Preprints  

In this work we examine the ability of global climate models in representing the atmospheric circulation in the upper troposphere, focusing on the eventual benefits of an increased horizontal resolution. Our results confirms that an higher horizontal resolution has a positive impact especially in those models in which the resolution is increased in both the atmosphere and the ocean, whereas when the resolution is increased only in the atmosphere no substantial improvements are found.

While the assumption of Gaussianity leads to many simplifications, ensemble forecasts often exhibit non-Gaussian distributions. This work has systematically identified the presence of a specific case of non-Gaussianity, bimodality. It was found that bimodality occurs in a large portion of global 2-meter temperature forecasts. This has drastic implications on forecast skill as the minimum probability in a bimodal distribution often lays at the maximum probability of a Gaussian distribution.

Inacurracies in representing processes occurring at spatial scales smaller than the grid scales of the weather forecast models are important sources of forecast errors. This is the case of deep convection representation in models with ten kilometers grid spacing. We performed simulations of a real extratropical cyclone using a model with different representations of deep convection. These forecasts lead to different behaviors in the ascending air masses of the cyclone and the jet stream aloft.

Diabatic processes affect the development of extratropical cyclones. This work provides a systematic assessment of the diabatic processes that modify potential vorticity (PV) in model simulations. PV is primarily produced by condensation and convection. Given favorable environmental conditions, long-wave radiative cooling and turbulence become the dominant process at the cold and warm fronts, respectively. Turbulence and long-wave radiative heating produce negative PV anomalies at the fronts.

In a warmer future climate, climate simulations predict that some land areas will experience excessive warming during summer. We show that the excessive summer warming is related to the vertical distribution of warming within the atmosphere. In regions characterized by excessive warming, much of the warming occurs close to the surface. In other regions, most of the warming is re-distributed to higher levels in the atmosphere which weakens the surface warming.

Forecasting temperature patterns beyond two-weeks is very challenging although occasionally, forecasts show more skill over Europe. Our study indicates that the level of skill varies concurrently for two distinct forecast systems. It also shows that higher skill occurs when forecasts are issued during specific patterns of atmospheric circulation that tend to be particularly persistent. These results could help forecasters estimate ‘a priori’ how trustworthy extended-range forecasts will be.

During the night of 04/05 June 2016 Campinas, Brazil, was hit by a tornado and by intense lightning activity. The day (Sunday) and time of the tornado occurrence probably contributed to the fact that only a small number of persons suffered injuries, without any fatalities. The phenomenon was evaluated by means of radar observations, lightning activity and damages. The occurrence of the phenomenon has shown that the municipal government and citizens are unprepared to face this kind of event.

This research demonstrates, using feature identification and tracking, that anticlockwise rotating vortices at about 7 km altitude called tropopause polar vortices frequently interact with storms developing in the Arctic region, affecting their structure and where they occur. This interaction has implications for the predictability of Arctic weather, given the long lifetime, but relatively small spatial scale of these vortices compared with the density of the polar observation network.

Extratropical cyclones are relevant in the western South Atlantic and influence the climate of ocean surface wave. Propagating atmospheric features from the South Pacific to the South Atlantic are relevant to the cyclones and waves and its intensified westerlies lead to more cyclones and, as a consequence, to higher wave heights. The opposite happens with its weakening. These features are similar to the so-called Pacific South-American patterns and present periods between 30–180 days.

We use machine learning to create a network that detects and classifies four types of synoptic scale weather fronts from ERA5 atmospheric reanalysis data. Our work can be used to reduce the amount of manual work previously needed for this task. Additionally our results show that multiple sources of training data are necessary to perform well on different regions, implying differences within those regions. Qualitative evaluation shows that the results are physically plausible.

Climate models still struggle to correctly represent blocking frequency over the North Atlantic-European domain. This study makes use of five large ensembles of climate simulations and the ERA-Interim reanalyses to investigate the Greenland blocking frequency and one of its drivers, namely cyclonic Rossby wave breaking. We particularly try to understand the discrepancies between two specific models, out of the five, that behave differently.

This study uses reanalysis data to investigate the role of atmospheric blocking, prevailing high-pressure systems, and midlatitude cyclones in driving high-Arctic wintertime warm extreme events. These events are mainly preceded by Ural and Scandinavian blocks, which are shown to be significantly influenced and amplified by cyclones in the North-Atlantic. It also highlights processes that need to be well captured in climate models for improving their representation of Arctic wintertime climate.

In Switzerland hail may occur several days in a row. Such multi-day hail events may cause significant damage and understanding and forecasting these events is, thus, important. Using reanalysis data we show that weather systems over Europe move slower before and during multi-day hail events compared to single hail days. Surface temperatures are typically warmer and the air more humid over Switzerland and winds are slower on multi-day hail clusters. These results may be used for hail forecasting.

Stratospheric air can at times intrude into the troposphere and is associated with cyclonic development throughout the atmosphere and at the surface. Through a highly idealised, systematic approach, the effect different intrusion characteristics have on surface cyclones are investigated. The proximity of stratospheric intrusions surface is shown to be the main contributing factor to the development of associated surface cyclones, confirming the results of recent cut-off low climatologies.

For the first time, a 25-yr climatology of temperature and humidity inversions for Neumayer Station, Antarctica, was presented that takes into account different levels of inversion occurrence and different weather situations. Distinct differences in inversion features and formation mechanisms were found depending on inversion level and weather situation. These findings will increase our understanding of the polar boundary layer and improve the current paleoclimatic interpretation of ice cores.

The dynamics of the equatorial troposphere is incredibly complex. Several types of waves constitute the core of the theoretical understanding of tropical dynamics. Signatures of these waves are observed in the spectrum of atmospheric variables. We decompose the atmospheric winds into their wave contributions. A spectral analysis of these fields reveal important departures from the linear theory of equatorial waves, suggesting a possible role of nonlinearity in the propagation of these waves.

Novel techniques from computer science are used to study extreme weather events. Inspired by the interactive 3-dimensional visual analysis of the most comprehensive climatological dataset available (ERA5), we improve commonly used metrics for measuring polar winter storms and outbreaks of cold air. The software (Met.3D) that we develop and apply is freely available and can be used generically for visual analysis of a broad variety of atmospheric processes in weather and climate data.

This work is assessing the forecast of the temperature over the Sahara, a key driver of the West African monsoon, at seasonal time scale. The seasonal models are able to reproduce the climatological state and some characteristics of the temperature during the rainy season in the Sahel. But, because of errors in the timing, the forecast skill scores are significant only for the first 4 weeks.

In this study, we introduce a novel method to objectively define and identify Arctic extreme seasons based on different surface variables. We find that such seasons are resulting from various combinations of unusual seasonal conditions. The occurrence or absence of different atmospheric processes strongly affects the character of Arctic extreme seasons. Further, changes in sea ice and sea surface temperature can strongly influence the formation of such a season in distinct regions.

Major sudden stratospheric warmings (SSWs) are some of the most dramatic events in the atmosphere, and are believed to help cause extreme winter weather events such as the 2018 Beast from the East in Europe and North America. Here, we use unique data from the European Space Agency's new Aeolus satellite to make the first-ever measurements at a global scale of wind changes due to an SSW in the lower part of the atmosphere, to help us understand how SSWs affect the atmosphere and surface weather.

The strongest cyclone intensification is associated with a strong dry-dynamical forcing. Moreover, strong forcing and strong intensification correspond to a tendency for poleward cyclone propagation, which occurs in distinct regions in the Northern Hemisphere. There is a clear spatial pattern in the occurrence of certain forcing combinations. This implies a fundamental relationship between dry-dynamical processes and the intensification as well as the propagation of extratropical cyclones.

We use an advanced statistical approach to investigate the dynamics of the development of sudden stratospheric warming events in the winter Northern Hemisphere. We identify distinct signals of these events and their event type at lead times beyond currently predictable lead times. These signals could potentially improve the predictability of sudden stratospheric warming events, leading to improved long-range weather prediction.

We re-examine the uncertainty of ENSO teleconnection to the North Atlantic by considering the November–December and January–February months in the cold season, in addition to the conventional DJF months. This is motivated by previous studies reporting varying teleconnected atmospheric anomalies and the mechanisms concerned. Our results indicate an improved confidence in the patterns of the teleconnection. The finding may also have implications on research in predictability and climate impact.

Given the recent focus on the influence of upper tropospheric structure in wind and temperature on midlatitude weather, we use an idealised model to investigate how structural modifications impact cyclone development. We find that cyclone intensification is less sensitive to these modifications than to changes in the amount of cloud condensation, suggesting that an accurate representation of the upper-level troposphere is less important for midlatitude weather than previously anticipated.

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.

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.

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.

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.

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.

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.

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.

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.