Targeting Studies for the Extratropical Transition of Hurricane Fabian: Signal Propagation, the Interaction between Fabian and Midlatitude Flow, and an Observation Strategy

Abstract

Abstract This study examines how the impact of targeted observations propagates during the extratropical transition (ET) of Hurricane Fabian. Signal (i.e., the forecast difference between denial experiments and the control experiment) propagation can reveal the interaction between the tropical cyclone (TC) and the midlatitude jet, and the energy dispersion or propagation of the TC undergoing ET also can be determined. The crucial role of an upper-level trough is discussed. Based on this study, a strategy issue regarding targeted observations of ET and several typical problems regarding the numerical prediction of ET are discussed. The results show that the greatest signals along with their propagation are associated closely with various types of instabilities. In general, the signal first appears at the location of the TC, and then it propagates to the midlatitude jet through the interaction between the TC and the jet itself. Thereafter, signals propagate downstream along the jet and downward to the lower troposphere at the same time by way of Rossby wave packets; the jet essentially acts as a waveguide. Through the signal propagation and development in the jet, the impact of targeted observations seems sensitive to the ET process. The interaction between the TC and the jet occurs as high θ (low potential vorticity) air flows out of the TC toward the northeast and into the jet below the tropopause. The interaction may be strengthened by an upstream trough at upper levels. The TC outflow enhances the potential vorticity (PV) gradient and baroclinity in the jet. Therefore, the jet becomes stronger and more baroclinically unstable. The signal propagation also indicates the energy dispersion of a TC undergoing ET. The strong southwesterly flow ahead of the upper-level trough steers Fabian to higher latitudes, and strengthens the advection process of low PV air into the jet. Therefore, the development of the upper-level trough and its proximity to the TC are crucial for the interaction between the TC and the jet, and the resulting signal propagation. Small deviations from this synoptic situation may result in great differences in the signal propagation and the ET forecast. The most suitable region for targeting is likely a region where crucial synoptic processes can magnify initial errors.