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Sam Hardy, David M. Schultz, and Geraint Vaughan

Abstract

Major river flooding affected the United Kingdom in late September 2012 as a slow-moving extratropical cyclone brought over 150 mm of rain to parts of northern England and north Wales. The cyclone deepened over the United Kingdom on 24–26 September as a potential vorticity (PV) anomaly approached from the northwest, elongated into a PV streamer, and wrapped around the cyclone. The strength and position of the PV anomaly is modified in the initial conditions of Weather Research and Forecasting Model simulations, using PV surgery, to examine whether different upper-level forcing, or different phasing between the PV anomaly and cyclone, could have produced an even more extreme event. These simulations reveal that quasigeostrophic (QG) forcing for ascent ahead of the anomaly contributed to the persistence of the rainfall over the United Kingdom. Moreover, weakening the anomaly resulted in lower rainfall accumulations across the United Kingdom, suggesting that the impact of the event might be proportional to the strength of the upper-level QG forcing. However, when the anomaly was strengthened, it rotated cyclonically around a large-scale trough over Iceland rather than moving eastward as in the verifying analysis, with strongly reduced accumulated rainfall across the United Kingdom. A similar evolution developed when the anomaly was moved farther away from the cyclone. Conversely, moving the anomaly nearer to the cyclone produced a similar solution to the verifying analysis, with slightly increased rainfall totals. These counterintuitive results suggest that the verifying analysis represented almost the highest-impact scenario possible for this flooding event when accounting for sensitivity to the initial position and strength of the PV anomaly.

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Sam Hardy, Juliane Schwendike, Roger K. Smith, Chris J. Short, Michael J. Reeder, and Cathryn E. Birch

Abstract

The key physical processes responsible for inner-core structural changes and associated fluctuations in the intensification rate for a recent, high-impact western North Pacific tropical cyclone that underwent rapid intensification [Nepartak (2016)] are investigated using a set of convection-permitting ensemble simulations. Fluctuations in the inner-core structure between ringlike and monopole states develop in 60% of simulations. A tangential momentum budget analysis of a single fluctuation reveals that during the ringlike phase, the tangential wind generally intensifies, whereas during the monopole phase, the tangential wind remains mostly constant. In both phases, the mean advection terms spin up the tangential wind in the boundary layer, whereas the eddy advection terms deepen the storm’s cyclonic circulation by spinning up the tangential wind between 1.5 and 4 km. Calculations of the azimuthally averaged, radially integrated vertical mass flux suggest that periods of near-constant tangential wind tendency are accompanied by a weaker eyewall updraft, which is unable to evacuate all the mass converging in the boundary layer. Composite analyses calculated from 18 simulations produce qualitatively similar results to those from the single case, a finding that is also in agreement with some previous observational and modeling studies. Above the boundary layer, the integrated contribution of the eddy term to the tangential wind tendency is over 80% of the contribution from the mean term, irrespective of inner-core structure. Our results strongly indicate that to fully understand the storm’s three-dimensional evolution, the contribution of the eddies must be quantified.

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