Abstracted/indexed

Heavy rainfall can cause dramatic societal and economic impacts. This study presents a global perspective on the role of wave breaking and moisture transport for heavy rainfall. We demonstrate the importance of these two atmospheric processes for heavy rainfall by several flood events and then quantify this linkage in a climatological analysis. These findings can have large implications for high impact weather prediction and climate studies on extreme precipitation changes in a warming climate.

Precipitation and moisture sources over an arid region in northeast Greenland are investigated from 1979 to 2017 by a Lagrangian moisture source diagnostic driven by reanalysis data. Dominant winter moisture sources are the North Atlantic above 45° N. In summer local and north Eurasian continental sources dominate. In positive phases of the North Atlantic Oscillation, evaporation and moisture transport from the Norwegian Sea is stronger, resulting in more precipitation.

Polar lows are intense meso-scale cyclones occurring at high latitudes. The research community has not agreed on a conceptual model to describe polar-low development. Here, we apply self-organising maps to identify the typical ambient sub-synoptic environments of polar lows and find that they can be interpreted as moist-baroclinic cyclones.

We show that the persistence of summer weather has increased throughout the mid-latitudes over the last 40 years, in both observations and CMIP6 models. Our results provide solid evidence that the weakening of the summer jet-stream has already made weather more persistent. We also show that future greenhouse-gases emissions will further increase weather persistence, creating risks from high-impact, stalling weather extremes like persistent heat waves and stalling cyclones.

While the key role of moist processes in blocking has recently been highlighted, their moisture sources remain unknown. Here, we investigate moisture sources for wintertime Euro-Atlantic blocks using a Lagrangian method. We show that the Gulf Stream and Kuroshio and their extensions, along with the northeast of Hawaii, act as the primary moisture sources and springboards for particle ascent. We find that the evolution of the particles’ properties is sensitive to the moisture sources.

Global warming not only affects the mean state of the climate (i.e. a warmer world) but also its variability. Here we analyze a set of future climate scenarios and show how some configurations of the atmospheric flow will become more frequent and persistent in the future, under continued greenhouse forcing. For example, over Europe models predict an increase of the NAO+ regime, which drives intense precipitation in Northern Europe and the British Isles, and dry conditions over the Mediterranean.

This study focused on exploring if off-season tropical cyclones, those that develop outside of the peak months, have been increasing over time in the Atlantic and Pacific Ocean basins and if that higher frequency could be explained by climate variability or change. We found that off-season tropical cyclones are exhibiting an increase in total numbers by decade in the North Atlantic, East Pacific and West Pacific basins, and that climate change explained much of the increasing trends over time.

Motivated by the European heat wave occurrences of 2015 and 2018, this study evaluates the influence of cold North Atlantic SST anomalies on European heat waves by using the ERA-5 reanalysis product. Our findings show that widespread cold North Atlantic SST anomalies may be a precursor for a persistent jet stream pattern and are thus important for the onset of high European temperatures.

Precipitation is a key climate variable with large impact on society, but also difficult to simulate as it depends largely on temporal and spatial scales. We look here on the effect of model resolution on precipitation in Europe, from coarse scale global model to small scale regional models. Higher resolution improves simulated precipitation generally, bu individual models may over- or underestimate precipitation even at higher resolution.

We analysed a tornadic supercell that tracked through the northern Highveld region of South Africa for 7 hours. We found that atmospheric conditions were conducive for tornado-associated severe storms over the region. A 4.4 km resolution model run by the South African Weather Service was able to predict this supercell, including its timing. However, it underestimated its severity due to underestimations of other important factors necessary for real-world development of these kinds of storms.

The so-called stratospheric polar vortex affects the weather in the mid-latitudes making it important to understand potential changes in response to global warming. However, climate model projections disagree on how the vortex will change in the future. Here we show that sea ice loss in the Barents and Kara (BK) Seas plays a central role in this. The time the BK Seas are ice-free yet differs between models which explains some of the the disagreement regarding the polar vortex projections.

In this study, we investigate the three-dimensional life cycle of cyclonic structures (so-called PV cutoffs) in the upper troposphere. PV cutoffs are frequent weather systems in the extratropics and contribute substantially to high impact weather in many regions of the globe. However, many unknowns exist regarding their evolution. Using a new method to track PV cutoffs in reanalysis data, their life cycle is studied in detail and a classification into three life cycle types is proposed.

Warm conveyor belts (WCBs) are important cloud and precipitation producing airstreams in extratropical cyclones. By combining satellite observations with model data from a new reanalysis dataset, this study provides detailed observational insight into the vertical cloud structure of WCBs. We find that the reanalyses essentially capture the observed cloud pattern, but the observations reveal many mesoscale structures not resolved by the temporally and spatially much coarser-resolution model data.

The study presents the first results from the airborne RASTA observations measured during North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX). Our combined Eulerian-Lagrangian analysis found three types of organized convection (frontal, banded and mid-level) in the warm conveyor belt of the Stalactite cyclone. It suggests that a dynamical linkage between these organized convective ascents and the mesoscale negative PV bands found in their vicinity.