Mediterranean Cyclones Research Articles

GPM-CO observations of Medicane Ianos: Comparative analysis of precipitation structure between development and mature phase

The present work exploits the satellite measurements from the NASA/JAXA Global Precipitation Measurement Core Observatory (GPM-CO) active and passive microwave sensors to analyze the precipitation structure of the most intense Mediterranean Hurricane (Medicane) on record, named Ianos, which swept across the Ionian Sea between 14 and 18 September 2020. Two GPM-CO overpasses, available during Ianos development and tropical-like cyclone (mature) phase, are analyzed in detail. GPM Microwave Imager measurements are used to carry out a comparative analysis of the medicane precipitation structure and microphysics processes between the two phases. The GPM-CO Dual-frequency Precipitation Radar overpass, available for the first time during a medicane mature phase, provides key measurements and products to analyze the 3D precipitation structure in the rainbands, offering further evidence of the main precipitation microphysics processes inferred from the passive microwave measurement analysis. Substantial difference in the rainband precipitation structure is observed, with deeper convection and stronger updraft features during development than at the mature phase, when also shallow precipitation/warm rain processes are observed in the inner region around the medicane eye. These features play a key role to explain the substantial drop in lightning activity during Ianos mature phase. Graupel-ice electrification process is inhibited due to the combined effect of strong horizontal wind and the observed limited growth of graupel. The study demonstrates the value of the GPM-CO not only to characterize Medicane Ianos precipitation structure and microphysics processes with unprecedented detail, but also to provide evidence of its exceptional intensity, tropical-like characteristics during its mature phase, and its similarities with tropical cyclones.

Multifractal fluctuations of the precipitation in Spain (1960–2019)

In this work, an analysis of multifractal parameters of daily precipitation series over the Iberian Peninsula was performed in two 30-year periods to explore whether these properties follow any pattern. Fluctuations of precipitation series show three different scaling regions. Only two distinct regimes for small and large timescales can be confirmed, while intermediate scales are part of a transition region. It is also observed a certain degree of multifractality, which is higher for small timescales. At these scales, there is a high persistence which follows the spatial gradient of the annual precipitation. Moreover, multifractal parameters of the precipitation are modified according to complex spatial and temporal patterns. Only persistence uniformly decreases in the last period. Other relevant findings are the changes in the asymmetry of multifractal spectra in the eastern belt at larger timescales, which might be related to the change in the behavior of the Mediterranean cyclones.

Mediterranean cyclones: current knowledge and open questions on dynamics, prediction, climatology and impacts

Abstract. A large number of intense cyclones occur every year in the Mediterranean basin, one of the climate change hotspots. Producing a broad range of severe socio-economic and environmental impacts in such a densely populated region, Mediterranean cyclones call for coordinated and interdisciplinary research efforts. This article aims at supporting these efforts by reviewing the status of knowledge in the broad field of Mediterranean cyclones. First, we focus on the climatology of Mediterranean cyclone tracks, their relationship to large-scale atmospheric circulation and their future trends. Second, we discuss the dynamics and atmospheric processes that govern the genesis and development of Mediterranean cyclones. Then, we present the different subtypes of Mediterranean cyclones, devoting special attention to medicanes, i.e. cyclones with tropical characteristics and subjects of numerous recent studies. In a subsequent section, we review the state of the art in forecasting cyclones and relevant high-impact weather, and we discuss in detail the challenges and recent efforts to increase their forecast skill. Finally, we discuss the main impacts produced by cyclones, namely heavy precipitation, windstorms, dust transport, storm surges and sea wave extremes. In the last section of this review article, we thoroughly outline the future directions of research that would advance the broader field of Mediterranean cyclones.

Flood characterization based on forensic analysis of bridge collapse using UAV reconnaissance and CFD simulations

Flash floods are common manifestations of extreme weather events and one of the most severe natural hazards. In Europe, they have been responsible for 359 fatalities and an economic loss totalling 67 million USD in the past decade (EM-DAT), while their increasing severity is linked to climate change. Nevertheless, flash floods remain a poorly documented natural phenomenon due to the lack of flow intensity data in many of the affected watersheds. Based on a thorough field investigation, including UAV-based 3D mapping and material characterization with on-site testing, we carry out a numerical study of a notable flood that caused the collapse of bridges and buildings in Central Greece, following a recent Mediterranean hurricane. Focusing on a carefully selected case study, we combine 3D modelling of flow–structure interaction with detailed mechanical modelling of the nonlinear structural response to reproduce the flood-induced fracture of a bridge abutment. Back-analysis of this failure responds to the fundamental problem of estimating the undocumented magnitude of this extreme event. The paper estimates a lower bound value of the flow velocity at the studied location. This can be valuable input for the interpretation of the extensive damage that took place downstream and for the re-assessment of flood risk in a region where similar events are expected to become more frequent because of climate change. The approach, where disaster forensics and engineering analysis are used to fill the gap of missing real-time measurements, can be implemented for the a posteriori estimation of flood intensity in similar events. The well-documented case study of a bridge failure due to extreme flooding can also be used for validation of future numerical and experimental methods and motivate investigations of the mechanisms governing flow–soil–structure interaction in river crossings.

Coastal Hazards of Tropical‐Like Cyclones Over the Mediterranean Sea

AbstractMedicanes, for Mediterranean hurricanes, are mesoscale cyclones with morphological and physical characteristics similar to tropical cyclones. Although less intense, smaller, and rarer than their Atlantic counterparts, medicanes are very hazardous events threatening islands and continental coasts within the Mediterranean Sea. The latest strong episode, Medicane Ianos (September 2020), resulted in severe damages in Greece and several casualties. This work investigates the oceanic response to these extreme events along the Mediterranean coasts under present‐day and future (late 21st century) climate conditions. To this end, a coupled hydrodynamic‐wave model is used to simulate both storm surges and wind‐waves generation and propagation in the Mediterranean Sea at high resolution (∼2 km) along the coastlines. A data set of thousands of medicanes synthetically generated from 20 global climate models and two atmospheric reanalyses is used to derive the atmospheric forcing fields. Regional coastal hazards assessment is performed for the present and future climates. For the first period, highest medicane‐induced waves are found in the central and the southwest part of the western Mediterranean, while greatest storm surges are found in the Adriatic Sea and regions characterized by wide and gently sloping continental shelves. Results obtained for future changes show amplitudes generally smaller than the associated uncertainty due to limited agreement among models (especially for coastal elevation). Though, model consensus is reached (60–75%) and relative intensity change is significant (10–20%) at some locations (e.g., 1 m increase of medicane‐induced significant wave height on average for south coasts of Sicilia).

Investigating the Siberian High and its Effects on the Mediterranean Cyclones During 1970-2020 Period

Investigating the Siberian High and its Effects on the Mediterranean Cyclones During 1970-2020 Period

Meteorological situations that generated exceptional discharges along the Danube River. (Case study- the situation of April 2006).

For Europe, the undisputed importance of the Danube can be rendered by some general data: Its hydrographic basin surface exceeds 817000 km2, i.e. about 10% of that of the continent, its length is 2857 km and its mean multiannual discharge is about 6500 m3/s, thus ranking second to Volga river. Romania is the country with the largest surface situated within Danube’s basin (97.4%), representing 29% of Danube’s hydrographic basin. The water resources of the Danube in Bazias section amount to 173 billion m3, 30 billion m3 of which are technically usable resources.Our analysis aimed at determining those complex meteorological situations at the European continent level that triggered exceptional discharges along the Danube river, resulting in severe flooding, causing in turn heavy damages, fatalities, population evacuations and considerable rehabilitation costs. A complex analysis was performed, of statistical-synoptic type and those complex meteorological situations were identified that determined the occurrence of such disasters. Discharges and levels of the Danube were used along the whole measuring period, data from the archive of the National Meteorological Administration, and data, map and image archives from Wetterzentrale (Kartenarchiv, NCEP, NCAR, AVN etc.). The complex meteorological situations at the level of the European continent that triggered exceptional discharges along the Danube correlate with intense cyclonic activity, of both the Icelandic and the Mediterranean cyclones, with the negative phase of the North-Atlantic Oscillation and with decreasing or minimum solar activity (according to data from NOAA's Space Environment Center). The most disastrous floods occurred in the spring of 2006. The paper is important for meteorologists, in their weather forecasting activity, for hydrologists, in their hydrological forecasting and for the institutions involvedin flood management.

Future projections of Mediterranean cyclone characteristics using the Med-CORDEX ensemble of coupled regional climate system models

Here, we analyze future projections of cyclone activity in the Mediterranean region at the end of the twenty-first century based on an ensemble of state-of-the-art fully-coupled Regional Climate System Models (RCSMs) from the Med-CORDEX initiative under the Representative Concentration Pathway (RCP) 8.5. Despite some noticeable biases, all the RCSMs capture spatial patterns and cyclone activity key characteristics in the region and thus all of them can be considered as plausible representations of the future evolution of Mediterranean cyclones. In general, the RCSMs show at the end of the twenty-first century a decrease in the number and an overall weakening of cyclones moving across the Mediterranean. Five out of seven RCSMs simulate also a decrease of the mean size of the systems. Moreover, in agreement with what already observed in CMIP5 projections for the area, the models suggest an increase in the Central part of the Mediterranean region and a decrease in the South-eastern part of the region in the cyclone-related wind speed and precipitation rate. These rather two opposite tendencies observed in the precipitation should compensate and amplify, respectively, the effect of the overall reduction of the frequency of cyclones on the water budget over the Central and South-eastern part of the region. A pronounced inter-model spread among the RCSMs emerges for the projected changes in the cyclone adjusted deepening rate, seasonal cycle occurrence and associated precipitation and wind patterns over some areas of the basin such as Ionian Sea and Iberian Peninsula. The differences observed appear to be determined by the driving Global Circulation Model (GCM) and influenced by the RCSM physics and internal variability. These results point to the importance of (1) better characterizing the range of plausible futures by relying on ensembles of models that explore well the existing diversity of GCMs and RCSMs as well as the climate natural variability and (2) better understanding the driving mechanisms of the future evolution of Mediterranean cyclones properties.

Analysis of the Transition of an Explosive Cyclone to a Mediterranean Tropical-like Cyclone

This study investigates the dynamics of the development phases of a Mediterranean tropical-like cyclone (medicane) in the southern Ionian Sea, on 28 September 2018 that caused high impact phenomena in the central and eastern Mediterranean, focusing on the transition from explosive cyclone to medicane. The symmetry and the warm core structure of the system have been demonstrated via phase space diagrams determining three phases of the system development that are then supported on a dynamical basis. During the first phase of the system, baroclinic instability triggered the formation of the explosive cyclone, when strong upper-level PV anomalies at the dynamic tropopause level moved towards a pre-existed area of enhanced low-level baroclinicity over the coastal areas of Libya along with positive SST anomalies. The surface frontal structure was enhanced under the influence of the upper-level dynamic processes. During the second phase when the medicane formed, low-level diabatic processes determined the evolution of the surface cyclone, without any significant support from baroclinic processes in the upper troposphere. The distortion of the low-level baroclinicity and the frontal structure began after the initial weakening of the upper-level dynamics. During the third phase, the system remained barotropic, being affected by similar mechanisms as in the second phase but with lower intensity. The transition mechanism is not only the result of the seclusion of warm air in the cyclone core but, mainly, the continuation of an explosive cyclone or an intense cyclone when the occlusion began to form.

WIND SHEAR ASSOCIATED WITH MEDITERRANEAN CYCLONES ACTIVITY AND THE IMPACT ON FLIGHT SAFETY DURING WINTER OPERATIONS AT HENRI COANDA OTOPENI AND AUREL VLAICU BANEASA AIRPORTS

The regulated airspace in which aircraft flights take place is part of the Earth's atmosphere. At the same time, the same airspace is the seat of meteorological process and phenomena that have no borders and whose activity is not regulated, but whose evolution in time and behavior is governed by their own laws. This study presents, in a descriptive manner, low level dangerous weather conditions associated with wind shear also called the invisible killer. The phenomenon can occur locally, extremely rarely (3-4 times per year) during winter operations, mainly in January, under the activity of Mediterranean cyclones and its uniqueness consists in duration and intensity. To highlight the impact on flight safety in winter operations, especially in the current context of global warming, the reference and analysis periods applicable to this study are indissolubly reduced to days and minutes. When we talk about flight safety, the immediate application of corrective actions by pilots, the reference period is indissolubly reduced to seconds, those seconds that can make the difference between life and death

Temporal Variability and Predictability of Intense Cyclones in the Western and Eastern Mediterranean

Our understanding of the time variability of intense cyclones in the Mediterranean region is still lacking despite its importance for the long-term forecast of climate anomalies. This study examines the month-to-month variability and predictability of cyclones, the intensity of which exceeded the 75th percentile (intense cyclones) and the 95th percentile (extreme cyclones), over the Western and Eastern Mediterranean. The locations of cyclones were obtained by applying the method of M. Yu. Bardin on the 6-hourly 1000 hPa geopotential height data from the NCEP/NCAR reanalysis for the period 1951–2017 (67 years). It was shown that annual frequencies of cyclones were higher in the Western Mediterranean due to the contribution of spring and autumn; monthly averages were higher in the Eastern Mediterranean in December/January–March for intense/extreme cyclones. In the context of global warming, no linear trends significant at the 90% confidence level were found in the variability of intense and extreme cyclones, except for a positive trend in autumn extreme cyclones over the Eastern Mediterranean. The time series of cyclones in both parts of the Mediterranean were characterized by a pronounced interannual variability with a noticeable decadal modulation. According to spectral analysis, these interannual periods were multiples of 2–3 years corresponding to the main global teleconnection patterns. Seasonally, the most energy was concentrated in winter spectra; spring and autumn spectra had lower comparable magnitudes. The correlation analysis between the frequency of cyclones and the indices of the main atmospheric patterns showed that the main synchronous patterns for intense and extreme Mediterranean cyclones in September–April were the Mediterranean Oscillation (with the opposite signs for the Western and Eastern Mediterranean), Scandinavia pattern (positive correlation), and East Atlantic Oscillation (negative correlation). Additional important synchronous teleconnection patterns for some months were the Arctic Oscillation and East Atlantic/West Russia pattern for the Western Mediterranean, and the Polar/Eurasia pattern and Tropical Northern Hemisphere pattern for the Eastern Mediterranean. The outcome of this paper was the use of an artificial neural network model with inputs of global teleconnection indices both in the atmosphere and ocean to describe the temporal variability of the frequency of intense cyclones in the Western and Eastern Mediterranean. The predictability of intense cyclones was shown with the possibility of forecasts with a lead time of 0, 2, 4, and 6 months for the Western Mediterranean in October, January, February, April, and May, and for the Eastern Mediterranean in January, February, March, April, and May. One of the applications of this model may be in forecasting the evolution of the monthly frequency of cyclones with a lead time of 2 to 6 months.

Influence of sea salt aerosols on the development of Mediterranean tropical-like cyclones

Abstract. Medicanes are mesoscale tropical-like cyclones that develop in the Mediterranean basin and represent a great hazard for the coastal population. The skill to accurately simulate them is of utmost importance to prevent economical and personal damage. Medicanes are fueled by the latent heat released in the condensation process associated with convective activity, which is regulated by the presence and activation of cloud condensation nuclei, mainly originating from sea salt aerosols (SSAs) for marine environments. Henceforth, the purpose of this contribution is twofold: assessing the effects of an interactive calculation of SSA on the strengthening and persistence of medicanes, and providing insight into the casuistry and sensitivities around their simulation processes. To this end, a set of simulations have been conducted with a chemistry–meteorology coupled model considering prescribed aerosol (PA) and interactive aerosol (IA) concentrations. The results indicate that IA produces longer-lasting and more intense medicanes. Further, the role of the initialization time and nudging strategies for medicane simulations has been explored. Overall, the results suggest that (1) the application of spectral nudging dampens the effects of IA, (2) the initialization time introduces a strong variability in the storm dynamics, and (3) wind–SSA feedback is crucial and should be considered when studying medicanes.

A process-based anatomy of Mediterranean cyclones: from baroclinic lows to tropical-like systems

Abstract. In this study, we address the question of the atmospheric processes that turn Mediterranean cyclones into severe storms. Our approach applies online potential vorticity (PV) budget diagnostics and piecewise PV inversion to WRF model simulations of the mature stage of 100 intense Mediterranean cyclones. We quantify the relative contributions of different processes to cyclone development and therefore deliver, for the first time, a comprehensive insight into the variety of cyclonic systems that develop in the Mediterranean from the perspective of cyclone dynamics. In particular, we show that all 100 cyclones are systematically influenced by two main PV anomalies: a major anomaly in the upper troposphere, related to the baroclinic forcing of cyclone development, and a minor anomaly in the lower troposphere, related to diabatic processes and momentum forcing of wind. Among the diabatic processes, latent heat is shown to act as the main PV source (reinforcing cyclones), being partly balanced by PV sinks of temperature diffusion and radiative cooling (weakening cyclones). Momentum forcing is shown to have an ambiguous feedback, able to reinforce and weaken cyclones while in certain cases playing an important role in cyclone development. Piecewise PV inversion shows that most cyclones develop due to the combined effect of both baroclinic and diabatic forcing, i.e. due to both PV anomalies. However, the stronger the baroclinic forcing, the less a cyclone is found to develop due to diabatic processes. Several pairs of exemplary cases are used to illustrate the variety of contributions of atmospheric processes to the development of Mediterranean cyclones: (i) cases where both baroclinic and diabatic processes contribute to cyclone development; (ii) cases that mainly developed due to latent-heat release; (iii) cases developing in the wake of the Alps; and (iv) two unusual cases, one where momentum forcing dominates cyclone development and the other presenting a dual-surface pressure centre. Finally, we focus on 10 medicane cases (i.e. tropical-like cyclones). In contrast to their tropical counterparts – but in accordance with most intense Mediterranean cyclones – most medicanes are shown to develop under the influence of both baroclinic and diabatic processes. In discussion of medicane-driving processes, we highlight the need for a physical definition of these systems.

Orographic Effect on Extreme Precipitation Statistics Peaks at Hourly Time Scales

AbstractOrographic impact on extreme subdaily precipitation is critical for risk management but remains insufficiently understood due to complicated atmosphere‐orography interactions and large uncertainties. We investigate the problem adopting a framework able to reduce uncertainties and isolate the systematic interaction of Mediterranean cyclones with a regular orographic barrier. The average decrease with elevation reported for hourly extremes is found enhanced at subhourly durations. Tail heaviness of 10‐min intensities is negligibly affected by orography, suggesting self‐similarity of the distributions at the convective scale. Orography decreases the tail heaviness at longer durations, with a maximum impact around hourly scales. These observations are explained by an orographically induced redistribution of precipitation toward stratiform‐like processes, and by the succession of convective cores in multihour extremes. Our results imply a breaking of scale‐invariance at subhourly durations, with important implications for natural hazards management in mountainous areas.

Oscillations in deep-open-cells during winter Mediterranean cyclones

Open cloud cells can be described in ideal form as connected clouds that surround spots of isolated clear skies in their centers. This cloud pattern is typically associated with marine stratocumulus (MSc) that form in the oceanic boundary layer. However, it can form in deeper convective clouds as well. Here, we focus on deep-open-cells (with tops reaching up to ~5–7 km) that form in the post-frontal regions of winter Mediterranean cyclones, and examine their properties and evolution. Using a Lagrangian analysis of satellite data, we show that deep-open-cells have a larger equivalent diameter (~58 ± 18 km) and oscillate with a longer periodicity (~3.5 ± 1 h) compared to shallow MSc. A numerical simulation of one Cyprus low event reveals that precipitation-generated convergence and divergence dynamic patterns are the main driver of the open cells’ organization and oscillations. Thus, our findings generalize the mechanism attributed to the behavior of shallow marine cells to deeper convective systems.

Initialization of WRF Model Simulations With Sentinel-1 Wind Speed for Severe Weather Events

The model initialization with high-resolution SAR wind data provided by the Sentinel-1 mission and its impact on the meteorological model WRF-ARW simulations is discussed. The activity is performed within the Horizon 2020 CEASELESS project, focusing on one of the target areas, the northern Adriatic Sea (northern-central Mediterranean). The Sentinel-1 SAR wind is ingested into LAPS, a numerical system developed at NOAA, specifically designed for data analysis and nowcasting issues, since it has the advantage of being faster and less computational demanding than advanced data assimilation methods. Here, LAPS analyses are used to perform a smarter initialization of the WRF-ARW model simulations than using simply global model fields. The impact of the Sentinel-1 SAR wind on the model simulations is evaluated for twenty cases, ranging through several atmospheric conditions occurring in different seasons of the years 2014–2018. For each case study, a reference WRF-ARW simulation is forced with GFS analysis and forecasts used as initial and boundary conditions, respectively. Additional model runs are initialized with the LAPS analyses, which include the information of Sentinel-1 SAR wind, METAR data and the SEVIRI/MSG (Eumetsat) brightness temperature. A statistical evaluation of the WRF-ARW simulations is performed versus an independent set of surface records, provided by the Friuli Venezia Giulia regional station network (northeastern Italy), and METAR data. The comparison is performed for 10 m wind, 2 m air and dew point temperature. The results show a positive, albeit modest, impact on the WRF model simulations initialized with the LAPS analyses. The initialization with the Sentinel-1 SAR wind show benefits for all surface variables. Finally, a Mediterranean tropical-like cyclone (Medicane), occurred in the Ionian Sea in November 2017, is considered in order to show how the use of Sentinel wind data can contribute to a better analysis and simulation of severe weather episodes in the Mediterranean. The improvement in the simulation of the pressure minimum location is remarkable.

Impact of radar data assimilation on simulations of precipitable water with the Harmonie model: A case study over Cyprus

Understanding the mesoscale spatial and temporal variability of precipitation (of the order of 1 to 100 km, and of minutes to hours, respectively) is essential for larger-scale studies, especially in highly heterogeneous areas, such as coastal regions, mountains and urban areas. The adequate capturing of the precipitation patterns in both diagnostic and prognostic terms remains an open challenge. On the one hand, commonly used ground measuring devices, such as rain gauges, disdrometers and other point instruments, exhibit an ineffective sampling for the whole range of scales and areas covered with precipitation. On the other hand, weather radars provide a much greater spatial and temporal resolution which, however, must be exploited with care in rain rate estimations, due to complicated and non-unique transfer functions used in the processing procedures. This study aims at bridging, to at least some extent, this scale gap and to demonstrate the role of fine-scale precipitation estimates derived from weather radar data, and to illustrate their impacts on larger scale weather patterns. The focus is on the precipitable water content and its changes during a Mediterranean cyclone evolution.In this paper, the first results with the regional radar signal processing chain that provides the radar data assimilation (RDA) in the Harmonie convection permitting numerical model are described. The numerical experiments exhibit effective simulation of the precipitable water at different stages of the frontal depression which affected Cyprus in the Eastern Mediterranean, during 31 December 2018–2 January 2019. In particular, it was found that the major volume of precipitable water was related to the cyclone frontal zone (which is in turn related to heavier precipitation), followed by a weaker field of precipitable water behind the frontal zone (and weaker precipitation). During the whole severe weather period, the precipitable water minimum was maintained over the island's central mountain range, reflecting the importance of interactions and feedbacks between the air flow and orography.The radar data assimilation influences the results of the simulations, depending on the particular area and cyclone stage. The largest innovations due to RDA occur with the approach of the cyclone's frontal zone. Enhancement of precipitable water due to RDA is observed over most of the whole area, but this is more pronounced in weaker precipitation locations. The impact is associated with a shift of the precipitable water maximum towards higher values in the precipitable water distribution, which is accompanied by enlarged sizes of areas with greater precipitable water. During the cyclone's mature stage and at the rear zone, the integrated RDA impact over the whole domain is almost negligible but takes the form of mesoscale cells of opposite signs; at this stage, radar measurements yield an alteration of both the positions and rates of spatial precipitable water fields, which result in intermittent mesoscale changes.

Modelling a tropical-like cyclone in the Mediterranean Sea under present and warmer climate

Abstract. This study focuses on a single Mediterranean hurricane (hereafter medicane), to investigate its response to global warming during the middle of the 21st century and assesses the effects of a warmer ocean and a warmer atmosphere on its development. Our investigation uses the state-of-the-art regional climate model WRF to produce the six-member, multi-physics ensembles. Results show that our model setup simulates a realistic cyclone track and the transition from an initial disturbance to a tropical-like cyclone with a deep warm core. However, the simulated transition occurs earlier than for the observed medicane. The response of the medicane to future climate change is investigated with a pseudo global warming (PGW) approach. This is the first application of the PGW framework to medicanes. The PGW approach adds a climate change delta (defined as difference between future and present climate) to WRF's boundary conditions which is obtained for all prognostic variables using the mean change in an ensemble of CMIP5 simulations. A PGW simulation where the climate change delta is added to all prognostic variables (PGWALL) shows that most of the medicane characteristics moderately intensify, e.g. surface wind speed, uptake of water vapour, and precipitation. However, the minimum sea level pressure (SLP) is almost identical to that under present climate conditions. Two additional PGW simulations were undertaken; One simulation adds the projected change in sea surface and skin temperature only (PGWSST) while the second simulation adds the PGW changes to only atmospheric variables (PGWATMS); i.e. we use present-day sea surface temperatures. These simulations show opposing responses of the medicane. In PGWSST, the medicane is more intense than PGWALL as indicated by lower SLP values, the stronger surface wind, and the more intense evaporation and precipitation. In contrast, the medicane in PGWATMS still transitions into a tropical-like cyclone with a deep warm core, but the PGWATMS medicane weakens considerably (SLP, surface wind, and rainfall decrease). This difference can be explained by an increase in water vapour driven by the warmer ocean surface (favourable for cumulus convection). The warmer and drier atmosphere in PGWATMS tends to inhibit condensation (unfavourable for cumulus convection). The warmer ocean and warmer atmosphere have counteracting effects which leads to only a modest enhancement of the medicane by global warming. The novel approach in this study provides new insights into the different roles of warming of the ocean and atmosphere in medicane development.

Investigating Changes of the Siberian High During 1970-2020 Period and its Effects on the Mediterranean Cyclones

Investigating Changes of the Siberian High During 1970-2020 Period and its Effects on the Mediterranean Cyclones

Role of moist and dry air advection in the development of Mediterranean tropical‐like cyclones (medicanes)

AbstractTwo tropical‐like cyclones in the Mediterranean Sea, aka medicanes, are analysed herein by means of numerical simulations. The cyclones, which were recently investigated in Miglietta and Rotunno, Quarterly Journal of the Royal Meteorological Society, 2019, 145, are reconsidered in the present study, in which we focus on their respective preconditioning phases. In particular, here we analyse how local evaporation and/or long‐range transport of air masses may precondition the environment where the Mediterranean cyclones form. Numerical simulations indicate a rather different behaviour for the two cyclones. The first medicane (December 13–15, 2005) developed over the southern Mediterranean Sea, in a region of high low‐level humidity content, which existed before the cyclone formation; the air, originally dry over the eastern Balkans and the African mainland, gained humidity during its transit over the sea surface. In contrast, the second medicane (October 6–10, 1996) strongly intensified while benefitting from the intense sea‐surface fluxes due to the outbreak of Tramontane and Cierzo winds near the Balearic Islands, where the cyclone developed. Although limited to just two case studies, these results identify two different mechanisms conducive to an environment favourable for the intensification of medicanes in the western or southern Mediterranean. In addition, the role of dry air near the cyclone core, associated with upper‐tropospheric dry intrusions, is analysed. Sensitivity experiments were performed, constraining the relative humidity in the initial and boundary conditions to values above 50%. The advantage of this strategy is the ability to change the humidity content in the dry regions without modifying the associated strong potential vorticity anomalies. For both cases, we find that the humidity increase had the effect of inducing an earlier onset of cyclone development and of producing stronger, longer‐lasting vortices.