The Role of Vorticity Aggregation in the Rapid Intensification of Super Typhoon Nepartak (2016)

Super Typhoon Nepartak (2016) devastated Taiwan and eastern China in early July 2016, with widespread flooding and destructive winds responsible for 86 deaths and $1.85 billion in total damages. Nepartak formed on 2–3 July 2016 as a tropical depression to the south of Guam in the western North Pacific, before moving north-westward around the southern flank of an extensive subtropical ridge and eventually making landfall in southern Taiwan on 7 July. Encountering extremely favorable large-scale environmental conditions (sea surface temperatures of 30–31ºC, and 200–850 hPa deep-layer shear persistently below 5 m s^-1), Nepartak underwent rapid intensification (RI; an increase in near-surface wind speed of over 15 m s^-1 in 24 h) between 0000 UTC 5 and 6 July, strengthening from a tropical storm to a category 5 super typhoon in only 24 h, and subsequently maintained category 5 intensity for 36 h.

Nepartak’s RI period is investigated in more detail using lagged and perturbed initial condition global and convection-permitting (CP) configurations of the Met Office Unified Model (MetUM), supported by analysis (European Centre for Medium Range Weather Forecasts [ECMWF] 0.125º analysis, ERA5 0.3º reanalysis) and observational (Global Precipitation Measurement (GPM) mission) data. The CP MetUM simulations capture Nepartak’s evolution most accurately, as expected, but also have several failings. Although the simulated cyclone rapidly intensifies between 0000 UTC 5 and 6 July, its intensification rate is only about half of that observed. Furthermore, the simulated cyclone reaches its maximum intensity (category 4) 24 h too late. The ECMWF analysis also fails to capture the timing or magnitude of Nepartak’s RI period. The global MetUM simulations and ERA5 dataset both produce almost steady-state category 1 and 2 typhoons, respectively. These preliminary results suggest that the accurate representation of RI in the MetUM is extremely sensitive to model resolution, to the extent that a CP configuration is a minimum requirement.

To diagnose the dynamical processes responsible for Nepartak’s early development and subsequent RI, the flux form of the vorticity equation is applied to the CP MetUM data to calculate the circulation budget around a box enclosing Nepartak during its evolution. This technique provides insightful dynamical information about the cyclone’s intensification without the need to analyze mesoscale variations in the vorticity field near the cyclone center. Indicative of vorticity aggregation and in line with previous studies of intensifying typhoons, convergence and stretching contributed most strongly to Nepartak’s increasing lower-tropospheric circulation, opposed by friction in the boundary layer. The tilting term was generally at least an order of magnitude smaller, making only negligible contributions to the total circulation tendency. Similar circulation budget analyses are performed for additional western North Pacific typhoons undergoing RI to determine the most important dynamical processes inhibiting RI, even in favorable conditions.

Keywords super typhoon, rapid intensification, vorticity aggregation, Met Office, circulation budget