Storm surge forecasting: quantifying errors arising from the double-counting of radiational tides

Abstract. Tide predictions based on tide-gauge observations are not just the astronomical tides, they also contain radiational tides – periodic sea level changes due to atmospheric conditions and solar forcing. This poses a problem of double-counting for operational forecasts of total water level during storm surges. In some surge forecasting, a regional model is run as tide-only, with astronomic forcing alone; and tide-and-surge, forced additionally by surface winds and pressure. The surge residual is defined to be the difference between these configurations and is added to the local harmonic predictions from gauges. Here we use the Global Tide and Surge Model based on Delft-FM to investigate this in the UK and elsewhere, quantifying the weather-related tides that may be double-counted in operational forecasts. We show that the global S 2 atmospheric tide is captured by the tide-surge model, and observe changes in other key constituents, including M 2 . We also quantify the extent to which the Highest Astronomical Tide , which is derived from tide predictions based on observations, may contain weather-related components.