Publications by Daniel Befort

Seasonal forecasts of the 20th century

Bulletin of the American Meteorological Society American Meteorological Society 101 (2020) E1413-E1426

A Weisheimer, D Befort, D Macleod, T Palmer, C O’Reilly, K Strømmen

<p>New seasonal retrospective forecasts for 1901-2010 show that skill for predicting ENSO, NAO and PNA is reduced during mid-century periods compared to earlier and more recent high-skill decades.</p> <p>Forecasts of seasonal climate anomalies using physically-based global circulation models are routinely made at operational meteorological centers around the world. A crucial component of any seasonal forecast system is the set of retrospective forecasts, or hindcasts, from past years which are used to estimate skill and to calibrate the forecasts. Hindcasts are usually produced over a period of around 20-30 years. However, recent studies have demonstrated that seasonal forecast skill can undergo pronounced multi-decadal variations. These results imply that relatively short hindcasts are not adequate for reliably testing seasonal forecasts and that small hindcast sample sizes can potentially lead to skill estimates that are not robust. Here we present new and unprecedented 110-year-long coupled hindcasts of the next season over the period 1901 to 2010. Their performance for the recent period is in good agreement with those of operational forecast models. While skill for ENSO is very high during recent decades, it is markedly reduced during the 1930s to 1950s. Skill at the beginning of the 20th Century is, however, as high as for recent high-skill periods. Consistent with findings in atmosphere-only hindcasts, a mid-century drop in forecast skill is found for a range of atmospheric fields including large-scale indices such as the NAO and the PNA patterns. As with ENSO, skill scores for these indices recover in the early 20th Century suggesting that the mid-century drop in skill is not due to lack of good observational data.</p> <p>A public dissemination platform for our hindcast data is available and we invite the scientific community to explore them.</p>

Representing model uncertainty in multi‐annual predictions

Geophysical Research Letters American Geophysical Union (AGU) (0)

DJ Befort, CH O'Reilly, A Weisheimer

Assessing the robustness of multidecadal variability in Northern Hemisphere wintertime seasonal forecast skill

Quarterly Journal of the Royal Meteorological Society Wiley 146 (2020) qj.3890

CH O'Reilly, A Weisheimer, D MacLeod, DJ Befort, T Palmer

Recent studies have found evidence of multidecadal variability in northern hemisphere wintertime seasonal forecast skill. Here we assess the robustness of this finding by extending the analysis to analysing a diverse set of ensemble atmospheric model simulations. These simulations differ in either numerical model or type of initialisation and include atmospheric model experiments initialised with reanalysis data and free‐running atmospheric model ensembles. All ensembles are forced with observed SST and seaice boundary conditions. Analysis of large‐scale Northern Hemisphere circulation indicesover the Northern Hemisphere (namely the North Atlantic Oscillation, Pacific North American pattern and the Arctic Oscillation) reveals that in all ensembles there is larger correlation skill in the late century periods than during periods in the mid‐century. Similar multidecadal variability in skill is found in a measure of total skill integrated over the whole of the extratropics. Most of the differences in large‐scale circulation skill between the skillful late period (as well as early period) and the less skillful mid‐century period seem to be due to a reduction in skill over the North Pacific and a disappearance in skill over North America and the North Atlantic. The results are robust across different models and different types of initialisation, indicating that the multidecadal variability in Northern Hemisphere winter skill is a robust feature of 20th century climate variability. Multidecadal variability in skill therefore arises from the evolution of the observed SSTs, likely related to a weakened influence of ENSO on the predictable extratropical circulation signal during the middle of the 20th century, and is evident in the signal‐to‐noise ratio of the different ensembles, particularly the larger ensembles.

Constraining projections using decadal predictions

Geophysical Research Letters American Geophysical Union 47 (2020) e2020GL087900

DJ Befort, C O'Reilly, A Weisheimer

There is increasing demand for robust, reliable and actionable climate information for the next 1 to 50 years. This is challenging for the scientific community as the longest initialized predictions are limited to 10 years (decadal predictions). Thus, to provide seamless information for the upcoming 50 years, information from decadal predictions and uninitialized projections need to be merged. In this study, the ability to obtain valuable climate information beyond decadal time-scales by constraining uninitialized projections using decadal predictions is assessed. The application of this framework to surface temperatures over the North Atlantic Subpolar Gyre region, shows that the constrained uninitialized sub-ensemble has higher skill compared to the overall projection ensemble also beyond ten years when information from decadal predictions is no longer available. Though showing the potential of such a constraining approach to obtain climate information for the near-term future, its utility depends on the added value of initialization.

Seasonal forecast skill for extra‐tropical cyclones and windstorms

Quarterly Journal of the Royal Meteorological Society Wiley 145 (2018) 92-104

DJ Befort, S Wild, Knight, JF Lockwood, HE Thornton, L Hermanson, PE Bett, A Weisheimer, GC Leckebusch

Extra‐tropical cyclones and their associated extreme wind speeds are a major cause of vast damage and large insured losses in several European countries. Reliable seasonal predictions of severe extra‐tropical winter cyclones and associated windstorms would thus have great social and economic benefits, especially in the insurance sector. We analyse the climatological representation and assess the seasonal prediction skill of wintertime extra‐tropical cyclones and windstorms in three multi‐member seasonal prediction systems: ECMWF‐System3, ECMWF‐System4 and Met Office‐GloSea5, based on hindcasts over a 20 year period (1992–2011). Small to moderate positive skill in forecasting the winter frequency of extra‐tropical cyclones and windstorms is found over most of the Northern Hemisphere. The skill is highest for extra‐tropical cyclones at the downstream end of the Pacific storm track and for windstorms at the downstream end of the Atlantic storm track. We also assess the forecast skill of windstorm frequency by using the North Atlantic Oscillation (NAO) as the predictor. Prediction skill improves when using this technique over parts of the British Isles and North Sea in GloSea5 and ECMWF‐S4, but reduces over central western Europe. This suggests that using the NAO is a simple and effective method for predicting wind storm frequency, but that increased forecast skill can be achieved in some regions by identifying windstorms directly using an objective tracking algorithm. Consequently, in addition to the large‐scale influence of the NAO, other factors may contribute to the predictability of wind storm frequency seen in existing forecast suites, across impact relevant regions of Europe. Overall, this study reveals for the first time significant skill in forecasting the winter frequency of high‐impact windstorms ahead of the season in regions that are vulnerable to such events.

Modelling serial clustering and inter-annual variability of European winter windstorms based on large-scale drivers

International Journal of Climatology 38 (2018) 3044-3057

MA Walz, DJ Befort, NO Kirchner-Bossi, U Ulbrich, GC Leckebusch

Different long-term trends of extra-tropical cyclones and windstorms in ERA-20C and NOAA-20CR reanalyses

Atmospheric Science Letters 17 (2016) 586-595

DJ Befort, S Wild, T Kruschke, U Ulbrich, GC Leckebusch

Estimating uncertainties from high resolution simulations of extreme wind storms and consequences for impacts

Meteorologische Zeitschrift 25 (2016) 531-541

T Pardowitz, DJ Befort, GC Leckebusch, U Ulbrich

Intraseasonal variability of the Indian summer monsoon: wet and dry events in COSMO-CLM

Climate Dynamics Springer Nature 47 (2016) 2635-2651

DJ Befort, GC Leckebusch, U Cubasch

Was the Extreme Storm Season in Winter 2013/14 Over the North Atlantic and the United Kingdom Triggered by Changes in the West Pacific Warm Pool?

Bulletin of the American Meteorological Society American Meteorological Society 96 (2015) S29-S34

S Wild, DJ Befort, GC Leckebusch

Regional moisture change over India during the past Millennium: A comparison of multi-proxy reconstructions and climate model simulations

Global and Planetary Change Elsevier BV 122 (2014) 176-185

S Polanski, B Fallah, DJ Befort, S Prasad, U Cubasch

Projections of global warming-induced impacts on winter storm losses in the German private household sector

Climatic Change Springer Science and Business Media LLC 121 (2013) 195-207

H Held, F-W Gerstengarbe, T Pardowitz, JG Pinto, U Ulbrich, K Born, MG Donat, MK Karremann, GC Leckebusch, P Ludwig, KM Nissen, H Österle, BF Prahl, PC Werner, DJ Befort, O Burghoff

Identification of storm surge events over the German Bight from atmospheric reanalysis and climate model data

Natural Hazards and Earth System Sciences Copernicus GmbH 15 (0) 1437-1447

DJ Befort, M Fischer, GC Leckebusch, U Ulbrich, A Ganske, G Rosenhagen, H Heinrich

<jats:p>Abstract. A new procedure for the identification of storm surge situations for the German Bight is developed and applied to reanalysis and global climate model data. This method is based on the empirical approach for estimating storm surge heights using information about wind speed and wind direction. Here, we hypothesize that storm surge events are caused by high wind speeds from north-westerly direction in combination with a large-scale wind storm event affecting the North Sea region. The method is calibrated for ERA-40 data, using the data from the storm surge atlas for Cuxhaven. It is shown that using information of both wind speed and direction as well as large-scale wind storm events improves the identification of storm surge events. To estimate possible future changes of potential storm surge events, we apply the new identification approach to an ensemble of three transient climate change simulations performed with the ECHAM5/MPIOM model under A1B greenhouse gas scenario forcing. We find an increase in the total number of potential storm surge events of about 12 % [(2001–2100)–(1901–2000)], mainly based on changes of moderate events. Yearly numbers of storm surge relevant events show high interannual and decadal variability and only one of three simulations shows a statistical significant increase in the yearly number of potential storm surge events between 1900 and 2100. However, no changes in the maximum intensity and duration of all potential events is determined. Extreme value statistic analysis confirms no frequency change of the most severe events. </jats:p>