Abstracted/indexed

This work examines the fidelity to reproduce regional and global monsoons climatological features using the Met Office Unified Model (MetUM) third and fourth generations Global Atmosphere (GA3.0) and (GA4.0), two configurations of the HadGEM3 system developed for use across climate and weather time scales. GA3.0 largely captures global monsoon features, including the monsoon precipitation patterns. GA4.0 and GA3.0 results display a close similarity, and compares reasonably well against CMIP5.

Using a simple general circulation model, we examine the structure of waves in the mid-tropospheric jet over North Africa. We show that waves occur in near-stationary groups or wavepackets. As they are not swept out of the jet, this may provide the opportunity for the packets to amplify via feedback from other energy sources like rain-producing cloud complexes and mineral dust that are known to operate here. Our results address the criticism that the easterly jet is too short to sustain waves.

Extratropical cyclones are important for the transport of moisture from low to high latitudes. In this study, we investigate, how the isotopic composition of water vapour is affected by horizontal temperature advection associated with extratropical cyclones using measurements and modelling. It is shown, that air–sea moisture fluxes induced by this horizontal temperature advection lead to the large variability observed in the isotopic composition of water vapour in the marine boundary layer.

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.

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.

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.

In this paper, we investigate air-sea interaction by looking at the relationship between spatial variability in surface heat flux and air temperature. We observe that their interaction characterizes different stages of storm evolution thus providing a new perspective on the role played by air-sea heat exchange.

This study addresses the origin and impacts of a source of large-scale atmospheric waves in the lower stratosphere, which has not been examined before. This wave source is caused by interactions of waves of smaller scales. Here we show that as it lies in the lower stratosphere, this wave source can precede extreme events in the stratosphere and that such events can then lead to a robust response of the tropospheric weather patterns several weeks later (potential for long-term forecasting).

This study explores the relationship between the North Atlantic Oscillation and the winter climate of eastern North America using reanalysis data. Results show that negative phases are linked with an increase of frequency of winter storms developing on the east coast of the United States, resulting in much heavier snowfall over the eastern United States. On the contrary, an increase of cyclone activity over southeastern Canada results in heavier precipitation and snowfall during positive phases.

To understand better the factors that impact the weather in North America, this study explores the influence of El Niño-Southern Oscillation on wintertime surface air temperature variability using reanalysis data. Results show that La Niña enhances subseasonal variability over western North America by amplifying the baroclinic conversion of energy from the winter-mean circulation to subseasonal eddies. Changes in the vertical structure of these eddies are crucial for this amplification.

Our study focuses on the impact of topographic structures such as Himalayas and Rocky mountains, so-called orographic gravity-wave hotspots. These hotspots play an important role in the dynamics of the middle atmosphere, in particular in the lower stratosphere. We study intermittency and zonally asymmetric character of these hotspots and their effects on the upper stratosphere and mesosphere using a new detection method in various modeling and observational datasets.

Most precipitation over Europe is linked to low-pressure systems, cold fronts, warm fronts, or high-pressure systems. Based on a massive computer simulation able to resolve thunderstorms, we quantify in detail how much precipitation these weather systems produced during 2000–2008. We find distinct seasonal and regional differences, such as fronts precipitating a lot in fall and winter over the North Atlantic, but high-pressure systems mostly in summer over the continent by way of thunderstorms.

We investigate the stratospheric influence on marine cold air outbreaks (MCAOs) using ERA-Interim reanalysis data. MCAOs are associated with severe Arctic weather, such as polar lows and strong surface winds. We find that the key ingredients of MCAO formation in the Barents Sea are linked to the surface impact of Sudden Stratospheric Warming (SSW) events. As SSWs have a long-term surface influence, this study sheds light on the predictability of MCAOs in the Arctic for winters with SSW events.

In this paper we investigate the role of the tropospheric forcing in the occurrence of the Sudden Stratospheric Warming (SSW) that took place in February 2018, its predictability and teleconnection with the Madden-Julian oscillation (MJO) by analysing the European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble forecast. The purpose of the paper is to present the results of the analysis of the atmospheric circulation before and during the SSW and clarify the driving mechanisms.

We find, by tracing backward in time, that air masses causing extensive melt of the Greenland Ice Sheet originate from further south and lower altitudes than usual. Their exceptional warmth further arises due to ascent and cloud formation, which is special compared to near-surface heatwaves in the midlatitudes or the Central Arctic. The atmospheric systems and transport pathways identified here are crucial in understanding and simulating the atmospheric control of the ice sheet in the future.

We study the interactions between the tropical convective activity and the mid-latitude circulation in the Northern Hemisphere during boreal summer. We identify two circumglobal wave patterns with phase shifted corresponding to the South Asian and the western North Pacific monsoon systems at intraseasonal time scale. These patterns show a two-way interactions in a causal framework at weekly time scale and assess how El Niño affects these interactions.

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.