Surface compound extremes and Mediterranean cyclones

The co-occurrence of extreme events, resulting generally in stronger surface impacts compared to isolated extreme events, constututes a hazard for the public and can be associated with severe damage to buildings, infrastructure and means of transport. Within the context of the MedCyclone initiative (see e. g. the Mediterranean cyclone track dataset defined in [1]), we investigate the "impact area" of the cyclones crossing the Mediterranean region by linking their passage with the appearance of hazards at the surface, in particular extremes of accumulated precipitation, near surface wind intensity, height of combined swell and wave. This analysis is expected to produce an estimate of the fraction of compound extremes - combinations of the extremes listed above - associated with the MedCyclones tracks in the recent climatological period, including geographical and seasonal details. The large-scale characteristics of the cyclones are taken into account by using a classification based on their upper-level potential vorticity pattern, and eventually by considering dynamical features with strong impacts on surface weather, such as cold fronts, warm conveyor belts and dry intrusions

Extreme weather and societal impacts in the eastern Mediterranean

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)

Size-dependent ice nucleation by airborne particles during dust events in the eastern Mediterranean

Reicher N, Budke C, Eickhoff L, et al. Size-dependent ice nucleation by airborne particles during dust events in the eastern Mediterranean. Atmospheric Chemistry and Physics (ACP) . 2019;19(17):11143-11158.The prediction of cloud ice formation in climate models remains a challenge, partly due to the complexity of ice-related processes. Mineral dust is a prominent aerosol in the troposphere and is an important contributor to ice nucleation in mixed-phase clouds, as dust can initiate ice heterogeneously at relatively low supercooling conditions. We characterized the ice nucleation properties of size-segregated mineral dust sampled during dust events in the eastern Mediterranean. The sampling site allowed us to compare the properties of airborne dust from several sources with diverse mineralogy that passed over different atmospheric paths. We focused on particles with six size classes determined by the Micro-Orifice Uniform Deposit Impactor ( MOUDI) cutoff sizes: 5.6, 3.2, 1.8, 1.0, 0.6 and 0.3 mu m. Ice nucleation experiments were conducted in the Weizmann Supercooled Droplets Observation on a Microarray (WIS-DOM) setup, whereby the particles are immersed in nanoliter droplets using a microfluidics technique. We observed that the activity of airborne particles depended on their size class; supermicron and submicron particles had different activities, possibly due to different composition. The concentrations of ice-nucleating particles and the density of active sites (n(s)) increased with the particle size and particle concentration. The supermicron particles in different dust events showed similar activity, which may indicate that freezing was dominated by common mineralogical components. Combining recent data of airborne mineral dust, we show that current predictions, which are based on surface-sampled natural dust or standard mineral dust, overestimate the activity of airborne dust, especially for the submicron class. Therefore, we suggest in-cluding information on particle size in order to increase the accuracy of ice formation modeling and thus weather and climate predictions

A dynamical link between deep Atlantic extratropical cyclones and intense Mediterranean cyclones

Breaking of atmospheric Rossby waves has been previously shown to lead to intense Mediterranean cyclones, one of the most prominent environmental risks in the region. Wave breaking may be enhanced by warm conveyor belts (WCBs) associated with extratropical cyclones developing over the Atlantic Ocean. More precisely, WCBs supply the upper troposphere with air masses of low potential vorticity that, in turn, amplify ridges and thus favor Rossby wave breaking. This study identifies the mechanism that connects Atlantic cyclones and intense mature Mediterranean cyclones through ridge amplification by WCBs, and validates its climatological relevance. Using European Centre for Medium‐Range Weather Forecasts (ECMWF) ERA‐Interim reanalyses and a feature‐based approach, we analyze the 200 most intense Mediterranean cyclones for the years 1989–2008 and show that their majority (181 cases) is indeed associated with this mechanism upstream. Results show that multiple Atlantic cyclones are associated with each case of intense Mediterranean cyclone downstream. Moreover, the associated Atlantic cyclones are particularly deeply intensifying compared with climatology

Enhancement of Indian summer monsoon rainfall by cross-equatorial dry intrusions

Abstract The Indian summer monsoon affects the lives of over 1/6 of the world’s population. Precipitation extremes during summer monsoons have dire socioeconomic impacts. Yet, the mechanisms leading to these extremes are poorly understood, making their accurate forecasts and reliable future projections a longstanding challenge. Using a Lagrangian-based method, we show that precipitation extremes link to dry air intrusions from the southern midlatitudes upper troposphere, crossing the equator, and reaching the Arabian Sea. By triggering intense ocean evaporation, these dry intrusions are associated with modulated moisture transport patterns toward India and enhanced precipitation by >17% on average, often embedding local extremes. A notable example is the excessive rain that caused the devastating Kerala flood of 2018. However, depending on the wind pattern, these dry intrusions may, in some cases, decrease rainfall over land. The emerging connection of rainfall variability with midlatitude weather systems opens opportunities for improving the forecast of precipitation extremes and understanding their future projections

Linking compound weather extremes to Mediterranean cyclones, fronts and air streams

Mediterranean cyclones are the primary driver of many types of surface weather extremes in the Mediterranean region, the association with extreme rainfall being the most established. Although smaller in size compared to Atlantic cyclones, they share a similar synoptic structure organised in distinct air streams, such as the warm conveyor belt and the dry intrusion, and are associated with low-level temperature fronts. The large-scale characteristics of a Mediterranean cyclone, the properties of the associated airflows, the interaction with the topography around the Mediterranean basin, and the season of occurrence, all contribute in determining its surface impacts. Here, we take these factors into account to establish statistical links between mediterranean cyclones and weather compounds of two types, namely co-occurring rain–wind and wave–wind extremes. Specifically, compound extremes are attributed to a cyclone if they fall within the system’s impact area, using a definition that is expressly tested on Mediterranean cyclones and on the compound selection. Our results show that the majority of Mediterranean compound rain–wind and wave–wind extremes occur in the neighbourhood of a Mediterranean cyclone, with peaks exceeding 80%; the proportion of cyclone-related compounds is highest when considering transition seasons, and rain–wind events. Winter cyclones show highest compound frequency, matching with the peak winter occurrence of distinctively baroclinic cyclones. A novelty of this work, the de-construction of the cyclones’ impact areas based on the presence of objectively-identified air streams and fronts, reveals a high incidence of both types of compound extremes below warm conveyor belt ascent regions, of wave-wind below dry intrusions

Dynamics of a Puelche foehn event in the Andes

In this numerical modelling study, we investigate a Puelche foehn event (25–26 March 2014) in the southern Andes – a region with sparse observations. The synoptic environment as well as the mesoscale structure and the dynamics of the easterly wind are examined with European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and a simulation with the mesoscale non-hydrostatic limited-area weather prediction model COSMO with a grid spacing of 2.2 km.The large-scale synoptic situation leading to this Puelche event is characterized by a mid-tropospheric cut-off low above the mountain range, the formation of a coastal surface low, as well as high pressure extending over the southern Andes. Easterly winds extend throughout the entire troposphere, indicative of a deep foehn flow. In the free troposphere, the easterlies are geostrophically balanced and develop in association with increasing pressure to the south. In contrast, within the planetary boundary layer, the easterly winds occur predominantly due to an increasing cross-range large-scale pressure gradient with only a weak geostrophic component. Kinematic trajectories indicate that a significant part of the Puelche air mass originates from above an inversion on the upstream side of the Andes. Some air parcels, however, ascend on the upstream side to crest height as the boundary layer deepens during daytime and/or flow through gaps across the mountain range. Hence, this Puelche event shares characteristics of both a blocked and a non-blocked foehn type.ISSN:0941-2948ISSN:1610-122

A storm-relative climatology of compound hazards in Mediterranean cyclones

Cyclones are responsible for much of the weather damage in the Mediterranean region, and while their association with individual weather hazards is well understood, their association with impactful multivariate compound hazards remains to be quantified. This study aims to establish a storm-relative climatology of three different multivariate hazards in Mediterranean cyclones. Namely, the co-occurrences of rain and wind, rain and wave, and particulate matter and heat are composited relative to storm centers. Composites are computed for various large-scale environments using a recent cyclone classification, which shows that few different large scale configurations lead to each compound event type. Compound rain and wind events are mostly associated with frontal cyclones and cyclones induced by anticyclonic Rossby wave breaking from late fall to early spring in the northern Mediterranean. Compound rain and wave events occur at similar times and locations, but are also associated with cyclonic Rossby wave breaking. Particulate matter and heat compound events are associated with heat lows, daughter cyclones and anticyclonic Rossby wave breaking in the warm season and over North-Africa. Next, we find that the probability of compounding associated with a cyclone class does not depend monotonically on the probabilities of the individual contributing hazards, but also on the goodness of their temporal and spatial correspondence. Finally, we find warm conveyor belts and cold fronts to frequently co-occur with rain and wind, and rain and wave events, while particulate matter and heat events are not strongly associated with dynamical features. These results, which systematically associate various large-scale environments and dynamical features to different compound event types, have implications for forecasting and climate risk predictions

The dynamical structure of intense Mediterranean cyclones

This paper presents and analyzes the three-dimensional dynamical structure of intense Mediterranean cyclones. The analysis is based on a composite approach of the 200 most intense cyclones during the period 1989-2008 that have been identified and tracked using the output of a coupled ocean-atmosphere regional simulation with 20km horizontal grid spacing and 3-hourly output. It is shown that the most intense Mediterranean cyclones have a common baroclinic life cycle with a potential vorticity (PV) streamer associated with an upper-level cyclonic Rossby wave breaking, which precedes cyclogenesis in the region and triggers baroclinic instability. It is argued that this common baroclinic life cycle is due to the strongly horizontally sheared environment in the Mediterranean basin, on the poleward flank of the quasi-persistent subtropical jet. The composite life cycle of the cyclones is further analyzed considering the evolution of key atmospheric elements as potential temperature and PV, as well as the cyclones' thermodynamic profiles and rainfall. It is shown that most intense Mediterranean cyclones are associated with warm conveyor belts and dry air intrusions, similar to those of other strong extratropical cyclones, but of rather small scale. Before cyclones reach their mature stage, the streamer's role is crucial to advect moist and warm air towards the cyclones center. These dynamical characteristics, typical for very intense extratropical cyclones in the main storm track regions, are also valid for these Mediterranean cases that have features that are visually similar to tropical cyclones