Structural Changes in Tropical Cyclones during Extratropical Transition in Idealized Experiments

Tropical cyclones (TCs) change their structures during extratropical transition at the mid-latitudes. Previous case studies have shown that transitioning TCs have different structures from typical TCs in the tropics, such as left-of-track precipitation, warm frontogenesis, and horse-shoe-shaped wind patterns. For example, heavy precipitation of Typhoon Hagibis (2019) was concentrated on its north side during extratropical transition, and caused a devastating disaster in Japan. On the other hand, the dynamics of extratropical transition vary from one case to another depending on several factors such as upper-tropospheric troughs, orography, and non-uniform environments. It is worth examining what processes of extratropical transition occur and their sensitivities to various environmental factors in simplified atmospheric conditions.

We conduct idealized numerical experiments using a Japan Meteorological Agency Non-Hydrostatic Model (JMA-NHM) with a horizontal grid spacing of 5 km. The horizontal domain is 12000 km and 7000 km in the zonal and meridional directions, respectively. The zonal boundaries are periodic, and the meridional boundaries are free-slip walls. Moist processes are represented by a 5-class bulk microphysics scheme and the Kain-Fritsch cumulus parameterization. The Coriolis parameter is given by a β-plane approximation at 45°N centered at y=4500 km. The environmental field has a zonally-uniform baroclinic zone centered at y=4500 km, with a westerly jet that is in thermal wind balance with a prescribed hyperbolic-tangent meridional distribution of temperature. The sea surface temperature is 1°C higher than the near-surface atmospheric temperature. A Rankine-type vortex with maximum tangential wind speed of 20 m s^-1 is initialized at y=1500 km.

During the time integration of 250 h, the vortex moves northward owing to the β drift first, and then northeastward due to the environmental westerly flow in the baroclinic zone. During the first 60 h, the vortex develops into an axisymmetric TC over the warm sea south of the baroclinic zone. When the cyclone approaches the baroclinic zone, it starts extratropical transition. In the early stage of the transition, precipitation associated with slantwise convection is concentrated to the north of the cyclone, where moist symmetric stability is reduced owing to the westerly vertical shear in the baroclinic zone. This convection is accompanied by a northward outflow in the upper troposphere toward the westerly jet axis. In the later stage of transition, precipitation associated with warm frontogenesis is concentrated to the northeast of the cyclone, where horizontal deformation enhances the horizontal gradient of potential temperature. This change in precipitation mechanisms from the early to late stages is consistent with that observed in Typhoon Hagibis (2019). The experiment also reproduces several characteristics of extratropical transition reported in case studies such as left-of-track precipitation, delta-shaped rain regions, and horse-shoe-shaped wind patterns. In additional model experiments, we examine the sensitivity of these dynamics to changing the structure of the baroclinic zone.