Author: Dandan Tao

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Published: 2015

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This dissertation explores the formation and predictability of sheared tropical cyclones (TCs) through a series of convection-permitting ensemble simulations using the Weather Research and Forecasting (WRF) model with different environmental vertical wind shear, sea-surface temperature (SST), and ambient moisture conditions. Small-amplitude random moisture perturbations are introduced in the lower troposphere as the initial-condition uncertainties to generate the ensembles under different environmental conditions. Both the composites and members of ensemble sets are analyzed in this study to examine the mean dynamics of sheared TCs as well as the predictability of tropical cyclone rapid intensification onset. From the ensemble-mean aspect, it is found that the environmental shear can significantly affect the timing of tropical cyclone formation by influencing the spatial distribution of convection and subsequently changing the positive feedback between diabatic heating and the TC vortex primary circulation. Except for the initial spin-up periods, the larger the vertical wind shear, the farther and weaker the convection from the TC center, which leads to a weakening TC vortex circulation and more time is needed to start the onset of rapid intensification (RI). The simulated tropical cyclones cannot start rapid intensification during a 9-day simulation if the shear between 200 hPa and 850 hPa approaches 7.5 m or higher for a constant SST of 27 °C. Increasing SST to 29 °C reduces the tilt magnitude and thus shortens the RI onset time because of faster generation of diabatic heating at the very beginning which strengthens vortex column to resist shear effect. Reduction in the environmental moisture content will eventually lead to weakened convection and delayed or failed precession in the later stages if the TC forms at all. In summary, the development of tropical cyclones is largely depending on the magnitude of vertical wind shear and diabatic heating, which can be further altered by other environmental conditions, such as the sea-surface temperature and ambient moisture content.From the ensemble aspect, it is found that the intrinsic predictability of the RI onset time is getting worse with increasing shear magnitude until the shear magnitude is large enough to prevent the TC formation. Based on ensemble sensitivity and correlation analysis, the RI onset timing within one set is largely related to the vortex tilt magnitude, the diabatic heating distribution and the strength of the vortex primary circulation. It appears that systematic differences amongst the ensemble members begin to arise right after the initial burst of moist convection associated with the incipient vortex. This difference from the randomness inherent in moist convection first changes the TC vortex structure subtly and then leads to the deviations in systematic scales and eventually in the development of the TC vortices. On average, a higher SST has a positive effect on the TC formation and reduces the uncertainty of development under all shear conditions, while a drier environment has a negative impact on the TCs development and either broadens the ensemble spread of RI onset time or prevents the storm from forming when the shear-induced tilt is large. It is also found that the shear modulation is quite significant that the effect of randomness of moist convection at the very beginning is overwritten by the difference in shear magnitudes.