Preprints  

In this study we analyse the complex seasonal evolution of the East Asian summer monsoon. Using reanalysis data, we show the importance of the interaction between tropical and extratropical air masses converging at the monsoon front, particularly during its northward progression. The upper-level flow pattern (e.g. the westerly jet) controls the balance between the airstreams, and thus the associated rainfall. This framework provides a basis for studies of extreme events and climate variability.

Why do some monsoon seasons start early while others start late? For the Australian monsoon, some previous research suggested the El Niño–Southern Oscillation in the months before the onset influenced the monsoon timing, and we wanted to test if that was still correct, and if other large-scale climate patterns also influenced onset timing. We found that a strong La Niña pattern usually coincided with an early onset but a weak La Niña and El Niño patterns didn’t show a consistent relationship.

Storm activity in the tropics is one of the key phenomena that provide weather predictability on extended time scale of about 10–40 days. The influence of tropical storminess on places like North America is sensitive to the overall average state of the climate system. In this study, we show that global warming will likely lead to decreases in the influence of these storms on weather over the North Pacific, but that there is a lot of uncertainty in how this influence will change.

We present a ten year analysis on the occurrence of enhanced wind shear in the northern hemisphere, focussing on the region around the transport barrier that separates the first two layers of the atmosphere. The major result of our analysis is that enhanced wind shear above a certain threshold occurs frequently and nearly exclusively in this region, which, as an indicator for turbulent mixing, might have major implications concerning the separation efficiency of the transport barrier.

Mistral wind is a renowned phenomenon in the Mediterranean, yet, its large-scale controlling mechanisms have not been systematically mapped. Here, using a new mistral database for 1981–2016, the upper-tropospheric flow patterns are classified by a self-organizing map algorithm, resulting in 16 distinct patterns related to Rossby wave life cycles. Each pattern has unique surface impact, having implications to understanding mistral predictability, air-sea interaction and their future projections.

The amount of precipitation over Europe largely varies from year to year, with impacts on crop yields and energy production. In this study, we show that it is possible to infer much of the winter precipitation and temperature signal over Europe by only knowing the frequency of occurrence of certain atmospheric circulation patterns. The results highlight the importance of (daily) weather to understand and interpret seasonal signals.

Air stagnation occurs when an air mass becomes settled over a region and precipitation is suppressed. The large-scale influence on European air stagnation is explored here. We show that around 60 % of the monthly variability in air stagnation can be explained by dynamical indices describing the large-scale circulation. The weather systems most related to stagnation are different for regions across Europe. The findings can be used to assess the representation of air stagnation in climate models.

Using a large sample of Atlantic tropical cyclone tracks, we diagnose the mean circulations in the extratropics that are associated with rapid changes in the storm speeds. We show that rapid accelerations and decelerations are associated with distinct phasing between the tropical cyclone and weather waves of the extratropics. We also show that over the past 5 decades, rapid acceleration and deceleration of tropical cyclones have reduced in magnitude.

This study investigates the development processes of extratropical cyclones in East Asia, particularly when they are passing through the Korean Peninsula. These extratropical cyclones originate from Mongolia and East China and travel towards the Korean Peninsula. The quantitative analyses in the potential vorticity perspective demonstrate that the former is mostly driven dry-dynamically and the latter is influenced significantly by moist processes.

Atmospheric blocking events are complex large-scale weather patterns which block the path of the jet stream. They are associated with heat waves in summer and cold snaps in winter. Blocking is poorly understood, and the effect of climate change is not clear. Here, we present a new method to study blocking using unsupervised machine learning. We show that this method performs better than previous methods used. These results show the potential for unsupervised learning in atmospheric science.

We classify by wave geometry the stratospheric polar vortex during the final warming that occurs every spring in both hemispheres due to a combination of radiative and dynamical processes. We show that the shape of the vortex can have impacts on both total column ozone and surface weather following the final warming. These results have implications for prediction and our understanding of stratosphere-troposphere coupling processes in springtime.

We study a range of dynamical aspects of the Asian monsoon anticyclone as the response of a simple numerical model to a steady imposed heating distribution with different background flow configurations. Particular focus is given on interactions between the monsoon anticyclone and active mid-latitude dynamics, which we find to have a zonally localising effect on the time mean circulation and to be able to qualitatively alter the temporal variability of the bulk anticyclone.

Stationary, long-lasting blocked weather patterns can lead to extreme conditions. Within this study the temporal evolution of the occurrence probability is analysed and the onset, offset, and transition probabilities of blocking within the past 30 years are modeled. Using Markov models combined with logistic regression we found large changes in summer, where the probability of transitions to so-called Omega blocks increase strongly, while the unblocked state becomes less probable.

Vertical motion on horizontal scales of a few hundred kilometers can influence cloud properties. This motion is difficult to measure directly, but can be inferred from the area-averaged mass divergence. The latter can be derived from horizontal wind measurements at the area’s perimeter. This study derives vertical properties of area-averaged divergence from an extensive network of atmospheric soundings and proposes an explanation for the variation of divergence magnitudes with area size.

Weather variability controls the transport of ozone from the stratosphere to the Earth's surface and water vapor from oceanic source regions to continental land masses. Forecasting these types of transport has high societal value because of the negative impacts of ozone on human health and the role of water vapor in governing precipitation variability. We use upper level wind forecasts to assess the potential for predicting ozone and water vapor transport 3–6 weeks ahead of time.

Much of the rainfall in tropical regions comes from organised cloud bands called convergence zones (CZs). These bands have hundreds of kilometers. In South America (SA), they cause intense rain for long periods of time. To study these systems, we need to define and identify them with computer code. We propose a definition of CZs based on the the pathways of air, selecting regions where air masses originated in separated regions meet. This method identifies important systems of rain in SA.

Tropical Pacific sea surface temperatures (El Nino) effect the global climate. The Pacific-to-Europe connection relies on interactions of large atmospheric waves, with winds and surface pressure. We looked at how mean errors in a climate model effect its ability to simulate the Pacific-Europe connection. We found that even large errors in the seasonal winds did not affect the response of the model to an En Nino event, which is good news for seasonal forecasts which rely on these connections.

High-resolution measurements of stable isotopes in near-surface vapour and precipitation show a W-shaped evolution during a 24-h landfalling atmospheric river event in southern Norway. We distinguish contributions from below-cloud processes, weather system characteristics, and moisture source conditions during different stages of the event. Rayleigh distillation models need to be expanded by additional processes to accurately predict isotopes in surface precipitation from stratiform clouds.

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.

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.

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.

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.