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We investigate the run-to-run consistency (jumpiness) of ensemble forecasts of tropical cyclone tracks from three global centers: ECMWF, the Met Office, and NCEP. We use a divergence function to quantify the change in cross -track position between consecutive ensemble forecasts initialized at 12-h intervals. Results for the 2019-21 North Atlantic hurricane season show that the jumpiness varied substantially between cases and centers, with no common cause across the different ensemble systems. Recent upgrades to the Met Office and NCEP ensembles reduced their overall jumpiness to match that of the ECMWF ensemble. The average divergence over the set of cases provides an objective measure of the expected change in cross-track position from one forecast to the next. For example, a user should expect on average that the ensemble mean position will change by around 80-90 km in the cross-track direction between a forecast for 120 h ahead and the updated forecast made 12 h later for the same valid time. This quantitative information can support users' decision-making, for example, in deciding whether to act now or wait for the next forecast. We did not find any link between jumpiness and skill, indicating that users should not rely on the consistency between successive forecasts as a measure of confidence. Instead, we suggest that users should use ensemble spread and probabilistic information to assess forecast uncertainty, and consider multimodel combinations to reduce the effects of jumpiness. SIGNIFICANCE STATEMENT: Forecasting the tracks of tropical cyclones is essential to mitigate their impacts on society. Numerical weather prediction models provide valuable guidance, but occasionally there is a large jump in the predicted track from one run to the next. This jumpiness complicates the creation and communication of consistent forecast advisories and early warnings. In this work we aim to better understand forecast jumpiness and we provide practical information to forec

It is well known from modelling studies that surface topography influences the large-scale atmospheric circulation and that several model biases are associated with incorrect representation of topography. The textbook explanation of topographic effects on large-scale circulation appeals to the theoretical relationship between surface forcing and vortex stretching along trajectories in single-layer models. The goal of this study is to design and use a simple diagnostic of the large-scale forcing on the atmosphere when air is passing over topography, directly from atmospheric fields, based on this theoretical relationship. The study examines the interaction of the atmosphere with the North American Cordillera and samples the flow by means of trajectories during Northern Hemisphere winter. We detect a signal of topographic forcing in the atmospheric dataset, which, although much less distinct than in the theoretical relationship, nevertheless exhibits a number of expected properties. Namely, the signal increases with latitude, is usually stronger upslope than downslope, and is enhanced if the flow is more orthogonal to the mountain ridge, for example during periods of positive Pacific-North American index (PNA). Furthermore, a connection is found between an enhanced signal of topographic forcing downslope of the North American Cordillera and periods of more frequent downstream European blocking.

© The Author(s) 2018. This paper was stimulated by an international workshop on atmospheric blocking in April 2016 with over 100 participants. Giacomo Masato instigated the workshop and it was taken forward by members of the WMO World Weather Research Programme’s WG on Predictability, Dynamics and Ensemble Forecasting (PDEF) with sponsorship from WCRP/SPARC, IAMAS/ICDM, NCAS-Climate, ERC, and the Universities of Bern and Reading. We thank Ben Harvey as the local organiser for the workshop, hosted by the University of Reading, and Ted Shepherd for his ERC support. We would like to acknowledge David Barriopedro for the analysis presented in Figs. 2, 3, and 4, and to thank two anonymous reviewers for their helpful comments. Reanalysis data was kindly provided by NCEP/NCAR, EMCWF and JRA. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output. For CMIP the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. TW was supported by UK NERC grant NE/ N01815X. JS is supported by the Norwegian Research Council funded project ClimateXL (grant no.243953). OM received funding from the Swiss National Science Foundation Grant Nr. 200021_156059. S.-W. Son is supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science and ICT (2017R1E1A1A01074889)., Purpose of Review Atmospheric blocking events represent some of the most high-impact weather patterns in the mid-latitudes, yet they have often been a cause for concern in future climate projections. There has been low confidence in predicted future changes in blocking, despite relatively good agreement between climate models on a decline in blocking. This is due to the lack of a comprehensive theory of blocking and a pervasive underestimation of blocking occurrence by models. This paper reviews the state of knowledge regarding blocking under climate change, with the aim of providing an overview for those working in related fields. Recent Findings Several avenues have been identified by which blocking can be improved in numerical models, though a fully reliable simulation remains elusive (at least, beyond a few days lead time). Models are therefore starting to provide some useful information on how blocking and its impacts may change in the future, although deeper understanding of the processes at play will be needed to increase confidence in model projections. There are still major uncertainties regarding the processes most important to the onset, maintenance and decay of blocking and advances in our understanding of atmospheric dynamics, for example in the role of diabatic processes, continue to inform the modelling and prediction efforts. Summary The term ‘blocking’ covers a diverse array of synoptic patterns, and hence a bewildering range of indices has been developed to identify events. Results are hence not considered fully trustworthy until they have been found using several different methods. Examples of such robust results are the underestimation of blocking by models, and an overall decline in future occurrence, albeit with a complex regional and seasonal variation. In contrast, hemispheric trends in blocking over the recent historical period are not supported by different methods, and natural variability will likely dominate regional variations over the n, Ministerio de Ciencia e Innovación (MICINN), WCRP/SPARC, IAMAS/ICDM, NCAS-Climate, ERC, Universities of Bern and Reading, World Climate Research Programme’s Working Group on Coupled Modelling, U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison, Global Organization for Earth System Science Portals, UK NERC, Norwegian Research Council project ClimateXL, wiss National Science Foundation, Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Depto. de Física de la Tierra y Astrofísica, Fac. de Ciencias Físicas, TRUE, pub

[Purpose of Review] Atmospheric blocking events represent some of the most high-impact weather patterns in the mid-latitudes, yet they have often been a cause for concern in future climate projections. There has been low confidence in predicted future changes in blocking, despite relatively good agreement between climate models on a decline in blocking. This is due to the lack of a comprehensive theory of blocking and a pervasive underestimation of blocking occurrence by models. This paper reviews the state of knowledge regarding blocking under climate change, with the aim of providing an overview for those working in related fields., [Recent Findings] Several avenues have been identified by which blocking can be improved in numerical models, though a fully reliable simulation remains elusive (at least, beyond a few days lead time). Models are therefore starting to provide some useful information on how blocking and its impacts may change in the future, although deeper understanding of the processes at play will be needed to increase confidence in model projections. There are still major uncertainties regarding the processes most important to the onset, maintenance and decay of blocking and advances in our understanding of atmospheric dynamics, for example in the role of diabatic processes, continue to inform the modelling and prediction efforts., [Summary] The term ‘blocking’ covers a diverse array of synoptic patterns, and hence a bewildering range of indices has been developed to identify events. Results are hence not considered fully trustworthy until they have been found using several different methods. Examples of such robust results are the underestimation of blocking by models, and an overall decline in future occurrence, albeit with a complex regional and seasonal variation. In contrast, hemispheric trends in blocking over the recent historical period are not supported by different methods, and natural variability will likely dominate regional variations over the next few decades.