Diagnosis of the Downstream Ridging Associated with Extratropical Transition Using Short-Term Ensemble Forecasts

Abstract

The dynamical mechanisms that led to downstream ridging during the extratropical transition (ET) of Typhoons Tokage and Nabi are evaluated using data drawn from a cycling ensemble Kalman filter coupled with the Weather Research and Forecasting Model (WRF). During both transitions, the ensemble covariances indicate that the 350-K potential vorticity (PV) at the apex of the ridge, which is used to define the ridge structure, is proportional to the amount of precipitation along the baroclinic zone to the northeast of the tropical cyclone (TC), and at some times to the upper-tropospheric divergence above the tropical cyclone. Multivariate regression calculations indicate that the frontal precipitation has the largest impact on the ridge amplitude and area during Tokage?s transition, while the TC divergence has roughly equal impact during some times of Nabi?s transition. The amount of precipitation along the baroclinic zone is modulated by the lower-tropospheric frontogenesis and moisture flux on the east side of the tropical cyclone, both of which are related to the TC winds. Although both of these metrics covary with the PV at the ridge apex, a one standard deviation perturbation to the moisture flux is associated with a larger change in the ridge PV. Diagnostic perturbations to the initial conditions confirm that increasing (decreasing) the initial moisture flux leads to comparatively lower (higher) PV at the ridge apex 12 h later. Assimilating a single hypothetical wind or moisture observation within the large moisture flux region leads to a 0.3 standard deviation change in the 12-h PV forecast when the observation innovation is comparable to the observation error. Overall, these results suggest that better wind and moisture analyses at the periphery of the TC could improve forecasts of the downstream ridging during ET.