Recent  

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.

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.

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.

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.

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.

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.