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Chris J. Short and Jon Petch

Abstract

Convection-permitting numerical weather prediction models are a key tool for forecasting tropical cyclone (TC) intensities, intensity changes, and precipitation. The Met Office has been routinely running a regional (4.4-km grid spacing), explicit convection version of its Unified Model (UM) over the Philippines since August 2014, driven by its operational global model. The principal aim of this study is to assess the performance of this model relative to the driving global model. By evaluating over a year’s worth of operational TC forecasts, it is shown that the Philippines regional model offers clear benefits for TC forecasting compared with the Met Office global model. In particular, it provides much improved predictions for the intensities of strong storms (category 3 and above) and can successfully capture some rapid intensification (RI) events, whereas the global model cannot predict RI at all. The spatial location of rainfall within intense TCs is also more skillfully predicted by the regional model, and the statistical distribution of rain rates is closer to that observed. Although the regional model adds value, notable biases are also identified, highlighting areas for future work to develop and improve the model.

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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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