Landfalling tropical cyclones accelerate due to land–sea thermal and roughness contrasts

Changes in the translation speed of landfalling tropical cyclones (TCs) pose great challenges in disaster preparedness. While some recent studies have discussed the increased chance of a reduction in the annual-mean translation speed of TCs after landfall, such changes before landfall have not been systematically investigated, especially for short-term variations (that is, hour-to-day timescales). Here we show, first based on observations, that globally, a TC about to make landfall tends to accelerate towards the coast, with an average acceleration of about 0.83 m s−1 per day, which means that the mean translation speed of a landfalling TC increases by ~48% during the 60-h period before landfall. Such an acceleration exists irrespective of TC intensity, seasonality and ocean basin, although its magnitude varies. Numerical simulations demonstrate that land–sea differences in surface roughness and thermal effect result in asymmetric circulation and convection in TCs, both of which are enhanced as the TC moves closer to the coast, leading to local changes in potential vorticity and thereby accelerating the storm. As this phenomenon is due to the land–sea contrast, a TC approaching the coast will probably have such an acceleration and hence it is inherent. Landfalling tropical cyclones generally accelerate as they approach coastlines due to changes in surface roughness and thermal properties, according to numerical modelling simulations and global observations.