Modulation of tropical cyclone rapid intensification by mesoscale asymmetries

Computer model simulations are one of the most important tools in current use for understanding tropical cyclone (TC) formation and rapid intensification (RI). These include "idealized" simulations in which a TC-like vortex is placed in a hypothetical environment with predefined sea surface temperature and vertical profiles of temperature, humidity, and wind that are either constant or slowly varying across a large domain. The vast majority of such simulations begin with a perfectly circular vortex as the precursor to a TC. However, most real TCs form or intensify while interacting with asymmetric wind fields either within or external to the vortex circulation. This study introduces a method to initialize idealized TC simulations with asymmetries, and investigates the extent to which such asymmetries might delay RI in favorable environments. It is shown that mesoscale asymmetries can delay RI and reduce the fastest rates of intensification, and that these effects are statistically significantly increased when relatively low values of vertical shear of the horizontal wind are present. In some cases the asymmetries tilt the vortex directly through advection. In other cases, the wind asymmetries increase the disorganization of the convection or increase the size of the inner-core wind field, and thus make the weaker TC more susceptible to environmental wind shear. The results suggest that mesoscale asymmetries of the wind field could be useful predictors for delay of RI in otherwise favorable environments. Rapid intensification of tropical cyclones presents one of the most dangerous risks in weather forecasting and emergency preparation. This study uses numerical simulations to assess whether asymmetries in and around the wind field of a developing tropical cyclone will delay or weaken rapid intensification. It is shown that such asymmetries do delay rapid intensification, and that this delaying effect is greatly enhanced when the storm is also experiencing low to moderate levels of environmental wind shear.image