Factors influencing downstream anti-cyclogensis during extratropical transition

Extratropical Transitions (ET) -- the process by which a warm-core tropical cyclone (TC) transforms into a cold-core baroclinic system -- present a major predictability problem for numerical weather prediction (NWP) models. The interaction between the transitioning TC in the western Pacific Ocean and the mid-latitude flow can generate significant errors in global models and spawn Rossby wave packets that will impact forecasts for both North America and Europe. Two mechanisms have been discussed in the literature to explain how the TC can modify the extratropical circulation: (i) adiabatic interaction between the PV of the TC and the mid-latitude jet and (ii) the diabatic reduction of upper tropospheric PV by convection in the TC; however, it is not clear how each of these processes contribute to the downstream anti-cyclogenesis and associated Rossby wave.

Here we investigate the factors that lead to downstream anti-cyclogenesis during two recent western Pacific ET events using a 90 member Weather Research and Forecasting (WRF) model EnKF. An analysis ensemble is generated every six hours by assimilating in-situ observations from surface stations, rawinsondes, aircraft and cloud motion vectors. The statistics of this ensemble are used to understand the role of the TC and mid-latitude flow in amplifying the downstream ridge. For the transition of Typhoon Tokage (2004), there are two main regions of precipitation near the storm, one associated with the TC itself and another associated with developing warm front on the poleward side of the TC. Ensemble statistics indicate that the building ridge is sensitive to the amount of precipitation along the warm front, but is relatively insensitive to the precipitation in the TC core. Moreover, these results show that the amplitude of the ridge is inversely proportional to the poleward moisture flux to the east of the TC. This research suggests that observations of lower-tropospheric moisture and winds during ET may improve forecasts of Rossby wave genesis in the western Pacific.