Initial condition sensitivities for Western Pacific extratropical transition events

Numerical Weather Prediction (NWP) models often have difficulty forecasting the transition of a warm-core tropical cyclone to a cold-core baroclinic system. These extratropical transition (ET) events are often associated with decreased model predictability because the forecast evolution is highly sensitive to the initial conditions. Previous studies have shown that small changes in the position of the tropical cyclone or the upstream mid-latitude features can lead to significant differences in the resulting extratropical cyclogenesis. Although ET events often produce widespread high-impact weather, few studies have investigated the sensitivity of ET forecasts to the initial conditions.

Here I investigate the initial condition sensitivity of Western Pacific ET events using a 90 member EnKF based on the Weather Research and Forecasting (WRF) model. Conventional observations are assimilated every six hours (no satellite radiances). All 90 ensemble members are integrated forward 48 hours to obtain ensemble forecasts of the ET event. The sensitivity of the ET forecast to the initial conditions is computed via linear regression of the ensemble estimate of various forecast metrics onto the analysis ensemble. Ensemble sensitivity calculations for Typhoon Tokage (2004) indicate that the ET forecast was sensitive to the position of the TC and the amplitude of two upper-level troughs upstream of the TC. Amplifying the upper-level trough that eventually phases with Tokage leads to a deeper ET storm; however, amplifying the upper-level trough to the north of the Tokage leads to a weaker ET storm. Intensifying the latter trough strengthened a downstream cyclone, thus limiting the baroclinic energy available to Tokage's remnants. These results suggest that improved ET forecasts may be obtained by creating a better analysis of the upstream mid-latitude state.