The Remote Role of North‐American Mesoscale Convective Systems on the Forecast of a Rossby Wave Packet: A Multi‐Model Ensemble Case‐Study

In this study, the medium‐range predictability of heatwave (HW) onsets in four midlatitude European regions is investigated statistically with the help of ensemble reforecasts for the period 2001–2018. The concept of Euro‐Atlantic weather regimes is adopted to characterise HWs (about 50 in each region) and to study whether forecast skill may depend on the large‐scale dynamical setup. HW onsets over the British Isles and Scandinavia are mainly associated with Scandinavian and European blocking regimes, whereas the “no regime” case is observed more frequently for Central Europe. Stratified by weather regime, the predictability of heatwave onsets is then studied by means of a multiple metric‐based analysis of European Centre for Medium‐Range Weather Forecasts (ECMWF) and Global Ensemble Forecast System Version 12 (GEFSv12) ensemble reforecasts. For two of the regions considered, Central Europe and the British Isles, a conclusive picture is obtained: medium‐range predictive skill is significantly higher for HW onsets associated with Scandinavian or European blocking compared with cases with no pronounced regime. This skill advantage mostly concerns the large‐scale flow and, to some extent, 850‐hPa temperatures, but is generally not reflected in the correct prediction of near‐surface temperatures. Finally, we investigate for two regions how exceptionally good or poor forecasts are related to the atmospheric state during or shortly after forecast initialisation. At 10 days lead time, poor large‐scale flow predictive skill for Central European HW onsets is linked to anomalously high baroclinicity further upstream and an intensified North Atlantic jet stream, whereas good forecasts on average feature an initial state close to climatology. Forecast skill for near‐surface temperatures is not affected by such dynamical precursors, but rather by pre‐existing soil‐moisture anomalies. For the British region, exceptionally good forecasts of both large‐scale flow and near‐surface temperatures are associated with an already established continental blocking. In contrast to Central Europe, pre‐existing soil‐moisture anomalies play less of a role there.

Investigating the medium‐range predictability of European heatwave onsets in relation to weather regimes using ensemble reforecasts

Lemburg,

Fink

2024

Predictability of midlatitude Rossby‐wave packets

Prestel‐Kupferer,

Riemer,

Schmidt

et al. 2024

Rossby‐wave packets (RWPs) along the midlatitude jet are a fundamental ingredient of extratropical dynamics. RWPs have been linked to enhanced atmospheric predictability and the occurrence of extreme weather events. We here investigate the predictability of Northern Hemispheric RWPs as physical entities, defined by enhanced values of the Rossby‐wave envelope field. Using a catalogue of RWPs identified and tracked in reanalysis data, we analyze RWP predictability in a 19‐year period of NOAA GEFSv12 reforecasts. Our analysis adopts the so‐called distance and amplitude score (DAS), a verification metric that avoids both the double‐penalty issue of field‐based verification of coherent features and the complications of an object‐based approach. Applied to the envelope field, forecast errors defined by this metric asymptote towards saturation, but do not completely reach saturation within the 10 days lead time available to this study. The growth rate of the median DAS is highest initially and decreases with lead time. This is a nontrivial result, because the underlying envelope field largely de‐emphasizes phase information, but still exhibits very similar error‐growth characteristics to fields that contain the full phase information. Variations in RWP predictability are dominated by the stage of the RWP life cycle, with higher predictability found for the propagation stage than the onset and decay stages. In addition, RWP predictability exhibits a seasonal cycle, with higher predictability in winter than in summer. Controlling for seasonality and the stage of the life cycle, we find that (i) high‐amplitude RWPs exhibit higher predictability than low‐amplitude RWPs up to 6 days lead time and (ii) there is a general pattern of relatively high predictability over Eurasia. Finally, predictability of the propagating stage is higher if forecasts are initialized after RWP onset than if initialized before onset. In this sense, RWP onset acts as a predictability barrier to the subsequent propagation stage.

Medium‐range predictability of temperature extremes and biases in Rossby‐wave amplitude

Doensen,

Fragkoulidis,

Magnusson

et al. 2024

This study investigates the medium‐range predictability of warm and cold extremes in the Northern Hemisphere and the role that upper‐tropospheric circulation biases play in this regard. Deterministic ERA5 reforecasts for the period 1979–2019 are evaluated based on the ERA5 reanalysis of the respective period, thus providing a large sample for verification and bias identification. The predictability of temperature extremes at 850 hPa is assessed based on the Gilbert Skill Score and other metrics and is shown to exhibit regional and seasonal variations. Summer is generally characterized by lower forecast skill scores than winter for both warm and cold extremes. Moreover, cold extremes in summer have slightly lower skill scores than warm extremes, while the opposite is true in winter. Biases in the frequency of temperature extremes are, to some extent, consistent with biases in mean temperature and indicate an underestimation in the total amount of extremes for much of the hemisphere in summer. Associated with the latter, biases also emerge in the standard deviation of the daily temperature distribution, with the summer values being largely underestimated over most of the hemisphere. The role of upper‐tropospheric circulation in these biases is then assessed by verifying the representation of Rossby‐wave packet (RWP) properties. It is found that the amplitude of RWPs is systematically underestimated in most of the hemisphere in summer, while it is overestimated in many parts of the midlatitudes in winter. Overall, the results suggest that the underestimation of RWP amplitude in summer hinders the medium‐range predictability of temperature extremes in the explored retrospective and operational forecasts. Although operational European Centre for Medium‐Range Weather Forecasts (ECMWF) forecasts gradually improve between 2013 and 2022 in terms of the 850‐hPa temperature and 300‐hPa RWP amplitude absolute errors, the aforementioned summer biases remain qualitatively similar.