<p><span>Cyclone clustering, the succession of multiple extratropical cyclones during a short period of time, has a huge impact on European weather extremes. The idea that several cyclones follow a similar track already dates back to the concept of cyclone families of Bjerknes and Solberg. To investigate the dynamical causes of cyclone clustering, one needs to diagnose where cyclone clustering occurs and determine their characteristics. So far most diagnostics either focused on either local impact-based diagnostics or on a statistical analysis of storm recurrence. While the first cannot be applied globally, the latter is difficult to relate to individual events. We therefore present a new way to globally detect cyclone clustering that is closer to the original concept of Bjerknes and Solberg that extratropical cyclones follow similar tracks.</span></p><div><span>Using this new cyclone clustering diagnostic based on spatio-temporal distance between cyclone tracks, we analyse cyclone clustering globally in Era-Interim for the period 1979 until 2016. We complement this analysis with a baroclinicity diagnostic based on the slope of isentropic surfaces. With the isentropic slope and its tendencies, the relative role of diabatic and adiabatic effects associated with extra-tropical cyclones in maintaining baroclinicity are assessed. We find that cyclone clustering mainly occurs along the climatological storm tracks. In general, clustered cyclones are stronger than non-clustered cyclones. Moreover clustered cyclones are more often related to atmospheric rivers and stronger isentropic slope, indicating that diabatic effects might be an important mechanism in the formation of cyclone clustering.&#160;</span></div>
Abstract The effect of modified equator‐to‐pole temperature gradients on the jet stream by low‐level polar warming and upper‐level tropical warming is not fully understood. We perform aquaplanet simulations to quantify the impact of different sea surface temperature distributions on jet stream strength, large wave amplitudes and extreme waviness. The responses to warming in the waviness metrics Sinuosity Index and Local Wave Activity are sensitive to the latitude range over which they are calculated. Therefore, we use a latitude range that accurately represents the position of the jet. The uniform warming scenario strengthens the jet and reduces large wave amplitudes. Reductions in meridional temperature gradients lead to weakened mid‐latitudinal jet strength and show significant decreases in large wave amplitudes and jet stream waviness. These findings contradict the mechanism that weakened jet streams increase wave amplitudes and extreme jet stream waviness. We conclude that weakened jet streams do not necessarily become wavier.
<p>The existence of cyclone clustering, the succession of multiple cyclones in a short amount of time, indicates that the baroclinicity feeding these storms undergoes episodic cycles. With the generally accepted paradigm of baroclinic instability for extratropical cyclones, one would anticipate that clustering coincides with increased baroclinicity, though simultaneously individual cyclones reduce baroclinicity to maintain their growth. This apparent contradiction motivates our hypothesis that some cyclones increase baroclinicity, which could be a pathway for cyclone clustering.</p><p>Using a new cyclone clustering diagnostic based on spatio-temporal distance between cyclone tracks, we analyse cyclone clustering for the period 1979 until 2016. We complement this analysis with a baroclinity diagnostic, the slope of isentropic surfaces. With the isentropic slope and its tendencies, the relative roles of diabatic and adiabatic effects associated with extra-tropical cyclones in maintaining baroclinicity are assessed. We first present a case study, for which a sequence of cyclones culminated in severe cyclones due to the fact that one of the storms significantly increased the background baroclinity along which the succeeding storms evolved. The life cycle of these storms is discussed in terms of how the storm changes and uses its environment to attain its intensity. We compare these findings to composites of clustered and non-clustered cyclones to quantify how consistent the proposed clustering-mechanism is.</p>
To understand the mechanisms behind precipitation extremes, one can determine the origin of this precipitation, i.e. its moisture sources. The temporal and spatial distribution of these sources provide insights into the synoptic situation of the extreme event, and the importance of land-atmosphere interactions and moisture recycling. To determine moisture sources, moisture tracking models are used and forced with gridded atmospheric data to track water vapour in the atmosphere backward in time to its origin. Moisture tracking models have become an increasingly popular tool in scientific studies in recent years, but diversify in their underlying assumptions. Validation of tracking models is difficult due to the scarcity of isotope measurements as a benchmark. Further, structured intercomparisons among different models are lacking.Here, we present our efforts to coordinate a moisture tracking intercomparison study. Therefore, we reached out to many members of the moisture tracking community, and asked them to run their own moisture tracking model for three selected extreme precipitation events that occurred in 2022. Those three selected events cover different precipitation mechanisms: a monsoon event in Pakistan, a convective precipitation case in Australia, and an atmospheric river driven precipitation case over Scotland. We aim to compare the sources for the three cases among the different models during a one-week workshop in May 2024 in Leiden, The Netherlands. To this date, this intercomparison study covers about eight different moisture tracking models, allowing us to address and quantify the uncertainty in the moisture sources. At the EMS Annual meeting, we will present our approach and preliminary results of this intercomparison. This coordinated model intercomparison facilitates the explanation and quantification of uncertainty, acting as a point of reference for future work and literature on moisture tracking.
Extreme precipitation events in Norway in all seasons are often linked to atmospheric rivers (AR). We show that during the period 1979–2018 78.5% of the daily extreme precipitation events in Southwestern Norway are linked to ARs, this percentage decreasing to 59% in the more northern coastal regions and ~40% in the inland regions. The association of extreme precipitation with AR occurs most often in fall for the coastal areas and in summer inland. All Norwegian regions experience stronger winds and 1–2°C increase of the temperature at 850 hPa during AR events compared to the climatology, the extreme precipitation largely contributing to the wet climatology (only considering rainy days) in Norway but also in Denmark and Sweden when the rest of Europe is dry. A cyclone is found nearby the AR landfall point in 70% of the cases. When the cyclone is located over the British Isles, as it is typically the case when ARs reach Southeastern Norway, it is associated with cyclonic Rossby wave breaking whereas when the ARs reach more northern regions, anticyclonic wave breaking occurs over Northern Europe. Cyclone-centered composites show that the mean sea level pressure is not significantly different between the eight Norwegian regions, that baroclinic interaction can still take place although the cyclone is close to its decay phase and that the maximum precipitation occurs ahead of the AR. Lagrangian air parcel tracking shows that moisture uptake mainly occurs over the North Atlantic for the coastal regions with an additional source over Europe for the more eastern and inland regions.
Abstract Contrary to the general notion that extratropical cyclones reduce baroclinicity, the baroclinicity is found to be enhanced in the wake of the extreme winter storm Dagmar. Thus, individual storms can increase baroclinicity, yielding a pathway to secondary cyclogenesis and cyclone clustering. We use a recently introduced diagnostic for baroclinicity—the tendency equation for the isentropic slope—and found that strong diabatic heating due to moisture supply from the subtropical Atlantic led to the enhanced baroclinicity in the rear of Dagmar. Storms ensuing Dagmar benefited from this increased baroclinicity. In contrast to previous studies on the mechanisms of cyclone clustering, we only find weak evidence for Rossby wave breaking and thus propose diabatic heating as an alternative pathway to cyclone clustering.
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