A Hypothesis for the Intensification of Tropical Cyclones under Moderate Vertical Wind Shear

An important roadblock to accurate tropical cyclone forecasts is the issue of how and why some tropical cyclones can intensify under moderate vertical wind shear. Recent studies suggest that thermodynamic factors are critical—tropical cyclones are likely to intensify under moderate shear if the environmental water vapor is large and uniformly distributed around the low-level center of circulation. This study seeks to expand those findings by diagnosing relevant physical processes driving intensity changes and exploring the sensitivity of those mechanisms to the amount and distribution of tropospheric water vapor. To this end, a set of idealized numerical experiments was produced with the Advanced Research WRF model. All experiments used average conditions representative of tropical cyclones in sheared environments, but the amount of water vapor was either increased or decreased to generate simulations with differing thermodynamic environments.

The simulations consistently show that convective symmetrization and vortex alignment precede the onset of intensification. Detailed analysis reveals that symmetrization and alignment are conducive to intensification because the vertical mass flux profile transitions from being predominantly top-heavy (deep convection + stratiform precipitation) to being more bottom-heavy (deep + shallow convection + stratiform precipitation). A vorticity budget analysis confirms that the increased lower-tropospheric vertical mass flux aids intensification via large near-surface vortex stretching underneath a midtropospheric vortex. Lastly, the comparison between different thermodynamic environments demonstrate that increased water vapor prevents dry air intrusions in the lower troposphere and promotes a deeper and stronger vertical mass flux, thus leading to a faster convective symmetrization than the experiments with decreased water vapor.