An overview of the development of heavy precipitation associated with the extratropical transition of tropical cyclones

Midlatitude coastal regions adjacent to all ocean basins that contain tropical cyclones are threatened by the poleward movement and extratropical transition of tropical cyclones. It is well known that a mature tropical cyclone contains bands of heavy precipitation that are also associated with strong winds. In most cases, the heavy precipitation occurs near the circulation center. However, when a tropical cyclone moves poleward and begins to interact with the midlatitude environment, the characteristics of the cyclone change and broad cloud and precipitation patterns develop. Whereas the expanding cloud field associated with a poleward-moving tropical cyclone includes high clouds due to the tropical cyclone outflow into the midlatitude westerlies, a significant impact on midlatitude regions is associated with heavy precipitation embedded in the large cloud shield. This heavy precipitation is not always anticipated as it begins far poleward of the center of the tropical cyclone, which is the focus of the media and public attention. Due to the expansion of the precipitation region when the tropical cyclone moves poleward, heavy precipitation can occur over land without the tropical cyclone center making landfall. If the heavy precipitation associated with the central region of the tropical cyclone then falls in the same region as the pre-storm precipitation, the potential for flooding is increased. However, the relatively small scale of the tropical cyclone and the complex physical processes that occur during the interactions between a tropical cyclone and the midlatitude environment present difficult forecast problems.

In this presentation, an overview of the changes in tropical cyclone structural characteristics during extratropical transition as they relate to expansion of heavy precipitation regions is examined. Extratropical transition has been defined to begin during a transformation stage and lead to an eventual re-intensification stage. Specific physical attributes of each stage are related to the expansion of the precipitation field as the tropical cyclone enters the midlatitudes. These attributes are discussed in terms of the dissipation of the central warm core of the tropical cyclone and development of frontal regions at the interface between the tropical circulation and the primary midlatitude baroclinic zone. Precipitation patterns are related to the degree of coupling between the decaying tropical circulation and the midlatitude environment into which it is moving.

The impacts associated with the expansion of heavy precipitation due to the poleward movement of a tropical cyclone are examined for each ocean basin that contains tropical cyclones. Characteristics common to all basins are associated with the transformation and re-intensification stages of extratropical transition. However, variations between basins exist due to the variability of midlatitude circulation characteristics.