Abstracted/indexed

Observations of the stratosphere span roughly half a century preventing analysis of multi-decadal variability in circulation using these data. Instead, we rely on long simulations of climate models. Here, we use a model to examine variations in northern polar stratospheric winds and find they vary with a period of around 90 years. We show that this is possibly due to variations in the size of winds over the equator. This result may improve understanding of equator-polar stratospheric coupling.

On 14–15 October 2018, in the Aude department (France), a heavy precipitation event produced up to about 300 mm of rain in 11 h. Simulations carried out show that the former hurricane Leslie, while involved, is not the first supplier of moisture over the entire event. The location of the highest rainfall appears to be driven primarily by the location of a quasi-stationary cold front and secondarily by the location of precipitation bands downwind of the mountains bordering the Mediterranean Sea.

Rare aircraft observations in the warm conveyor belt outflow associated with an extratropical cyclone are complemented with convection-permitting simulations. They reveal a complex tropopause structure with two jet stream cores, from which one is reinforced by bands of negative potential vorticity. They show that negative potential vorticity takes its origin in midlevel convection, which indirectly accelerates the jet stream and thus may influence the downstream large-scale circulation.

Rossby Wave packets impact all aspects of midlatitude weather systems, from their climatological distribution to predictability. Case studies suggest an important role of latent heat release in clouds. We investigate thousands of wave packets with a novel diagnostic. We demonstrate that, on average, the impact of moist processes is substantially different between troughs and ridges and that dry conceptual models of wave packet dynamics should be extended.

Comparing simulations with realistic and smoothed SSTs, we find that the intensification of individual cyclones in the Gulf Stream and Kuroshio regions is only marginally affected by reducing the SST gradient. In contrast, we observe a reduced cyclone activity and a shift in storm tracks. Subdividing the winter climatology into dates with/without cyclones, we find that cyclones play only a secondary role in explaining the mean states differences between the SST experiments.

The interaction of clouds in the trade wind region with the atmospheric flow is complex and at the heart of uncertainties associated with climate projections. In this study, a natural tracer of atmospheric circulation is used to establish a link between air originating from dry regions of the midlatitudes and the occurrence of specific cloud patterns. Two pathways involving transport within midlatitude weather systems are identified, by which air is brought into the trades within 5–10 days.

Convection embedded in the stratiform cloud band of strongly ascending airstreams in extratropical cyclones (so-called warm conveyor belts) can influence not only surface precipitation but also the upper-tropospheric potential vorticity (PV) and waveguide. The comparison of intense vs. moderate embedded convection shows that its strength alone is not a reliable measure for upper-tropospheric PV modification. Instead, characteristics of the ambient flow co-determine its dynamical significance.

The representation of an extratropical cyclone in simulations of two climate models is studied by comparing them to observations of the international field campaign NAWDEX. We show that the current resolution used to run climate model projections (more than 100 km) is not enough to represent the cyclone life cycle accurately but the use of 50 km resolution is good enough. Despite these encouraging results cloud properties (partitioning liquid/solid) are found to be far from the observations.

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.

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.

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.

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.