Recent  

Wintertime lightning in Central Europe is rare but has a large damage potential for tall structures such as wind turbines. We use a data-driven approach to explain why it even occurs when the meteorological processes causing thunderstorms in summer are absent. In summer with strong solar input, thunderclouds have a large vertical extent, whereas in winter thunderclouds are shallower in the vertical but tilted and elongated in the horizontal by strong winds that increase with altitude.

In this work a special form of a foehn wind in an Alpine valley with a large-scale northwesterly flow is investigated. The study clarifies the origin of the air mass and the mechanisms by which this air enters the valley. A trajectory analysis shows that the location where the main airstream passes the crestline is more suitable for a foehn classification than the local or large-scale wind direction. Mountain waves and a lee rotor were crucial for importing air into the valley.

Tropical cyclones are amongst the most dangerous weather events. Here we develop an empirical model that allows to estimate the number and strengths of tropical cyclones for given atmospheric conditions and sea surface temperatures. An application of the model shows that atmospheric circulation is the dominant factor for seasonal tropical cyclone activity. However, warming sea surface temperatures have increased the likelihood of extremely active hurricane seasons in the past decades.

It is important to understand what makes it hot in Australia in spring so that we can be better prepared to deal with harmful impacts. We look at how the higher latitudes and tropics change the atmospheric circulation in early to late spring, and how that changes maximum temperatures in Australia. We find that the relationship between maximum temperatures and the tropics is stronger in late spring than early spring. These findings could help improve forecasts of hot months in Australia in spring.

Observational data suggests that the extent of Arctic sea ice influences mid-latitude winter weather. However, climate models generally fail to reproduce this link, making it unclear if models are missing something or if the observed link is just a coincidence. We show that if one explicitly represents the effect of unresolved sea ice variability in a climate model, then it is able to reproduce this link. This implies that the link may be real but that many models simply fail to simulate it.

Polar lows are small, but intense cyclones and constitute one of the major natural hazards in the polar regions. To be alert when and where polar lows occur, this study maps polar lows globally by utilizing new atmospheric datasets. Polar lows develop in all marine areas adjacent to sea ice or cold landmasses, mainly in the winter half-year. The highest frequency appears in the Nordic Seas. Further, it is found that polar lows are rather similar in the different ocean sub-basins.

El Niño events are known to effect the variability of the wintertime stratospheric polar vortex. The observed relationship differs from what is seen in climate models. Climate models have errors in their average winds and temperature, and in this work we artificially reduce those errors to see how that changes the communication of El Niño events to the polar stratosphere. We find reducing errors improves stratospheric variability, but does not explain the differences with observations.

A revisited space/time spectral decomposition allows to determine which harmonics dominate the upper tropospheric flow evolution over a given time period as well as their propagation. This approach is used to identify Rossby wave patterns with a circumglobal extent, affecting weather evolution over different region of the Northern Hemisphere. The results cast light on the processes originating and supporting these wave patterns, advocating at the same time for the usefulness of the technique.

Eleven operational forecast models that run on subseasonal timescales (up to two months) are examined, to assess the errors in their simulated large-scale stationary waves in the Northern Hemisphere winter. We found that models with finer resolved stratosphere generally do better in simulating the waves both in the stratosphere (10–50 km) and troposphere below. Moreover, a connection exists between errors in simulated time-mean convection in tropical regions and errors in the simulated waves.

Atmospheric blocking is associated with stationary, self-sustaining, long-living high pressure systems. They can cause or at least influence surface weather extremes, such as heat waves, cold spells, heavy precipitation events, droughts, or wind extremes. The location of the blocking determines where and what type of extreme event will occur. These relationships are also important for weather prediction and may change due to global warming.

Water vapour profoundly impacts the Arctic, for example by contributing to sea ice melt. A substantial portion of water vapour in the Arctic originates at mid-latitudes and is transported poleward in a few episodic and intense events. This transport is accomplished by low and high-pressure systems occurring in specific regions or following particular tracks. Here, we explore how the type of weather system impacts where the water vapour is coming from and how it is transported poleward.

This is a collective effort to describe the state of the art in Mediterranean cyclone dynamics, climatology, prediction (weather and climate scales) and impacts. More than that, the paper focuses on the future directions of research that would advance the broader field of Mediterranean cyclones as a whole. Thereby, we propose interdisciplinary cooperation, additional modelling and forecasting strategies and highlight the need for new impact-oriented approaches on climate prediction.

Recent studies have identified the weather systems in observational data, where wave patterns with high magnitude values that circle around the whole global in either wavenumber 5 or wavenumber 7, are responsible for the extreme events.In conclusion, we find that the climate models are able to reproduce the large-scale atmospheric circulation patterns, as well as their associated surface temperature, precipitation, sea level pressures characters.

Northern Hemisphere winter weather is influenced by the strength of westerly winds 30 kilometres above the surface, the so-called polar vortex. Eurasian autumn snow cover is thought to modulate the polar vortex. So far however, the modelled influence of snow on the polar vortex did not fit the observed influence. By analysing a model experiment for the timespan of 110 years, we could show that the causality of this impact is indeed sound and snow cover can weaken the polar vortex.

We assess with an Observation System Simulation Experiment the potential of mid-tropospheric water isotopologue data for constraining uncertainties in meteorological analysis fields in the tropics. Our assimilation experiments indicate that isotopologue observations have the potential to reduce the uncertainties of diabatic heating rates and meteorological variables in the tropics and in consequence offer potential for improving meteorological analysis in the tropical regions.

Mesocyclones are the rotating updraft of supercell thunderstorms that present a particularly hazardous subset of thunderstorms. A first time characterization of the spatio-temporal occurrence of mesocyclones in the Alpine region is presented, using 5 years of Swiss operational radar data. We investigate parallels to hailstorms, particularly the influence of large-scale flow, daily cycle and terrain. Improving understanding of mesocyclones is valuable for risk assessment and warning purposes.

Western Disturbances are mid-latitude, high altitude low pressure areas that brings orographic precipitation in the Upper Indus Basin. Using statistical tools, we show that the interaction between Western Disturbances and relief explains the near surface, cross-barrier wind activity. We also reveal the existence of a moisture pathway from the nearby seas. Overall, we offers a conceptual framework for Western Disturbance activity, particularly in terms of precipitation.

In climate models, improvements obtained in the winter mid-latitude circulation following horizontal resolution increase are mainly caused by the more detailed representation of the mean orography. A high resolution climate model with a low resolution orography might underperform a low resolution model with a low resolution orography. The absence of proper model tuning at high resolution is considered as the potential reason behind such lack of improvements.

Potential future changes of the Hadley cells (HCs), either to their strength or their meridional extent, will have a profound impact on the global distribution of precipitation. An objective analysis of past and future changes of HCs requires a unified index of the their strength. The measures, which rely on the single point values, produce unreliable long-term trends of the HC strength. Thus, two new measures are proposed, which alleviate the spatial inhomogeneities of HC strength trends.

This study assesses the ability of the new Met Office IND1 numerical model to simulate tropical cyclones and their associated hazards, such as high winds and heavy rainfall. The new system consists of both atmospheric and oceanic models coupled together, allowing us to explore the sensitivity of cyclones to important air–sea feedbacks. We find that the model can accurately simulate tropical cyclone position, structure, and intensity, which are crucial for predicting and mitigating hazards.

Greenhouse gases concentrations continue to increase, while the Antarctic ozone hole is expected to recover during the twenty-first century. We separate the effects of ozone recovery and of greenhouse gases on the Southern Hemisphere atmospheric and oceanic circulation and we find that ozone recovery is generally reducing the impact of greenhouse gases, with the exception of certain regions of the stratosphere during spring, where the two effects reinforce each other.

We study the impact of climate change on wintertime atmospheric blocking over Europe. We focus on the frequency, duration, and extension of blocking events. The blocking events are identified via a particular methodology, the weather type decomposition methodology. Analysing these events, we find that their frequency and duration will not change during the 21st century, while their extension will increase; such increase is related to the global warming rather than atmospheric dynamics changes.