Diagnosing the Sources of Operational Forecast Model Errors in Tropical-Extratropical Interactions

The atmospheric response to variations in latent heating in the tropics is known to extend well beyond its source region and therefore it is thought that a reduction of tropical forecast errors is beneficial global forecast skill over remote regions such as North America. The general mechanism for tropical extra-tropical teleconnections is that latent heating in the tropics are sources of Rossby wave-trains that then propagate to higher latitudes, where the type of wave patterns and their path depend on interactions between the horizontal and vertical distribution of the tropical heat source due to precipitation and the basic state large-scale flow. The work presented here is focused on the hypothesis that a positive correlation between the performance of tropical short range and extra-tropical forecast at later lead times implies that when tropical heat sources are well (poorly) predicted, extra-tropical skill is gained (lost) due to properly (improperly) triggered Rossby-like wavetrains. This analysis is applied to operational forecasts from the National Centers for Environmental Prediction Global Forecast System (NCEP GFS) and European Centre for Medium-Range Weather Forecasts Integrated Forecast System (ECMWF IFS). The two models are chosen to contrast the link between tropical and extratropical skill in a model that is known to perform relatively well in the tropics (IFS) to a model with much lower tropical skill (GFS). One interesting result from this analysis is that, for both IFS and GFS, when the metric for tropical skill is based on verification of forecast upper level divergence against the model analysis, the likelihood of high (low) extratropical medium skill increases (decreases) when the very short range tropical QPF is good (poor). However, when the metric for tropical skill is based on verification of quantitative precipitation forecasts (QPF) against satellite estimates of precipitation, such relationship is found only in the IFS. It is also shown than tropical QPF and dynamical short range skill are highly correlated in the IFS, but not in the GFS, implying that model physics, rather than dynamics, control the link between tropical and extra-tropical skill. The results presented here are in agreement with previous studies showing that tropical errors related to the coupling between precipitation and large-scale circulation play some role in the differences in mid-range extra-tropical skill between the IFS and GFS.