The Weakening of Intense Tropical Cyclones Due to Shallow Layers of Vertical Wind Shear

The extent to which a shallow layer of environmental vertical wind shear in the upper-troposphere is capable of weakening a tropical cyclone (TC) remains unclear. Upper-level shear is less effective than deep-layer shear at tilting a TC vortex, but may be more effective at transporting relatively cool air from the environment into (i.e. “ventilating”) the warm core. Recently, Hurricane Delta (2020) – a small and intense TC observed as part of the TCRI field campaign – unexpectedly weakened in a seemingly favorable environment prior to its first landfall in Mexico. We hypothesize that this rapid and unexpected weakening was a result of ventilation from a shallow layer of vertical wind shear in the upper troposphere. This study uses a series of idealized WRF simulations inspired by Hurricane Delta to examine the weakening response of an intense TC exposed to a shallow layer of moderate, upper-level vertical wind shear. In a control simulation, a small TC initialized in a uniform environment with 2 m/s of easterly shear between 6 and 12 km undergoes rapid intensification. Gradually increasing easterly shear within this layer to 8 m/s near the end of rapid intensification, while simultaneously decreasing relative humidity (RH) by up to 25% in the same layer, causes the storm to begin rapidly weakening 12 hours after the shear starts to increase. Similarly rapid weakening occurs without reducing RH, which indicates a limited role of upper-level dry air in weakening the TC. The simulated TC does not tilt substantially in response to increasing upper-level shear, but we find evidence of shear-induced ventilation of the warm core just beneath the outflow layer. In additional experiments, we increase the size of the initial vortex such that the RMW is 66% and 166% larger than in the original experiment when the shear is imposed. The larger TCs weaken less in response to the same shear due to weaker ventilation of the warm core. We also conduct a simulation in which we impose vertical wind shear in a lower layer (3–7 km) and find that the TC weakens by a much smaller amount than in upper-level shear. These results suggest that small, rapidly intensifying TCs are especially prone to weaken upon encountering shallow layers of vertical wind shear in the upper troposphere.