An Investigation of Large Track Errors of North Atlantic Tropical Cyclones

An Investigation of Large Track Errors of North Atlantic Tropical Cyclones

Nicholas Leonardo and Brian A. Colle

¹ School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY

This study analyzes the large error tropical cyclone (TC) track forecasts for two global ensembles (21-member GEFS, and 51-member ECMWF) for the 2008-2016 North Atlantic seasons, using NHC’s best-track data as the verifying analysis. This study focuses on large track errors for the day 3-5 forecasts for two regions to help separate extratropical transitions: a “north” (“south”) region if the verifying best-track (never) crosses north of 30N. The 20% most negative and positive along- (cross-) track errors are considered “slow” and “fast” (“left” and “right”), respectively. The ECMWF (GEFS) fields of each case are analyzed with standardized differences of the 10 (5) “slowest” or “leftmost” ensemble members and the 10 (5) “fastest” or “rightmost” members. Model fields are also evaluated against the CFSR reanalysis. The following questions are addressed: (1) are there specific flow patterns or synoptic features with these large errors, (2) how sensitive are the tracks to errors in these features, and (3) does TC intensity also affect the growth of large track errors?

The mean along-track biases of both the GEFS and ECMWF are significantly more negative (slow bias) for north TC’s than for south TC’s at all lead times. For the ECMWF, the slow north cases on average are associated with inherently stronger storms than the fast at < 48 h, though the intensity errors of slow and fast cases are comparable. From vortex-relative composites, the slow north cases are associated with more amplified flow patterns than the fast cases. Both the upstream troughs and downstream ridges underamplify in >70% of cases at ~60-72 h. The under-amplification of the ridge is accelerated by a significant under-prediction of the divergent outflow of the TC and its interaction with ambient PV gradients.

Both ensembles have slightly larger 120 h cross-track biases for South TC’s than for North, with the GEFS having a positive (right) bias and the ECMWF having a negative (left) bias. The largest absolute cross-track errors are associated with inherently weaker TC’s at 0 h. Focusing on the GEFS, most right cases start to drift northward while approaching the western extent of a subtropical ridge at 24-60 h. By ~48 h, the northward drift in >70% of these right cases is negatively correlated with the strength of the ridge to the north. It will be tested whether the local weakening of the ridge corresponds to the inflow mass transport into the TC. The GEFS left cases appear to have a wider variety of synoptic features, some of which involve the TC approaching a split flow pattern such that a small drift in the <24 h track can greatly impact the >72 h position. Difference plots of upper-level vorticity will reveal any cases with tracks sensitivity to shear-induced near-storm asymmetries, which can be especially important in split flow regimes. The track sensitivity of both left and right cases to TC radius and depth errors will also be quantified for comparison against errors in environmental steering. These possible causes of large cross-track errors will also be diagnosed in the ECMWF.