Recent  

The effect of deep convection representation on the jet stream above the cold front of an extratropical cyclone is investigated in the global numerical weather prediction model ARPEGE. Two simulations using different deep convection scheme are compared with (re)-analysis datasets and NAWDEX airborne observations. A deeper jet stream is observed with the less active scheme. The diabatic origin of this difference is interpreted with backward Lagrangian trajectories and potential vorticity budgets.

Southern Australian rainfall and temperatures have experienced dramatic systematic changes that became evident for extreme rainfall and streamflow in the south-west since the 1960s and in the south-east since the late 1990s. Notable increases in average and extreme surface temperatures over all of Australia have occurred since the start of the 21^st century. Southern Australian climate has transitioned into a drier warmer state associated with shifts in the Southern Hemisphere circulation.

Precipitation and temperature are two of the most important variables describing our weather and climate. The relationship between these variables has been studied extensively, however, the role of specific weather systems in shaping this relationship has not been analysed yet. We therefore analyse whether intense precipitation occurs on warmer or on colder days and identify the relevant weather systems. In general, weather systems strongly influence this relationship, especially in winter.

We use the newest set of climate model experiments to investigate changes to mid-latitude storm tracks and cyclones from global warming. The overall number of cyclones will decrease, however in winter their will be more of the most intense cyclones and these intense cyclones are likely to be stronger. Cyclone wind speeds will increase in winter and as a result the area of strongest wind speeds will increase. By 2100 the area of strong wind speeds may increase by over 30 %.

Strong winds caused by extratropical cyclones represent a costly hazard for European countries. Here, based on CESM-LENS coupled climate simulations, we show that future changes of such strong winds are characterized by an increased magnitude and extended footprint southeast of the cyclone center. This intensification is related to a combination of increased diabatic heating and changes in upper-level wave dynamics.

A south-westward propagating moist wavetrain with quasi-biweekly period is shown to exist in the South-West Indian Ocean during winter. This oscillation is interpreted in terms of an equatorial Rossby wave. Differential planetary rotation is responsible for its propagation, and moist convection (via stretching) reduces its speed. Its circulation provides favourable conditions for cyclogenesis, with potential implications for the forecast of cyclones that affect Madagascar and southeast Africa.

We investigate how well current state-of-the-art climate models reproduce the wintertime weather of the North Atlantic and Western Europe, by studying how well different 'regimes' of weather are captured. Historically, models have struggled to capture these regimes, making it hard to predict future changes in wintertime extreme weather. We show models can capture regimes if the right method is used, but they show biases, partially as a result of biases in jet speed and eddy strength.

Local weather is governed by the atmospheric circulation, which can be viewed as the air flow on a large scale and at high elevation. In this work, we show some recent evolutions of the air flows coming from the Atlantic ocean and from the Mediterranean Sea to Western Europe, but also for Anticyclonic conditions (reflecting no air flow). These recent evolutions could have implications for temperature and precipitation variability and extremes over Western Europe.

Tropopause polar vortices (TPVs) are a high-latitude atmospheric phenomenon that impact weather inside and outside of polar regions. Using a set of long-lived TPVs to gain insight into the conditions that are most supportive of TPV survival, we describe patterns of vortex formation and movement. In addition, we analyze the characteristics of these TPVs and how they vary by season. These results help us to better understand TPVs which, in turn, may improve forecasts of related weather events.

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.