On the limits of measuring the maximum wind speed in hurricanes

While the average errors of hurricane track forecasts have been steadily declining over the last two decades, intensity forecasts have only marginally improved. In particular, the 24-hour intensity forecast has shown no improvement, with mean errors of peak wind speed forecasts remaining around 10 knots for the last 20 years. Some recent studies have suggested that, given the current "observing system" of satellites, aircraft reconaissance, and subjective analysis, the actual peak wind speed cannot be measured to an accuracy greater than about 10 knots. In this study, we use an Observing System Simulation Experiment (OSSE) approach to test the limitations of even nearly perfect observing systems to capture the peak wind speed occurring within a tropical storm or hurricane. The data set is provided by a 1 km resolution simulation of an Atlantic hurricane with surface wind speeds saved every 10 seconds. An optimal observing system consisting of a dense field of fixed anemometers is placed in the path of the storm; this provides a perfect measurement of the peak 1-minute wind speed. In reality, reliable surface observations are very rare in hurricanes. Therefore suboptimal observing systems consisting of a small number of anemometers are sampled and compared to the truth provided by the optimal observing system. Results show that a single, perfect anemometer experiencing a direct hit by the right side of the eyewall will still underestimate the actual peak intensity by 7-9 m/s. Even an unusually large number of anemometers (e.g., 3-5) experiencing direct hits by the storm will together underestimate the peak wind speeds by 4-6 m/s. If this negative bias were already known, the underestimate could be corrected, but mean errors of 2-3 m/s would remain. Thus, it appears that the 24 hour forecast error is already close to the realistic limits of measuring hurricane wind speeds.