On the role of the height and depth of environmental vertical wind shear in tropical cyclone intensification

We present two large sets of idealized tropical cyclone (TC) simulations demonstrating sensitivity of TC intensity to both the height and depth of a vertically sheared layer in the environmental wind profile. Our simulations suggest that vertical wind shear confined to a shallow layer and maximized lower in the troposphere is more destructive to a TC than vertical shear distributed over a deeper layer and maximized higher in the troposphere. Decreasing the depth of the sheared layer, or shifting the sheared layer toward lower altitudes excites stronger asymmetries in convection and tangential winds that are associated with vortex tilting. The more tilted vortices are characterized by slower cyclonic precession and, consequently, delayed realignment and intensification. In most cases, the prescribed height and depth of environmental vertical wind shear are skillful predictors of TC intensity change: model TCs consistently intensify when the imposed shear is maximized above 450 hPa, and consistently fail to intensify when shear is maximized below 500 hPa. However, for shear maximized between 450 and 500 hPa, intensity predictability with respect to the imposed wind profile is greatly reduced in our modeling framework.

These results underscore the importance of high vertical resolution wind observations around TCs for both improving TC intensity forecasts, and anticipating when such forecasts are subject to large errors. Given that all of our simulations have the same westerly deep-layer (200-850-hPa) shear of 10 m/s, these findings also support a growing body of evidence suggesting the deep shear magnitude may not fully describe the role of vertical wind shear in TC intensity change. Lastly, we discuss more suitable techniques for uncertainty quantification, as the distinct groups of developing and non-developing TCs in each set of simulations expose shortcomings in ensemble spread-based metrics.