the histogram, when compared with the results from only one cycle data. However, this must be taken with caution, longer term statistics are still needed.
An interesting type of histogram is the one obtained for the Gulf of Tehuantepec region. This is rather wide compared to those presented for the Gulf of Mexico and the continental shelf in South America. This area is characteristically open to the great Pacific Ocean, therefore swell is regularly present. Besides, locally generated waves are also expected as northerlies are frequent in this region.
The assistance of S. Ramos and J. Garcia is gratefully acknowledged. ERS-1 data has been provided by ESA through Pilot Project PP2-JP2(MXOT1361). Support has been provided by the Commission of the European Communities (CI1* - CT93-0061) and CONACyT, México (3993-T9402).
Young, I. R., 1994:
Chelton, D. B., Hussey, K. J. and Parke, M. E., 1981:
Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry
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