The role of equatorial waves in the skill of NCEP and ECMWF forecast systems

George Kiladis and Juliana Dias

NOAA/ESRL PSL

Tuesday, Oct 03, 2017, 2:00 pm
DSRC Room 2A305

Abstract

Despite decades of research on the roles of moist convective processes in large-scale tropical dynamics, tropical forecast skill in operational models is still deficient, even at short lead times. Here we start by contrasting tropical and Northern Hemisphere (NH) forecast skill during the El Nino Rapid Response field campaign (January, 2016 - March, 2016), focusing on quantitative precipitation forecasts (QPF) of the NCEP Global Forecast System (GFS) and ECMWF Integrated Forecast System (IFS) models. The analysis is then extended to January, 2015 - March, 2016 and results reveal that in general initial conditions are reasonably well estimated in both forecast systems, as indicated by relatively good scores for the 6-12 hour forecasts. However, overall, tropical forecasts in both systems are not considered useful by typical metrics much beyond 4 days. To quantify the relationship between QPF and dynamical skill, we calculate space-time spectra and coherence of rainfall and divergence fields. It is shown that while tropical variability is too weak overall in both models, the IFS is much superior in allowing tropical waves to propagate for longer lead times. As shown in previous work, this study indicates that the differences in physics used in each system, in particular, how moist convective processes are coupled to the large scale flow through these parameterizations, appear to be one major source of forecast errors.