Recent  

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.