Harmful algal bloom forecast system for SW Ireland. Part I: Description and validation of an operational forecasting model

Abstract

A 3D primitive equation coastal ocean model for southwest Ireland, called the Bantry Bay model, was developed and implemented operationally. Validated model outputs have multiple uses. One of the incentives to develop the model was to explore the possible transport pathways that carry harmful algae blooms (HAB) into Bantry Bay. The model is nested offline in a regional North East Atlantic operational model. Surface forcing is taken from the half-degree Global Forecasting System, available at three-hourly intervals. Heat fluxes are calculated from the bulk formulae. Surface freshwater fluxes are obtained from the prescribed rainfall rates and the evaporation rates calculated by the model. Freshwater discharges from five rivers are included in the model. Model validation and the model skill in representing the water level, currents, temperature and salinity in the bay are reported. A scoring system based on the average adjusted relative mean absolute error for the predicted currents was used. An upgrade to a higher score was achieved through the incorporation of local winds into the surface forcing and by varying the bottom roughness coefficient. The model, designed to work in forecast mode, can replicate the main oceanographic features in the region. The model forecast is used in a decision support system for HAB alerts. An operational HAB alert system did not exist in Ireland prior to the use of this model.