Predictability and dynamics of tropical cyclones: sensitivity to environmental shear, sea-surface temperature and moisture

The effect of vertical wind shear on the formation and predictability of tropical cyclones (TC) is studied under different environmental conditions using cloud-permitting simulations with the Weather Research and Forecasting (WRF) model. A series of ensemble experiments are performed with varying magnitudes of vertical wind shear under different sea-surface temperature (SST) and different environmental moisture conditions. Each ensemble simulation is initialized using the same idealized weak TC-like vortex but with different set of small-scale, small-amplitude random perturbations added to the innermost domain. Both the members and composites of the ensemble sets are studied. It is found that the larger the shear, the less predictable the formation and rapid intensification until the shear is stronger enough to prevent the TC formation. The ensemble spread (and thus the uncertainty) is very sensitive to small differences in shear magnitude itself around critical shear value (~ 6m/s under SST of 27°C and ~ 10m/s under SST of 29°C). With an increased SST, the storm can develop and survive under shear of 10m/s and moist surroundings but in a dry environment a 10m/s shear will prevent the storm to develop. Overall the larger the shear, the longer the duration time for RI onset and the larger the uncertainty until the shear exceeds the maximum resistible value of the given vortex. The analysis shows that the environmental shear can significantly affect the timing of tropical cyclone formation through organizing spatial distribution and intensity of the convection and then changing the positive feedback between diabatic heating and TC vortex strength. The variation in diabatic heating release and later the vortex mean circulation strength leads to the divergence in the timing of precession and vortex alignment. The higher SST condition enhances diabatic release and thus strengthens the mean vortex circulation, which later shortens the precession time. The environmental dry air dilutes the boundary layer inflow air moisture, which leads to less diabatic heating release and weaker vortex strength and finally longer precession time.