Comparing short term intensity fluctuations and an Eyewall replacement cycle in Hurricane Irma (2017) during a period of rapid intensification

Abstract. An eyewall replacement cycle that occured after a period of rapid intensification of Hurricane Irma (2017) between 07 September and 08 September was investigated in a detailed modelling study using Met Office Unified Model (MetUM) convection permitting ensemble forecasts. The eyewall replacement cycle was then compared to intensity fluctuations that occurred during the period of rapid intensification between 04 September and 06 September. Both the short term fluctuations and eyewall replacement cycle involved an initial dynamical response to localized convection from inner rainbands in the former case and outer rainbands in the later case. One key difference between the intensity fluctuations and the eyewall replacement cycle is that, in the case of the intensity fluctuations, the small radial distance between the eyewall and the inner rainbands meant that a moat region did not form and hence the intensity fluctuations did not lead to an eyewall replacement.