Preprints  

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.

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.

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.

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.

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.

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.

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.

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.

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.

We create a climatology of mid-latitude cyclones and windstorms in Northern Europe and investigate how sensitive the minimum pressure and maximum gust of windstorms are to four precursors. Windstorms are more common in the cold season than the warm season whereas the number of mid-latitude cyclones has no annual cycle. The low-level temperature gradient has the strongest impact of all considered precursors on the intensity of windstorms in terms of both the minimum pressure and maximum gust.

The coupling between the large-scale atmospheric circulation and the clouds in the trade-wind region is complex and not yet fully understood. In this study, the formation pathway of two anomalous cloud layers over Barbados during the field campaign EUREC⁴A are described. The two case studies highlight the influence of remote weather systems on the local environmental conditions in Barbados.

Temperature and humidity profiles in the Arctic atmosphere in winter are affected by both the large scale dynamics and the local processes, such as radiation, cloud formation and turbulence. Hence, the influence of different large scale flows on temperature and humidity profiles in the Arctic must be viewed as a progressing set of processes.

The interactions between the dry and moist components of the atmosphere and the influence of, for example, the phase transition of water on the atmospheric circulation are often studied from the potential vorticity (PV) framework. Changes in the PV due to, for example, condensation can relate to changes in the static stability or vorticity. To better the interaction between these two drivers of PV changes, we explore the usefulness of a novel vorticity-and-stability diagram.

Attribution of the causes of extreme temperature events has become an active research due to the wide-ranging impacts of recent heatwaves and cold spells. Here we show that a purely observational approach based on atmospheric circulation analogues and resampling provides a robust quantification of the various dynamic and thermodynamic contributions to specific extreme temperature events. The approach can easily be integrated in the toolbox of any real-time extreme event attribution system.

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.

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.

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.

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.

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.

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.

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.