The 23–26 September 2012 UK floods: Influence of diabatic processes and upper-level forcing on cyclone development

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Abstract

This thesis was funded by the Natural Environment Research Council (NERC) and is presented in the alternative format. The thesis comprises two separate journal articles that together form a coherent body of work. In this thesis, the key physical processes responsible for the 23–26 September 2012 UK floods are investigated using a case study approach. The cyclone responsible for the floods developed near the Azores on 20¬–22 September following the interaction between an equatorward-moving potential vorticity (PV) streamer and tropical storm Nadine. Convectively-driven latent heat release associated with the developing cyclone reduced upper-level PV and resulted in the fracture of the PV streamer into a discrete anomaly as the cyclone intensified. In Paper 1, convection-permitting model simulations and diabatic heating rate and PV tendency calculations along trajectories demonstrate that deposition heating strongly reduced upper-level PV in the vicinity of the PV streamer, contributing to its fracture into a discrete anomaly. The cyclone deepened further over the UK on 23–26 September, ahead of a second upper-level PV anomaly. In Paper 2, sensitivity simulations of the storm are presented. PV inversion is used to modify the strength and position of the PV anomaly in the initial conditions and to examine whether the event could have been even more extreme with different upper-level forcing. Results show that quasigeostrophic forcing for ascent ahead of the PV anomaly contributed to the maintenance of the rainfall band over the UK. Counterintuitively however, strengthening the upper-level forcing produced a shallower cyclone with lower rainfall totals. Instead of moving eastward over the UK to interact with the cyclone, the strengthened anomaly rotated cyclonically around a large-scale trough over Iceland, resulting in a fragmented rainfall band. The counterintuitive results suggest that the verifying analysis represents almost the highest-impact scenario possible for this flooding event.

Layman's Abstract

When clouds form in the atmosphere as water vapour turns into liquid water droplets, heat is released by condensation. Heat is also released higher up in the atmosphere when ice and snow crystals develop via a process called vapour deposition. We know that heat released by condensation can intensify large-scale storms that affect the UK. However, the intensifying effect of deposition heating on these storms is less well-understood. The first part of this thesis addresses that question by studying the development of a high-impact storm that caused major flooding over the UK in late September 2012. An important feature of the atmosphere is the presence of eastward-moving waves at the top of the troposphere, where the winds are strongest. Large-scale storms often intensify as an upper-level wave approaches. In some cases, these upper-level waves can interact with eachother as well as with the large-scale storms. This process can be unpredictable and is poorly understood. In the second part of this thesis, the interactions between upper-level waves and their role in the intensification of the storm responsible for the September 2012 floods over the UK are investigated in detail.

Keyword(s)

Diabatic heating ; Extratropical cyclone; Flooding ; Ice-phase microphysics ; Potential vorticity ; United Kingdom

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