Rapid Weakening of Intense Tropical Cyclones in Moderate Vertical Wind Shear

This study describes idealized simulations that emulate the rapid weakening (RW) of category-4 Hurricane Delta (2020) after encountering moderate vertical wind shear. One set of simulations exposes a small, Delta-like vortex to wind profiles with moderate shear concentrated in an upper layer, a middle layer, or a deep layer of the troposphere. RW occurs for each wind profile and is due to shear enhancing storm-relative inflow in the mid-and upper troposphere. This inflow causes deep downdrafts to form on the upshear side of the eyewall and to the left of the vortex tilt. Backward trajectories reveal that the increasing shear causes a nearly fivefold increase in the number of air parcels radially transported from the surrounding midtroposphere into the inner core, resulting in cooling of the eyewall and the formation of downdrafts upshear. Both the downdrafts and ventilation signatures in the eyewall are evident 1 h before the onset of RW and while the vortex tilt magnitude is small (<5 km). Upper-level shear causes the vortex tilt to increase more gradually than the other shear profiles but consistently causes an eyewall replacement cycle to occur shortly after the onset of RW. In a sec-ond set of simulations, increasing the size of the vortex exposed to moderate shear reduces the amount and duration of weakening. The larger vortices resist moderate shear more effectively than the smaller vortices by maintaining more coher-ent eyewall convection, driven by higher surface enthalpy fluxes in the inner core. SIGNIFICANCE STATEMENT: We use idealized hurricane simulations to understand how vertical wind shear weakens intense tropical cyclones (TCs). The simulations are based on Hurricane Delta (2020)}a small and intense TC that rapidly and unexpectedly weakened in moderate shear. We find that for three different wind profiles with the same moderate shear, small simulated TCs rapidly weaken like Delta. Rapid weakening is due to strong downward motion in the eyewall and the inward transport of cool and dry air from the environment. Increasing the size of the TCs causes them to weaken less in response to the imposed shear, suggesting that rapid weakening depends more on the size of the vortex than on the details of the wind profile around the TC.