Predictability associated with the downstream impacts of the extratropical transition (ET) of tropical cyclones

The poleward movement of a decaying tropical cyclone (TC) often results in a rapidly-moving, explosively-deepening midlatitude cyclone. The re-intensification of the remnant TC as an extratropical cyclone depends on the phasing between the decaying TC and a midlatitude environment that is favorable for midlatitude cyclogenesis. Furthermore, the export of low potential vorticity air to the midlatitudes from the upper-levels of the poleward-moving TC may result in excitation and dispersion of Rossby waves that have far-reaching impacts on downstream atmospheric conditions. Therefore, an extratropical transition (ET) of a TC perturbs the midlatitude flow patterns across individual ocean basins with the potential for nearly hemispheric-scale impacts.

Because of the typical rapid translation speed of the decaying TC, accurate extended-range prediction of the phasing between the remnant tropical circulation and the midlatitude environment into which it is moving is critical. However, the complex physical interactions that occur during the structural changes associated with extratropical transition (ET) of the decaying tropical cyclone often contribute to large errors in numerical forecasts from operational global forecast models. Furthermore, forecast errors associated with the ET may propagate rapidly downstream of the ET location.

In this study, measures of the relative predictability in global numerical weather forecasts with respect to the downstream impacts of ET events are computed based on operational (NCEP and ECMWF) global ensemble prediction systems (EPS). There is evidence that increased variability among ensemble members, which increases downstream of an ET event, is related to decreased forecast accuracy. Variability among EPS members is characterized by an empirical orthogonal function (EOF) analysis that is applied to 500 hPa geopotential heights and potential temperature on the dynamic tropopause. Principal components computed from the EOF analysis are defined based on the projection of individual ensemble members on the EOFs. The principal components provide a framework for a fuzzy cluster analysis, which is applied to identify principal downstream impact scenarios. To identify the temporal change in the variability associated with the forecasts of the ET event and its impacts on the downstream flow patterns, the EOF and cluster analyses are applied successively between 120 h and 24 h prior to the ET time. It is hypothesized that the number of forecast scenarios, which are identified as individual clusters of EPS members, decreases as the uncertainty associated with the downstream impact of the ET is reduced. Furthermore, the EOF and cluster analyses facilitate comparison between ensemble members produced by the NCEP and ECMWF systems. The EOF/cluster methodology is applied to the ensemble forecasts of two cases in 2005, one in the western North pacific and one in the North Atlantic. The characteristics of the development for these cases were very similar in that both systems merged with larger-scale midlatitude cyclones after ET and appeared to have a minor impact on the extratropical development. The deterministic forecast, however, was very different for these cases and exhibited large errors. Before the merger with the midlatitude flow the life-cycles of these two systems were quite different, which may be attributed to the fact that the midlatitude flow during their occurrences was quite zonal in one case and very non-zonal in the other. The ensemble forecast for these ET events gives insight to the reliability of the deterministic forecast on one hand and shows other possible dynamically consistent developments of the atmosphere on the other.