Prediction and diagnosis of the motion and rapid intensification of Typhoon Sinlaku during TCS08 (Tropical Cyclone Structure Experiment, 2008)

We investigate the motion and rapid intensification of Typhoon Sinlaku, which occurred during the 2008 Tropical Cyclone Structure Experiment (TCS08) over the Northwest Pacific. For the period 9 to 12 September 2008, Sinlaku underwent large motion and internal structure changes, including the development of multiple eyewalls and possibly multiple wind maxima at radii varying between 200 and 20 km.

We demonstrate that Sinlaku intensified as large scale cloud features propagating from the west, east and south collided to the east of the Philippines. These features were associated respectively with an easterly wave (the precursor to Sinlaku), the Madden-Julian Oscillation (or northwest Pacific Monsoon), and possibly a Planetary Rossby Wave from the Southern Hemisphere. An amplifying tropical upper tropospheric trough seemed also to be associated with the intensification.

Accurate prediction of the short-term track and intensity of tropical cyclones (TCs) currently relies on vortex specification to define the inner-core at the observed location. Here we investigate the sensitivity of prediction of track, structure and intensity to specification of initial vortex structure. Sensitivity experiments are performed at 15- and 5-km grid spacing with 29 vertical levels using the full-physics version of the operational Australian Bureau of Meteorology's Tropical Cyclone Limited Area Prediction System, TC-LAPS. The system includes sophisticated vortex specification, data assimilation, advanced numerics, a bulk explicit microphysics scheme and a mass flux convection parameterization.

We show that Sinlaku's environment evolved towards one of weak steering and weak vertical wind shear. We illustrate that the use of an initial vortex with a large radius of maximum wind (RMW) greatly improves the track prediction. We suggest that this is related to the increased influence of the beta-effect associated with strong winds at the large RMW, under weak environmental steering.

Most of the 15-km forecasts produce the observed intensification. Results at 5-km suggest that the pathway to RI, for the same environmental forcing, is dependent on initial storm structure. Evidence suggests that initial vortex structure plays a critical role in eyewall cycles. With an initially large RMW, the 5-km forecast is able to capture the observed double eyewall structure and the evolution of structure and intensity more skilfully. We illustrate the process and discuss some possible dynamical interpretations.

Implications for the initialization of vortex structure to take into account inner-core structure and processes that might occur during rapid intensification are discussed.