IMPROVING THE SHORT-RANGE FORECAST OF STORM SURGES IN THE SOUTHERN-WEST ATLANTIC CONTINENTAL SHELF USING ENSRF DATA ASSIMILATION.

DINAPOLI, M.; SIMIONATO, C. G.

Exeter

Workshop; EuroSea & OceanPredict Workshop on Ocean Prediction and Observing system design; 2022

MetOffice

In this study the assimilation of tide gauge and altimetry data into a 2D-barotropic numerical model for the Southern-West Atlantic Continental Shelf (SWACS) was developed. To do this, the preoperative 4-day storm surges ensemble prediction system developed by Dinápoli et al. (2021) was implemented for the SWACS. This new configuration, called “Model for Storm Surge Simulations'' (MSSS), considers a curvilinear grid that covers the SWACS with higher resolution along the shoreline (from 2 km to 10 km). MSSS was forced with the astronomical tide from TPXO9; daily continental discharge observations and the atmospheric variables forecasted by the Global Ensemble Forecast System of the National Centers for Environmental Prediction (GEFS). Tidal gauge and altimetry data were sequentially assimilated every 1 hour using the Ensemble Square Root Filter (EnSRF).Results showed that EnSRF’s innovations produce a positive impact upon the forecast skill up to 1.5 days, hence the 4-day forecast can be divided into two parts: the first 2 days with a stronger dependence on the initial conditions and the other 2 days purely driven by the external forcing. In addition, it was found that an assimilation window of 6 h was long enough to produce a proper model initial condition. Under this configuration, EnSRF removed biases and improved the timing of the MSSS forecasted solutions. Larger improvements were observed at the northern SWACS where non-deterministic processes, such as the atmospheric circulation, explain a large part of the sea surface height variability. There, data assimilation reduced the forecast errors up to 5%. At the southern SWACS no larger improvements were found because of the strong and deterministic tidal dynamic. Our results proved that the incorporation of EnSRF into MSSS can significantly improve the forecast of sea surface height in the SWACS and, evenmore, the accuracy of the short-range (~6 h) detection of storm surges.