Intensity fluctuations in Hurricane Irma (2017)

During a period of rapid intensification Hurricane Irma (2017) experienced two episodes of weakening that coincided with an expansion in the storm's radius of maximum wind speed. Such fluctuations in intensity can substantially alter the impacts of a storm, but are poorly understood. A modelling study was conducted using convection-permitting ensemble forecasts from the regional configuration of the Met Office Unifed Model. The model was able to reproduce similar intensity fluctuations to those observed. The cause of the weakening was determined to be a balanced response to the diabatic heating and absolute angular momentum source associated with vortical hot tower like structures (VHTs) within the inner rainbands near the eyewall. The VHTs promoted boundary layer convergence and an unbalanced spin-up process which led to an increase in tangential wind speed outside of the eyewall and weakening at the radius of maximum wind resulting in the reorganisation of the eyewall. The intensity fluctuations were compared to a full eyewall replacement cycle that occurred later. The intensity fluctuations resemble a secondary eyewall formation event occurring close to the original eyewall, without the large region of suppressed convection typically seen in eyewall replacement cycles. The absence of this moat region results in a shorter period of weakening where the eyewall convection is disrupted but not replaced. The intensity fluctuations in Hurricane Irma differ from previously observed fluctuations, highlighting the need for further convection-permitting ensemble modelling case studies to determine the frequency, characteristics and impact of such fluctuations.