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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.

This study shows, with numerical experiments, that the influence of the Arctic sea-ice loss on atmospheric circulation in mid-latitudes depends on persistent sea-surface temperature in the North Pacific.

We investigate responses of Etesian winds, which is one of the most stable summer circulation regimes in the Eastern Mediterranean, to large volcanic eruptions. Using model simulations of the last millennium climate, we find a robust reduction of the average wind speed in late summer months and the total number of days with Etesian winds. These signatures are explained by reductions in the intensity of the Indian Summer Monsoon in the summers following the eruption.

Our modeling approach has improved significantly (~30 % to 60 %) the simulation of North Indian Ocean tropical cyclones’ track and intensity. The results are promising. This technique can be used for real-time cyclone predictions over the study region to provide more accurate cyclone related warnings and alerts for better planning and preparedness.

This article shows that climate models largely differ in their representation of persistent dry spells that co-occur with extremely high temperatures. Some models simulate dry spells that are too short and too cool compared to observations, while others simulate dry spells that are too long and too hot. This information is important when interpreting the likelihood of such events in a warmer climate so that we can assess the plausibility of their future projections.

High-resolution climate models simulate better extreme precipitation. We investigate the dependency of these improvements with precipitation thermodynamical process. We revisited an index, to detect extreme events and derived statistically, environmental conditions leading to those extremes. We found that high-resolution simulations simulate larger (lower) precipitation at low terrain if moisture transports and instability are likewise larger (lower) than in coarser resolutions.

This paper establishes robust links between the stratosphere, via a semi-periodical oscillation and tropical surface climate. Surface responses as well as several features of the tropical circulation are found to be robustly linked to the variability in the tropical stratosphere in a long simulation of a state-of-the-art climate model which suggests that processes exist that couple the two bottom layers of the atmosphere and that can affect the tropical surface.

Forecast models that are used to predict weather often struggle to represent the Earth’s stratosphere. This may impact their ability to predict surface weather weeks in advance, on subseasonal-to-seasonal (S2S) timescales. We use data from many S2S forecast systems to characterize and compare the stratospheric biases present in such forecast models. These models have many similar stratospheric biases, but they tend to be worse in systems with low model tops located within the stratosphere.

Variations in the stratospheric polar vortex, so-called vortex events, can improve predictions of surface weather and climate. There are various ways to detect such events, and here we use the amount of wave energy that propagates into the stratosphere. The the new definition is tested against so-called stratospheric sudden warmings (SSWs). We find that the wave definition has important advantages over SSWs, for example in terms of a stronger surface response that follows the events.