03 Jan 2022
03 Jan 2022
Status: this preprint is currently under review for the journal WCD.

Intensity fluctuations in Hurricane Irma (2017) during a period of rapid intensification

William Stanley Torgerson1, Juliane Schwendike1, Andrew Ross1, and Chris Short2 William Stanley Torgerson et al.
  • 1School of Earth and Environment, University of Leeds, LS2 9JT, Leeds, UK
  • 2Met Office, FitzRoy Road, Exeter, EX1 3PB, UK

Abstract. Intensity fluctuations observed during a period of rapid intensification of Hurricane Irma (2017) between 04 September and 06 September were investigated in a detailed modelling study using an ensemble of Met Office Unified Model (MetUM) convection permitting forecasts. These intensity fluctuations consisted of alternating weakening and strengthening phases. During weakening phases the tropical cyclone temporarily paused its intensification. It was found that weakening phases were associated with a change in the potential vorticity structure, with a tendency for it to become more monopolar. Convection during strengthening phases was associated with isolated local regions of high relative vorticity and vertical velocity in the eyewall, while during weakening phases the storm became more azimuthally symmetric with weaker convection spread more evenly. The boundary layer was found to play an important role in the cause of the intensity fluctuations with an increase in the agradient wind within the boundary layer causing a spin--down just above the boundary layer during the weakening phases whereas during the strengthening phases the agradient wind reduces. This study offers new explanations for why these fluctuations occur and what causes them.

William Stanley Torgerson et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wcd-2021-81', Roger Smith, 12 Feb 2022
  • RC2: 'Comment on wcd-2021-81', Anonymous Referee #2, 16 Feb 2022
  • AC1: 'Comment on wcd-2021-81', William Torgerson, 30 May 2022

William Stanley Torgerson et al.

William Stanley Torgerson et al.


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Short summary
We investigated intensity fluctuations that occurred during the rapid intensification of Hurricane Irma (2017) to understand their cause and effects on the storm structure. Using high resolution model simulations, we found that the fluctuations were caused by local regions of strong ascent just outside the eyewall that disrupted the storm leading to a larger and more symmetrical storm eye. This alters the location and intensity of the strongest winds in the storm and hence the storm's impact.