On the impact of initial conditions in the forecast of Hurricane Leslie extratropical transition

TL;DR

This study investigates how different initial conditions from IFS and GFS models affect the simulation accuracy of Hurricane Leslie's extratropical transition, highlighting the importance of data assimilation and model resolution.

Contribution

It compares the impact of two initial condition datasets on hurricane transition simulations, emphasizing the role of data assimilation and model resolution in forecast accuracy.

Findings

IFS initial conditions yield less trajectory error.

Differences in geopotential height and thickness affect simulation.

IFS analysis is more precise due to better data assimilation.

Abstract

Hurricane Leslie (2018) was a non-tropical system that lasted for a long time undergoing several transitions between tropical and extratropical states. Its trajectory was highly uncertain and difficult to predict. Here the extratropical transition of Leslie is simulated using the Model for Prediction Across Scales (MPAS) with two different sets of initial conditions (IC): the operational analysis of the Integrate Forecast System (IFS) and the Global Forecast System (GFS). Discrepancies in Leslie position are found in the IC patterns, and in the intensity and amplitude of the dorsal-trough system in which Leslie is found. Differences are identified both in the geopotential height at 300 hPa and the geopotential thickness. Potential temperature in the dynamic tropopause shows a broader, more intense trough displaced western when using the IC-IFS. The IC-IFS simulation shows lesser trajectory errors but wind speed overestimation than the IC-GFS one. The complex situation of the extratropical transition, where Leslie interacts with a trough, increases the uncertainty associated with the intensification process. The disparities observed in the simulations are attributed to inaccuracies in generating the ICs. Both ICs generate different atmospheric configurations when propagated in time. Results suggest that during an extratropical transition in a highly baroclinic atmosphere, the IFS model's data assimilation method produced a more precise analysis than GFS due to the greater number of observations assimilated by the IFS, the greater spatial resolution of the model and the continuous adjustment of the simulations with the field of observations.