The relative roles of diabatic and dynamic processes in downstream development following the extratropical transition of tropical cyclones over the western North Pacific

Periods of reduced forecast accuracy from operational global models often occur during the extratropical transition of tropical cyclones. It is hypothesized that this reduction in forecast skill is due to an incomplete understanding of the relative roles of various mechanisms that govern the ET process. Generally, the ET of a tropical cyclone involves complex interactions between the TC and the midlatitude circulation into which it is moving. Efforts to accurately characterize these interactions have typically focused on the role of various TC- and midlatitude-related processes in the transition of the TC. Here the focus is on the impact of these processes on the downstream development of the longwave pattern.

Previous studies have shown that warm-air advection and associated rising motion and latent-heat release impact the synoptic-scale longwave pattern. In a TC undergoing ET, such diabatic processes act to build the ridge immediately downstream from the decaying TC and initiate downstream development through the excitation of diabatic Rossby waves. In this study, the relative impact of this diabatic process is investigated in relation to that of upper-level dynamic processes on the midlatitude flow during ET. Dynamically, the proximity of the outflow from the decaying tropical cyclone to the right entrance region of the midlatitude jet may provide a sustained energy source to that jet as well as to the midlatitude pattern. This import of energy into the midlatitude flow may initiate downstream development without an amplifying ridge immediately downstream of the decaying TC.

The ET of Super Typhoon Man-yi (04W) in the western North Pacific is examined to determine the relative roles of the lower-level diabatic and upper-level dynamic processes as mechanisms for downstream development. Although the ET of Man-yi was not related to favorable phasing of the TC and an upstream synoptic-scale trough, the ET did result in a strong extratropical cyclone and high-amplitude downstream wave pattern across the entire North Pacific. Although warm-air advection and related diabatic processes were present, they were not the primary mechanisms that led to amplification of the downstream longwave pattern. Instead, the outflow of the transitioning TC provided the kinetic energy necessary for downstream development. This upper-level dynamic process deepened the adjacent trough and passed kinetic energy downstream which amplified the longwave pattern.

Since reduced forecast skill is often associated with increased variability in numerical forecasts, understanding the various mechanisms associated with tropical-extratropical interactions may reduce the variability associated with ET events and may increase forecast accuracy. Additionally, techniques such as the adaptive sampling of satellite observations may be used to improve initial conditions in areas associated with the particular mechanisms responsible for the forecast variability, such as in the TC outflow and midlatitude jet in the case of Man-yi.