Effect of environmental shear, sea-surface temperature, and ambient moisture on the formation and predictability of tropical cyclones: An ensemble-mean perspective

The formation and predictability of sheared tropical cyclones (TCs) are explored 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; the composites of each ensemble are analyzed in this study to examine the mean dynamics of sheared TCs. 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 exceeds 7.5 m/s 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 the increased diabatic heating closer to the TC center. Reduction in the environmental moisture content will eventually lead to weakened convection and delayed or failed precession in the latter 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. Key Points The larger the shear, the longer it takes the TCs to form and intensify Shear affects development of TCs through modulating convective intensity and spatial distribution Higher SST and moisture allow TCs to form faster and resist stronger shear