Our Inaugural Scholar for the BRIDGE Series is Dr. Rosimar Rios-Berrios
Tropical Cyclone Intensification under Moderate Vertical Wind Shear
National Center for Atmospheric Research, Boulder, Colorado
Tropical cyclones, also known as hurricanes or typhoons, are amongst the most powerful atmospheric phenomena. Accurate forecasts of where a tropical cyclone will move towards (i.e., track), how strong the accompanying winds will be (i.e., intensity), and how much precipitation will accumulate are crucial to minimize the loss of lives and property. These forecasts, however, become particularly uncertain when a tropical cyclone moves through a region of vertical wind shear. Although vertical wind shear is generally negative for tropical cyclones, many tropical cyclones can intensify under moderate shear—the range of shear magnitudes that are neither too weak nor too strong (5–10 m s−1). Explaining why and how tropical cyclones can intensify when conditions are seemingly unfavorable is a necessary step towards improved tropical cyclone prediction.
A potential hypothesis to explain tropical cyclone intensification under moderate shear is that other factors—associated with both the tropical cyclone and its environment—can help offset the negative effects of wind shear and aid intensification. This hypothesis was tested through case studies, a climatological analysis, and idealized numerical simulations. These approaches consistently showed that the three-dimensional distribution of thermodynamic quantities is key for intensity changes in sheared environments: tropical cyclones are likely to intensify under moderate shear when water vapor and precipitation are uniformly distributed around the surface center of circulation. Process-based diagnostics demonstrate that those conditions suppress dry air intrusions and favor near-surface convergence of absolute vorticity, thus promoting intensification. Altogether, these findings suggest that three-dimensional observations of thermodynamic fields are important for understanding and predicting tropical cyclone impacts.