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The stratosphere contains under a fifth of the atmosphere's mass but through its variability has a significant influence at ground level. Features of the variability are high-latitude sudden stratospheric warmings and a tropical quasi-biennial oscillation. Much of the variability is dynamical due to waves from the troposphere interacting with the mean flow. Dynamical stratospheric variability, its representation and predictability in models and coupling to the troposphere are reviewed.

While the Arctic is warming rapidly, some counterintuitive regional impacts have emerged, including the pronounced cooling over the Eurasian subcontinent that has been linked to retreating Arctic sea ice. However, there is strong disagreement among the scientific community around the strength and even existence of this link, as well as the possible causes of Eurasian cooling. Here we provide a review of the state of understanding and a perspective that helps reconcile apparent disagreements.

More than half of India’s summer monsoon rainfall arises from low-pressure systems: storms originating over the Bay of Bengal. In observation-based data, we examine how the generation and pathway of these storms are changed by the “Boreal summer intraseasonal oscillation” – the chief means of large-scale control on the monsoon at time scales of a few weeks. Our study offers new insights for useful prediction of these storms, important for both water resources planning and disaster early warning.

Mid-latitude weather is dominated by the formation and decay of large cyclonic and anticyclonic eddies, mostly formed during the growth and breaking of baroclinic waves. One way to study these phenomena is via idealised numerical life-cycle simulations, which are typically characterised by cyclonic or anticyclonic wave breaking, depending on the details of the system. We show that all systems exhibit predominantly anticyclonic character if a range of wave modes is allowed to grow simultaneously.

Understanding the location and intensity of hazardous weather across the Arctic is important for assessing risks to infrastructure, shipping, and coastal communities. This study describes the typical lifetime and structure of intense winter and summer Arctic cyclones. Results show the composite development and structure of intense Arctic summer cyclones is different to intense winter Arctic and North Atlantic Ocean extra-tropical cyclones, and to conceptual models.

Assessing annual time series of local Rossby wave packet amplitude, phase, and phase speed on reanalysis data reveals that trends outweigh interannual variability in several regions and seasons. While no covariance emerges between amplitude and phase speed at decadal scales, the frequency of DJF large-amplitude quasi-stationary waves increases in several areas during 1999–2019. These outcomes prompt further investigations on the causes and implications of local variations in RWP properties.

Mesoscale high-wind features within extratropical cyclones can cause immense damage. Here, we present RAMEFI, a novel approach to objectively identify the wind features based on a probabilistic random forest. RAMEFI enables a wide range of applications such as probabilistic predictions for the occurrence or a multi-decadal climatology of these features, which will be the focus of Part II of the study, with the goal of improving wind and specifically wind gust forecasts in the long run.

The atmosphere is in motion and hereby transporting warm, cold, moist and dry air to different climate zones. In this study, we investigate how this transport of energy organises in different manners. Outside the tropics, atmospheric waves of sizes between 2,000 and 8,000 km, which we perceive as cyclones from the surface, transport most of the energy and moisture poleward. In the winter, large-scale weather situations become of high importance for transporting energy into the polar regions.

Supercells are amongst the most complex and dangerous severe convective storms due to their associated phenomena (lightning, strong winds, large hail, flash-floods, or tornadoes). In this survey we study the supercell synoptic configurations and convective environments in Spain using the atmospheric reanalysis ERA5. Moreover, supercells are grouped in hail (greater than 5 cm) and non-hail events in order to compare and analyze both groups. The results reveals statistical significant differences.

This study presents a five-year climatology of three main Foehn types and three Deep Foehn subtypes. The main types differ in the large-scale and Alpine-scale weather conditions, the subtypes in terms of the amount and extent of precipitation on the Alpine south side. They are found to strongly affect the local meteorological characteristics at Altdorf. The study concludes by setting the new classification into a historic context.

The El Nino-Southern Oscillation (ENSO) and Arctic Oscillation (AO) are phenomena that affect the weather and climate of North America. Although ENSO harkens from the tropical Pacific and the AO high above the North Pole–the spatial patterns of their influence on a North American tornado environment index are remarkably similar in computer models. We find that when ENSO and the AO act in concert, their impact is large, and when they oppose each other their impact is small.

Extreme events like heat waves are accompanied by changes in atmospheric circulation across scales. Our study shows that heat waves over Eurasia have fingerprints in the probability density functions (PDFs) of the global balanced energy. We analyzed energy anomalies in the global balanced circulation and its components (the zonal mean state and Rossby waves at different zonal scales) and found an increase in the skewness of the PDFs of the planetary scale Rossby waves during surface heat waves.

We report on a set of laboratory experiments that reproduce a global warming scenario. The experiments show that the decreased temperature difference between the poles and equator slows down the eastward propagation of the mid-latitude weather patterns. Another consequence is that the temperature variations diminish and hence temperature extreme events might become milder in a global warming scenario. Our experiments also show that the frequency of such events increases.

Two gap flow events in the Great Rift Valley in Ethiopia were investigated with observations, ERA5 reanalysis data and simulations with the numerical weather prediction model WRF. The main focus was on strong winds in the area around Lake Abaya since the winds may generate waves on the lake which help to sustain the lakes ecology. That is important in terms of food supply for the local population. The gap winds exhibit a diurnal cycle and a seasonal dependence.

Over 10 km above the ground, there is a region of the atmosphere called stratosphere. While there is very little air in the stratosphere itself, its interactions with the lower parts of the atmosphere – where we live – can affect the weather. Here we study a specific example of such interaction, whereby processes occurring at the boundary of the stratosphere can lead to a continent-wide drop in temperatures in North America during winter.

This study of changes in temperature and wind since 1979 met its twin aims of (i) increasing confidence in some findings of the latest IPCC assessment and (ii) identifying changes that had received little or no previous attention. It reports a small overall intensification and shift in position of the North Atlantic jet stream and associated storms, and a strengthening of tropical upper-level easterlies. Increases in low-level winds over tropical and southern hemispheric oceans are confirmed.

This study quantifies the impact of poorly constrained parameters used to represent aerosol-cloud-precipitation interactions in an operational weather forecasting model on precipitation and cloud forecasts. Uncertainties in these parameters have a non-negligible impact on daily precipitation amount, and largely change the amount of cloud. The comparison between different weather situations reveals that the impact becomes more important when convection is triggered by local effects.