Storm wave monitoring in Rio de Janeiro

Sofar Ocean

This blog was written by Lieutenant Tobias Ramalho, Felippe Galdino, Lieutenant Felipe Léllis, Luiz Alexandre A. Guerra, and André Lima T. Mendes of the Brazilian Navy.

The Marine Meteorological Service of Brazil’s (SMM’s) mission is to generate and disseminate weather forecast products, and weather warnings for the maritime area known internationally as METAREA V, in accordance with Brazil’s obligations to the International Convention for the Safety of Life at Sea, 1974/1988. 

On February 4, the SMM--part of the Marine Hydrography Center (CHM) of the Brazilian Navy--published a High Surf Warning for the South and Southeast coast of Brazil. To monitor and record storm conditions, the Brazilian Navy mobilized a team immediately to deploy a Spotter buoy for one week.

This deployment served as an experiment to determine the feasibility of using the Spotter to monitor storm waves near the shore. As a point of comparison, the Spotter was attached to a metocean buoy from the Brazilian Coast Monitoring System (SiMCosta) in front of Copacabana Beach (Figure 1). This arrangement would make it possible to compare data collected by two buoys that use different technologies for wave measurements. Specifically, the Spotter uses GPS signals whereas the metocean buoy WatchKeeper RJ-3 uses inertial sensors. 

Figure 1: Location of the buoy deployment, closed to WatchKeeper RJ-3 buoy ( Deployment depth: 18 meters.

Figure 2: Arrangement of the Spotter attached to a reference metocean buoy. You can see the silhouette of the mountains of the City of Rio de Janeiro and the Sugarloaf Mountain in the background.

On February 6th, the analysis fields of the Wavewatch III model showed large waves hitting the coast, due to an extratropical cyclone between Uruguay and South Brazil (Figure 3).

Figure 3: Significant wave height (Hs) from the Wavewatch III model driven by winds from the atmospheric model COSMO (Consortium for Small-scale Modelling) for 00Z February 6 and 7, 2021. The CHM runs both wave and atmospheric models as part of the forecasting routine.

Data transmitted by the Spotter buoy were processed in real-time using an automated quality control algorithm, and published on a web page exclusively created for this purpose. In addition to the environmental parameters, the team could also check the buoy’s position in real-time, which helped to maintain confidence that the buoy had not come off its mooring or gone missing.

Analysis of the non-directional wave spectrum showed the arrival of the swell on February 5, at 10 pm (Figure 4). A noticeable increase of wave energy occurred with peak periods between 12 and 14 seconds until February 7. During the whole storm event, the swell was dominant. On February 11, the wave energy decreased, and the period returned to 10 seconds, typical of the local sea.

Figure 4: Temporal variation of the nondirectional wave spectrum during the occurrence of the high surf event in February 2021.

With the analysis of the wave parameters sampled by the Spotter, it was possible to observe a Significant Wave Height (Hs) of up to 2.25 meters and a mean period of 11 seconds, coming from the South quadrant. The maximum individual wave height was estimated as high as 4 meters.

The comparison of the data collected by Spotter shows a strong correlation with the data from the RJ-3 buoy which was used as a reference, with Significant Wave Height (Hs) being the best-correlated variable (R = 0.987). Another variable with a good correlation (R = 0.975) was the mean period (Figure 5). However, the values measured by RJ-3 and Spotter buoys remained with an approximately constant difference during the experiment (1.2 seconds on average). Differences may be related to the different measurement methodologies and the algorithms for calculating the average period. It is essential to state that this noted difference is negligible to evaluate the effects of waves on surf events.

Figure 5: Average direction, period, and Hs measured by Spotter buoy during the high surf event off the coast of Rio de Janeiro. It is also possible to observe the comparison with the data measured by the RJ-3 buoy from SiMCosta.

In addition to the good quality of the data collected and agreement in measurement when compared to a reference metocean buoy system, the ease of handling the Spotter and its low cost showed that it is ideal for data collection over short periods that demand a quick mobilization.  The reduced size and weight of the Spotter buoy facilitates a fast response in situations such as the one faced on February 4th, 2021 by the SMM, and allowed them to deploy immediately from vessels of opportunity. Finally, the lower weight of the buoy implies a lower impact on the mooring, thereby reducing the potential for mooring breakage. Data from the buoy can be used for calibration and data validation of numerical models, especially in highly energetic events such as high surf and storm surges. Improving forecasts and warnings of bad weather contributes directly to our goals of Safety of Navigation and the Safeguard of Human Life at Sea--with the additional benefit of positively impacting economic and leisure activities.

We greatly appreciate the permission from Petróleo Brasileiro S.A. (PETROBRAS) and Brazilian Navy Hidrografic Centre (CHM) to publish this material from a Brazilian Petroleum Agency (ANP) funded Project. We thank to Projeto SiMCosta for sharing RJ-3 Buoy data and letting the attachment of Spotter buoy to it. Thank should also be directed to CHM Numerical Forecast Division for model results and to CAMR for the logistic support. Acknowledgement is also made to Prof. Luciano Pezzi (INPE/DSR) for useful information and comments during experiment design phase.

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