Abstracted/indexed

In this study we quantify how much the coldest, middle and hottest third of all days during extremely hot summers contribute to their respective seasonal mean anomaly. This extreme summer substructure varies substantially across the Northern Hemisphere and is directly related to the local physical drivers of extreme summers. Furthermore, comparing re-analysis (i.e., measurement based) and climate model extreme summer substructures reveals a remarkable level of agreement.

In this paper, we take a second look at the development an intense storm that made landfall in Norway a few hours into the new year of 1992, focussing on the effect of the Scandinavian mountains on the storm. We find that the cyclone core evolves largely unaffected, although both the warm and the cold front decay rapidly while passing over the mountains. This result suggest that the fronts of a cyclone can become detached from their cyclone core as part of the cyclone’s occlusion process.

Arctic sea ice loss has been linked to changes in midlatitude weather and climate. However, the literature offers differing views on the strength, robustness, and even existence of these linkages. We use a statistical tool (Causal Effect Networks) to show that one proposed pathway linking Barents-Kara ice and the midlatitude circulation is intermittent in observations, and likely only active under certain conditions. This result may help explain apparent inconsistencies across previous studies.

Sting jets (SJs) descend into the frontal-fracture region of intense extratropical cyclones leading to strong surface winds. In this study we look at idealised simulations of SJ-containing cyclones produced using an improved initial state and a wide set of sensitivity experiments. The results shed light on SJ evolution and the role of dry instabilities in SJ dynamics, supporting a recent conceptual model. The simulations also highlight the robustness of SJ occurrence in these intense cyclones.

Sudden stratospheric warmings (SSWs) are extreme events that influence surface weather up to two months after onset. We remove EEIs in vertical sections of a general circulation model to investigate the role of eddy-eddy interactions (EEIs) in SSW formation. We show that the effects of EEIs depend strongly on the pressure levels where they occur and the zonal structure of the wave forcing in the troposphere. Our results highlight the importance of upper level processes in stratospheric dynamics.

The mesoscale convective system (MCS) that affected Germany at Pentecost 2014 was one of the most severe for decades. However, the predictability of this system was very low. By moving the model domain by just one grid point changed whether the MCS is successfully simulated or not at all. The main reason for the different model results seems to be the proximity of the track of the initial convective system to the coast and colder sea surface.

The 2013/14 winter sea surface temperature (SST) was anomalously cool in the mid-North Atlantic region. We investigate the processes by which cyclones can lead to SST cooling and their contribution towards the 2013/14 SST anomaly. We find that cyclones induce a cold wake, which extends along the cyclones cold front. Cyclones account for over half of the observed cooling in the mid-North Atlantic. Thus, cyclones play a major role in determining the extreme 2013/2014 winter SST cooling.

Warm conveyor belts (WCBs) are important, mainly stratiform cloud forming airstreams in extratropical cyclones that can include embedded convection. This WCB case study systematically compares the characteristics of convective versus slantwise ascent of the WCB. We find that embedded convection leads to regions of significantly stronger precipitation. Moreover, it strongly modifies the potential vorticity distribution in the lower and upper troposphere, where its also influences the waveguide.

In September 2018 a Mediterranean cyclone with structural similarities to a hurricane, a so called Medicane, caused severe damages in Greece. The development of the Medicane was uncertain, even a few days ahead. We find that the reason for this was an uncertain state of the jet stream over North America two days earlier. This uncertainty grew and propagated into the Mediterranean where it lead to uncertainty in the location of the cyclone formation and, as a result, its structure and intensity.

We studied how mid-latitude cyclones are likely to change in the future. We used a state-of-the-art numerical model and performed a control and a warm experiment. The total number of cyclones did not change with warming and neither did the average strength, but there were more stronger, and more weaker storms, in the warm experiment. Precipitation associated with the most extreme mid-latitude cyclones increased by up to 50 % and occurred in a more poleward location in the warmer experiment.