Publications

The model representation of dry intrusions (DIs) and the marine boundary layer (MBL) is analyzed in the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS). For this purpose, a DI classification at the Azores is combined with observation, short-term background forecast and analysis data from the IFS data assimilation system. The background exhibits a cold bias in the descending DI, which is possibly related to a cold bias in the MBL below through vertical mixing. At the surface, simulated wind speeds are underestimated and directions are veered compared to the observations. The errors are reduced in the analysis except for near-surface wind and humidity biases. We hypothesize that these biases are connected through underestimated surface latent heat fluxes. Such persistent biases potentially influence local weather and midlatitude weather evolution as cyclones are supplied with moisture from the cold sector influenced by DIs.

Deep convection occurs periodically in the Gulf of Lion, in the northwestern Mediterranean Sea, driven by the seasonal atmospheric change and Mistral winds. To determine the variability and drivers of both forcings, multiple 1 year ocean simulations were run, spanning from 1993 to 2013. Two sets of simulations were performed: a control and seasonal set, the first forced by unfiltered atmospheric forcing and the other by filtered forcing. The filtered forcing was bandpass filtered, retaining the seasonal and intraday aspects but removing the high frequency phenomena. Comparing the two sets allows for distinguishing the effects of the high frequency component of the Mistral on the ocean response. During the preconditioning phase, the seasonal forcing was found to be the main destratifying process, removing on average 46% of the stratification needed for deep convection to occur, versus the 28% removed by the Mistral. Despite this, each forcing triggered deep convection in roughly half of the deep-convection events. Sensible and latent heat fluxes were found to be the main drivers of the seasonal forcing during deep-convection years, removing 0.17 and 0.43 m
²s
⁻² of stratification, respectively. They were themselves driven by increased wind speeds, believed to be the low frequency signal of the Mistral, as more Mistral events occur during deep-convection winters (34% vs. 29% of the preconditioning period days). An evolving seasonal forcing in a changing climate may have significant effects on the future deep convection cycle of the western Mediterranean Sea.

Cold temperature extremes, causing damage to industry, agriculture and human health, are often associated with midlatitude cyclones. Here, we quantify the relation between cyclones and cold extremes by highlighting two cyclone-associated dynamical features that may, differently, induce cold extremes. Namely, slantwise-descending dry intrusions (DIs) advect cold air equatorward into the cyclone's cold sector, and upper-tropospheric troughs, diagnosed as cyclonic potential vorticity (PV) anomalies, induce cold anomalies directly below. By objectively-identified DI outflows and cyclonic PV anomalies for 1979-2019 in ERA5 reanalysis, their climatological association with the 5% lowest 2-m temperature is globally quantified. We find that in midlatitudes upper PV anomalies dominate cold extremes, while DIs dominate extremes in the subtropics and even the tropics. The rare overlap between the two features can potentially act as a strong predictor for cold extremes, accompanying ~30% of extremes at 30° latitude with local hotspots of 80%-100%.

Dust events can be hazardous to society and human health and can cost hundreds of millions of dollars each. Inhalable suspended particulate matter with a diameter of under 10 μm (PM10) is of particular concern since it can cause an array of adverse health effects following short- and long-term exposure. Understanding recurring episodes of dust events will enable accurate and timely forecasts, helping mitigate their impacts and allow for better societal protection. Previous expert-based studies highlighted several dust sources and transport pathways into the Eastern Mediterranean region and further identified several weather systems that sustain these transport routes. But since supervised methods, i.e., manual classifications, may have disadvantages, it is often preferred to use unsupervised methods, or systematic classifications. Here, a novel climatological understanding of the link between weather systems and dust transport is achieved by systematically classifying dust events. Using ground PM10 measurements in Israel to objectively identify a climatological set of extreme dust events between 2003 and 2020, we combine atmospheric data from ERA5 and CAMS reanalysis data sets and apply a new, unsupervised, and unbiased method to classify dust events in the Eastern Mediterranean. Six coherent types emerge, corresponding to events governed by shallow and deep Mediterranean cyclones, Mediterranean dipoles, Sharav thermal lows, Arabian anticyclones, and local factors, respectively. Having different seasonality, these classes are insightful in mapping the meteorological conditions and weather systems governing dust emission and transport towards the Eastern Mediterranean. In this context, slantwise-descending dry intrusions are shown to be a key precursor dynamical feature common to the buildup of elevated dust concentrations in three of the clusters of the highest PM10 concentrations.

There is limited understanding of temperature and atmospheric circulation changes that accompany an Atlantic Meridional Overturning Circulation (AMOC) slowdown beyond the North Atlantic realm. A Peqiin Cave (Israel) speleothem dated to the last interglacial period (LIG), 129116 thousand years ago (ka), together with a large modern rainfall monitoring dataset, serve as the base for investigating past AMOC slowdown effects on the Eastern Mediterranean. Here, we reconstruct LIG temperatures and rainfall source using organic proxies (TEX86) and fluid inclusion water d-excess. The TEX86 data show a stepwise cooling from 19.8±0.2° (ca. 128126 ka) to 16.5±0.6°C (ca. 124123 ka), while d-excess values decrease abruptly (ca. 126 ka). The d-excess shift suggests that rainfall was derived from more zonal Mediterranean air flow during the weakened AMOC interval. Decreasing rainfall d-excess trends over the last 25 years raise the question whether similar atmospheric circulation changes are also occurring today.

Large dust storms in the Saharan desert and the subsequent transport of airborne dust over large distances are a major meteorological hazard. Several mechanisms associated to dust emission, occurring on a range of scales, have been previously documented, notably involving Rossby wave breaking and a low-level jet. However, the mechanistic link between the different features and actual dust concentrations has not been coherently established. Here, using a Lagrangian approach, and the conceptual view of extratropical cyclone airstreams, the role of the dry intrusion airstream for translating the influence of the upper-tropospheric Rossby wave perturbation to near-surface flow conductive for the highest dust concentrations is examined. To this end, four large-scale dust storms that were accompanied by dry intrusions in west Africa are studied. Data from the Copernicus Atmospheric Monitoring Service (validated against AERONET station data) are combined with atmospheric data from reanalysis and objectively-identified Lagrangian trajectories of dry intrusions. Common, coherent structures highlighting the role of dry intrusions link the upper-tropospheric Rossby wave breaks with the lower-tropospheric dry and cold jets. These conditions favor dust uplift and transport along an arc-shaped cold front trailing from a Mediterranean cyclone. Consequently, the southwest side of the front is characterized by the highest near-surface dust concentrations ahead of the dry intrusion outflow, where the Intertropical Discontinuity shifts equatorward by 3 to 7°. The northeast part of the front is, however, accompanied by southerly, warm conveyor belt-like flow transporting the dust northward to the Mediterranean, Middle East and/or Europe at mid and upper-tropospheric levels. While case-to-case variations exists, the central role of dry intrusions in all cases calls for systematic investigation of its occurrence as a predictive tool for large dust transport events.

Scientists and stakeholders came to discuss together the most recent research results in understanding processes and impacts of Mediterranean cyclones and train the new generation of scientists.

Persistent dry winter events over the Eastern Mediterranean (EM) disrupt the rainy (winter) season precipitation patterns and dramatically reduce water availability in the region. Here we objectively identify persistent dry, warm winter events over Israel, and apply a Lagrangian approach to three case studies, aiming to understand the relation between the synoptic setting, precursor Rossby waves, and how the dry, warm conditions emerge. Self-organizing map classification of atmospheric profile data over Israel shows that the most persistent (at least 5days) dry and warm winter events are induced by a stagnant upper tropospheric ridge over the EM, pronounced trough or cutoff low over the western/central Mediterranean and blocking over the North Atlantic. The leading mechanisms of the warm and dry conditions are: adiabatic heating during slantwise subsidence, heating by sensible heat fluxes from the surface, and advection of warm and dry continental air. The relative contributions of the mechanisms and geographical locations of the back trajectories vary greatly both within and among the events. In addition, the Atlantic blocking and EM ridge are supported by upstream diabatic heating in warm conveyor belts of North Atlantic cyclones and Mediterranean cyclones, respectively. A quantitative classification of the back trajectories above the Atlantic and the Mediterranean has shown transitions from adiabatic to diabatic contributions and vice versa along these paths. The sequential relation between Atlantic ridge (or block), trough over Europe and ridge over EM and/or west Russia places local persistent warm and dry extremes into a large-scale context and thus provides new opportunities for understanding their predictability at weather and climate scales.

Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on society and ecosystems, is key to supporting future risk reduction and preparedness measures. Here, we provide an overview of the state of the art, knowledge gaps and key open questions in the study of extreme weather events over the vulnerable eastern Mediterranean. This region is situated in a transition zone between subtropical and mid-latitude climates. The large-scale atmospheric circulation and its interaction with regional synoptic systems (i.e., Cyprus Lows, Red Sea Troughs, Persian Troughs, "Sharav"Lows) and high-pressure systems mainly govern extreme weather. Complex orographic features further play an important role in the generation of extreme weather. Most extreme weather events, including heavy precipitation, cold spells, floods and windstorms, are associated with Cyprus Lows or active Red Sea Troughs, whereas heat waves are related with either Persian Troughs and sub-tropical high-pressure systems in summer or the Sharav Low during springtime. In future decades, heat waves and droughts are projected to significantly increase in both frequency and intensity. Changes in heavy precipitation may vary in sign and magnitude depending on the scale, severity and region of interest. There are still relatively large uncertainties concerning the physical understanding and the projected changes of cold spells, windstorms and compound extremes, as these types of events received comparatively little attention in the literature. We further identify knowledge gaps that relate to the societal impacts of extreme weather. These gaps mainly relate to the effects extreme weather may have on mortality, morbidity and infrastructure in the eastern Mediterranean. Research is currently limited in this context, and we recommend strengthening the database of analyzed case studies. We trust that this can only be suitably accomplished by inter-disciplinary and international regional collaboration (in spite of political unrest).

Long-range transport of continental emissions has a far-reaching influence over remote regions, resulting in substantial change in the size, morphology, and composition of the local aerosol population and cloud condensation nuclei (CCN) budget. Here, we investigate the physicochemical properties of atmospheric particles collected on board a research aircraft flown over the Azores during the winter 2018 Aerosol and Cloud Experiment in the Eastern North Atlantic (ACE-ENA) campaign. Particles were collected within the marine boundary layer (MBL) and free troposphere (FT) after long-range atmospheric transport episodes facilitated by dry intrusion (DI) events. Chemical and physical properties of individual particles were investigated using complementary capabilities of computer-controlled scanning electron microscopy and X-ray spectromicroscopy to probe particle external and internal mixing state characteristics. Furthermore, real-time measurements of aerosol size distribution, cloud condensation nuclei (CCN) concentration, and back-trajectory calculations were utilized to help bring into context the findings from offline spectromicroscopy analysis. While carbonaceous particles were found to be the dominant particle type in the region, changes in the percent contribution of organics across the particle population (i.e., external mixing) shifted from 68% to 43% in the MBL and from 92% to 46% in FT samples during DI events. This change in carbonaceous contribution is counterbalanced by the increase in inorganics from 32% to 57% in the MBL and 8% to 55% in FT. The quantification of the organic volume fraction (OVF) of individual particles derived from X-ray spectromicroscopy, which relates to the multi-component internal composition of individual particles, showed a factor of 2.06±0.16 and 1.11±0.04 increase in the MBL and FT, respectively, among DI samples. We show that supplying particle OVF into the κ-Köhler equation can be used as a good approximation of field-measured in situ CCN concentrations. We also report changes in the κ values in the MBL from κMBL, non-DI=0.48 to κMBL, DI=0.41, while changes in the FT result in κFT, non-DI=0.36 to κFT, DI=0.33, which is consistent with enhancements in OVF followed by the DI episodes. Our observations suggest that entrainment of particles from long-range continental sources alters the mixing state population and CCN properties of aerosol in the region. The work presented here provides field observation data that can inform atmospheric models that simulate sources and particle composition in the eastern North Atlantic.

The boundary layer (BL) profile over the coastal plain of Israel, Eastern Mediterranean (EM), varies considerably during winter. Although in the context of air pollution, the characteristics of the BL height (BLH) was intensively investigated, a quantitative classification of the BL profile regimes has not been performed. Here, we seek to reveal the dominant, recurring regimes of the BL profiles, their quantitative characteristics and links to regional synoptic-scale patterns. An objective unsupervised classification of winter BL radiosonde profiles is performed for the first time by multi-parameter self-organizing map (SOM) analysis. The analysis uses high-resolution, 12:00-UTC data of wind, temperature, humidity and pressure measurements during December to February 2007-2018, and yields 30 distinct profile regimes. Composite analysis using ERA5 reanalysis suggests strong association between the profile regimes and synoptic weather systems and highlights four groups: (a) Deep winter cyclones with strong westerly wind and precipitation; (b) Strong surface anticyclones and Red Sea troughs (RST) with a mid-tropospheric ridge, moderate dry easterly wind and extreme temperatures. (c) Moderate pressure gradients under shallow cyclones, anticyclone to the west and RST to the east of Israel. (d) Active RSTs, accompanied by upper-tropospheric trough/cutoff low and heavy precipitation. For the first time, general objective classification observes the active RST without requiring specific criteria. Consistent with previous knowledge, the new classification exhibits distinct categories of thermal stability, BLH and turbulence. Importantly, we show that the automatic objective classification of profile data from a single station can be a sensitive discriminator of winter synoptic regimes in the EM, and therefore explains the variability of the BL profile. It facilitates the study of the interaction between the BL and the free troposphere and may improve the prediction of air pollution or future BL profile regimes based on long time series from historical data or climate models.

The paleo-synoptic conditions that prevailed during the past ∼80 kyr in northeastern China are inferred from the elemental and SrNd isotopic compositions of Lake Sihailongwan Maar sediments. The detrital fraction in the lake sediments is dominated by aeolian input of felsic-rock origin, with little contribution of local volcanic material. Based on the isotopic SrNd composition of the lake core-sediments, we postulate that the deserts of northern China are the main source of allochthonous particles to the lake throughout the past ∼80 kyr. Northwesterly winds associated with the East Asian winter monsoon and high latitude westerlies are the main carriers of dust from these deserts to the lake. The deserts of central China are an additional minor dust source. The episodic dust input from these deserts results from anomalous dry southwesterly winds. These could be related to either El Niño conditions, or to delays in the onset of the East Asian summer monsoon rains.

Cold fronts are a primary feature of the day-to-day variability of weather in the midlatitudes, and feature in conceptual extratropical cyclone models alongside the dry intrusion airstream. Here the climatological frequency and spatial distribution of the co-occurrence of these two features are quantified, and the differences in cold front characteristics (intensity, size, and precipitation) when a dry intrusion is present or not are calculated. Fronts are objectively identified in the ECMWF ERA-Interim dataset for the winter seasons in each hemisphere and split into three sub-types: central fronts (within a cyclone area); trailing fronts (outwith the cyclone area but connected to a central front); and isolated fronts (not connected to a cyclone). These are then associated with dry intrusions identified using Lagrangian trajectory analysis. Trailing fronts are most likely to be associated with a DI in both hemispheres, and this occurs more frequently in the western parts of the major storm track regions. Isolated fronts are linked to DIs more frequently on the eastern ends of the storm tracks, and in the subtropics. All front types, when co-occurring with a DI, are stronger in terms of their temperature gradient, are much larger in area, and typically have higher average precipitation. Therefore, climatologically the link with DIs increases the impact of cold fronts. There are some differences in the statistics of the precipitation for trailing and isolated fronts that are further investigated in Part II of this study from the front-centred perspective.

A new method for identifying high impact large-scale wind and precipitation events in the extended Mediterranean region is outlined and applied to the European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis dataset ERA-Interim for the years 1979-2012. The method highlights large-scale 10 m gust and precipitation events that classify as extreme if integrated over a spatial scale of 1000 km and a temporal scale of 3 days. The method detects successfully high impact events, and reveals clear seasonal differences among the subregions of the Mediterranean. Western Mediterranean precipitation extremes are more intense, and occur mainly in autumn, while eastern Mediterranean events occur in winter. Composite dynamical analyses of large-scale wind and precipitation extremes, and a combination of them, highlight coherent dynamical flow structures associated with the extremes in the different subregions of the Mediterranean. Precipitation events are preceded by an upper-level trough and strong jet on its western flank, followed by cyclogenesis (mainly in the western Mediterranean), and/or a merging of the polar with the subtropical jet over northeastern Africa (in the eastern Mediterranean). Strong surface wind extremes develop around cyclones that intensify south of a deep parent cyclone near the exit of a strong anticyclonically curved jet, propagate eastwards and create a cold and dry northerly wind anomaly at the surface. Furthermore, combined large-scale wind and precipitation extremes often occur simultaneously near cyclones, either North Atlantic cyclones, which project the wind and precipitation into the western Mediterranean, or Mediterranean cyclones. The latter produce wind extremes over a localized area, which often overlaps entirely with the region that receives extreme precipitation.

Tropical plumes (TPs) reflect tropical-extratropical interaction associated with the transport of moisture from the Tropics to extratropical latitudes. They are observed in satellite images as continuous narrow cloud bands ahead of upper-level subtropical troughs at times when the subtropical jet is highly perturbed. Rainstorms usually develop in the exit regions of TPs, so their presence over northern Africa has an impact on the precipitation regime in the southeastern Mediterranean. Based on satellite images and rainfall measurements from Israel, 10 TPs over eastern North Africa between 1988 and 2005 in which considerable rain was recorded were selected. Using the NCEP-NCAR reanalysis data, the structure and evolution of these TPs were characterized and their regional canonical features were identified. A typical TP that occurred in March 1991 is described in detail. The main canonical characteristics are as follows: the TP development is preceded by an incubation period, expressed either as a stationary upper-level trough, persisting 2-6 days, or as two consecutive TP pulses; the preferred location for TP origin is 5°-15°N, 5°W-15°E; the TP is separated from the underlying dry Saharan PBL; the subtropical trough undergoes a phase locking with the lower tropical trough; the cloudiness in the TP-induced rainstorm is mostly stratified with continuous moderate rain, originating from midlevel moisture; and the TP tends to be followed by a midlatitude cyclogenesis over the eastern Mediterranean. This analysis proposes several explanations for the efficiency of the TPs in transporting moisture over a 2000-km distance.