Physical Mechanisms of Tropical Cyclone Intensification in Environments with Temporally Varying Vertical Wind Shear

Accurate forecasts for tropical cyclones (TCs) are essential for the mitigation of loss of life and property. Track forecasts have substantially improved in the last couple of decades, but intensity forecasts have lagged behind in skill. Intensity forecasts are particularly challenging when a TC interacts with vertical wind shear (VWS) that is neither too strong nor too weak. Although VWS often results in a weakening of the TC, sometimes TCs intensify in spite of VWS magnitudes between 5-10 m/s. These issues have inspired many idealized studies to better understand TC-VWS interactions; however, idealized simulations typically prescribe VWS that does not vary with time. Observed VWS changes substantially on time scales of 1-2 days, thus motivating the use of a time-varying shear to study TC intensity changes due to VWS. A series of experiments using the non-hydrostatic Cloud Model 1 (CM1) were performed to analyze the response of a simulated TC's intensity in an idealized environment with time-varying VWS. The simulations showed that the introduction of moderate VWS in the early stages of the TC's life cycle delays development, but, ultimately does not significantly limit the intensity. Conversely, VWS added to the environment of a TC, near the time of maximum intensity, produced a weakened storm that does not recover. Understanding how the intensity of a TC is affected by VWS is important for progress in intensity forecast improvements.