Does Jet Stream Sharpness Modulate the Downstream Response to Recurving Tropical Cyclones?

Global forecasting systems struggle to maintain the strong or “sharp” isentropic potential vorticity (PV) gradients that are associated with upper-tropospheric jet streams. Smoothing such PV gradients has been shown to reduce both the eastward propagation and amplitude of Rossby waves throughout the forecast period. Recurving tropical cyclones (TCs) that interact with the mid-latitude jet streams occasionally excite Rossby waves that amplify the upper-tropospheric flow several thousand kilometers downstream on times scales of 5-10 days and have been associated with periods of low forecast skill or “busts.” Both the excitation and development of a downstream response to recurving TCs fundamentally depend on the structure of the jet stream waveguide, such that changes in the width or sharpness of the jet stream would conceivably change the downstream response to recurving TCs. In order to understand the implications of maintaining jet sharpness for accurately predicting the downstream response to recurving TCs, we conduct a series of idealized simulations using a full-physics, atmosphere-only version of the COAMPS-TC model in which an initial zonal jet is progressively smoothed in the meridional and vertical directions. The amount of smoothing applied to the jet in the different experiments is designed to be consistent with what is found to occur over the course of an operational medium-range forecast from a global modeling system. How the jet sharpness affects the dynamics of Rossby wave initiation and downstream propagation is discussed, and the sensitivity of downstream wave activity to the initial jet sharpness is quantified. In an additional set of simulations that deactivate condensational heating, we also explore the extent to which diabatic heating in moist convection acts to sharpen the PV gradients across the tropopause.