Evolution of Tropical Cyclone Structures during Extratropical Transition in Idealized Experiments

Abstract Tropical cyclones (TCs) undergoing extratropical transition (ET) in the midlatitudes have several structural differences from typical TCs. The link between specific structural changes and large-scale conditions during ET remains to be fully elucidated because processes can vary from one case to another, depending on various factors such as upper-tropospheric troughs and orography. To explore the fundamental evolution of ET, we conduct idealized simulations using a nonhydrostatic atmospheric model on the β plane with a horizontal grid spacing of 5 km and examine in detail the ET characteristics produced by the interaction between a northward-moving TC and a zonally uniform baroclinic zone. The control experiment reproduces the wind and precipitation patterns unique to ET and subsequent reintensification as an extratropical cyclone, which are in good agreement with observational studies. Two asymmetric processes govern the evolution and dynamics of structures during ET. Warm frontogenesis in the northeastern part of the cyclone is evident in the late ET period (frontal stage), which locally enhances wind and precipitation. In addition to this well-known frontal process, precipitation is enhanced in the slantwise convection in the northern part of the cyclone in the early ET period (prefrontal stage), where conditional symmetric stability is reduced in strong westerly vertical shear associated with the combination of the TC circulation and the westerly jet stream. Sensitivity experiments demonstrate that warm frontogenesis and reintensification are enhanced in the environment with stronger baroclinicity and that the slantwise convection north of the cyclone occurs even with spatially uniform baroclinicity. Significance Statement Tropical cyclones undergoing extratropical transition in the midlatitudes have asymmetric structures different from typical tropical cyclones. Although understanding the structures unique to extratropical transition is important for disaster mitigation in midlatitude countries, their dynamics vary from one case to another depending on complicating factors such as preexisting weather disturbances and orographic effects. To explore the fundamental evolution and dynamics of extratropical transition, we conduct simplified model experiments of the interaction between a northward-moving tropical cyclone and a jet stream. This simplified experiment successfully reproduces several characteristics unique to extratropical transition reported in previous studies. We demonstrate that the causes of asymmetric wind and precipitation patterns change from the early to late transitioning stages.