Inertial Instability and Tropical Cyclogenesis

  • Callum Thompson

Student thesis: Phd


This thesis was funded by the Natural Environment Research Council (NERC) and is presented in the alternative format. The thesis comprises three separate journal articles that together form a coherent research project. The climatology of tropospheric inertial instability and the instability's effect on tropical cyclogenesis are determined in three papers. In the first paper, a hierarchy of instability criteria are derived, from which, two are used to construct global climatologies of tropospheric inertial instability using the ERA-Interim reanalysis. Seasonal occurrence maps reveal that instability is most frequent in the tropical upper troposphere of the winter hemisphere, with several local maxima of instability occurrence also identified. Synoptic composites reveal that these local maxima are associated with the midlatitude jet stream, cross-equatorial flow, and the Somali jet, as well as two previously unrecognised environments: gap wind outflow and tip jets. Furthermore, long-lived and synoptic-scale regions of instability are identified in the midlatitudes, in contrast to statements in contemporary textbooks. The effect of inertial instability on tropical cyclogenesis is then investigated via statistical and idealised numerical modelling approaches. For the statistical approach, best-track data and the ERA-Interim reanalysis are used to determine statistical relationships between inertial instability aloft and the 24-h change in mean sea-level pressure for East Pacific and North Atlantic hurricanes. No correlation is found between the spatial extent of 250-hPa instability and the 24-h intensification rates in either basin. Additionally, hypothesis testing reveals no statistical difference between the 24-h intensification rates of storms that originate in environments where the upper-tropospheric flow is predominantly inertially stable vs. storms that originate in environments where the upper-tropospheric flow contains inertial instability. Subsequently, idealised numerical modelling is used to explicitly control the inertial stability of the nascent synoptic environment via an initialised zonal jet. Although nascent synoptic environments of inertial instability are more conducive for intensification, no evidence is found to support the hypothesis that this is because the instability promotes convective outflow. Instead, we find that intensification is modulated by the environmental vertical wind shear associated with the evolution of each jet.
Date of Award1 Aug 2019
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorGeraint Vaughan (Supervisor) & David Schultz (Supervisor)


  • Climatology
  • Modelling
  • Tropical Cyclogenesis
  • Tropical Cyclone
  • Reanalysis
  • Dynamic Instability
  • Inertial Instability
  • Hydrodynamic Instability
  • Instability
  • Dynamic Meteorology
  • Meteorology
  • Atmospheric Science

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