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 5 days) dry and warm winter events are induced by a stagnant upper tropospheric ridge over the EM, a pronounced trough or cut‐off 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.
A Lagrangian approach was applied to three objectively identified persistent dry and warm winter events over Israel in order to understand the relation between the synoptic setting, precursor Rossby waves, and the emergence of dry and warm conditions. The warm and dry surface conditions were found to develop by adiabatic heating of descending air, surface sensible heat fluxes and advection of warm and dry continental air. The upper East Mediterranean tropospheric ridge was supported by diabatic inflow from upstream Mediterranean cyclones and North Atlantic blocking.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Cyclones impacting the densely populated Mediterranean region have been a continuous research focus, mainly for investigating either the associated heavy precipitation or the damaging wind gusts. In ...this study we examine five Mediterranean cyclones with combined large‐scale impact of strong 10 m gusts and heavy precipitation. The selected events occurred in (i) December 2003 in the northeastern Mediterranean; (ii) October 2007 in the central Mediterranean; (iii) January 2009, known as storm ‘Klaus’, in the western Mediterranean; (iv) December 2010 in the eastern Mediterranean; and (v) October 2011 in the central‐northern Mediterranean. European Centre for Medium‐range Weather Forecasts (ECMWF) reanalyses and 7 km resolution regional model simulations (COSMO) are analysed for each event. A Lagrangian viewpoint is employed to focus on interacting mechanisms that contribute to the joint impact on different spatial and temporal scales. In all cases, widespread strong wind gusts occur in the southwestern parts of the cyclone, while the precipitation field has localized peaks, with variable distribution in the central, southern, eastern and northern parts of the cyclone. Convective precipitation, significant in the cases in 2007, 2010 and 2011, is limited to the southern areas. In all cases, non‐convective precipitation is associated with ascent in a warm conveyor belt. Intense gusts are found within unstable air, below a low tropopause in a region with strong vertical wind shear, favouring downward momentum flux by turbulent mixing. Strongly descending dry intrusions are located coherently to the south and west of strong gusts. Much variability exists with regard to the emergence of convection, where strong winds and convective precipitation co‐occur: In the 2007 case, the dry intrusion is central in producing shallow convection in the cold frontal region. In the 2010 and 2011 cases, convective activity at high topography and in coastal regions leads to co‐location of both types of impact.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The mistral is a northerly gap‐wind regime blowing through the Rhone Valley in Southern France. It is held responsible for the sea‐surface cooling necessary to produce deep convection in the Gulf of ...Lion through turbulent ocean heat loss. The mistral is tightly connected to lee‐cyclogenesis in the Gulf of Genoa, where topography forces substantial downward motion. Dry intrusions (DIs) are airstreams forming the descending branch of extratropical cyclones. Known to induce cold and dry surface anomalies, DIs are potential contributors to enhanced surface evaporation during mistral. In this study, a climatological database (ERA‐INTERIM, 1981–2016) of mistral–DI co‐occurrence is constructed, allowing quantification of the impact of DIs on the mistral evaporative hot spot for the first time. We find that, on average, mistral–DI evaporation rates are doubled, compared to mistral without DIs. Moreover, cluster‐composite analysis reveals amplifications exceeding 300% between dynamically similar mistral events, with response to DIs. Daily latent heat‐flux anomalies in the Gulf of Lion are decomposed into contributions from the various parameters to analyse the mistral evaporation response to DIs. Mistral–DI events are shown to produce extreme evaporation rates through increased mistral wind speeds. The results highlight the downward momentum flux delivered by DIs to the mistral at the Gulf of Lion as the primary driver of the evaporation amplification mechanism. We further explore the variability between different mistral–DI events and conclude that extreme mistral–DI evaporation events are linked to descending air trajectories entering the Gulf of Lion at an early stage of their lifetimes. These DIs charge the mistral with maximum vertical momentum fluxes, which act to intensify surface winds and hence evaporation rates.
Ocean evaporation in the Gulf of Lion is greatly enhanced when the mistral wind is embedded in large‐scale dry intrusions at the rear of the Genoa low. Using a climatological set (1981–2016) of mistral events and contrasting it with mistral events occurring within dry intrusions, we find that downward momentum flux along the dry intrusions enhances mistral winds. The mistral wind speeds dominate the variability of ocean evaporation according to further decomposition of evaporation to its main drivers.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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 self‐organizing map (SOM) classification of profile data from a single station is a sensitive discriminator of winter synoptic regimes in the Eastern Mediterranean.• A full quantitative classification of the winter BL profile regimes at Beit Dagan has been performed.• Strong association between the regional synoptic systems and the profile regimes highlighted four major groups: 1. Deep winter cyclones 2. Strong winter anticyclones and Red Sea trough (RST) 3. Moderate pressure. 4. Active RST.• First objective classification of active RST, without the need to formulate specific criteria.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The Red‐Sea Trough (RST) extends toward the Levant from the south and is mostly associated with dry conditions. This study analysed the rarer type, rain‐producing RSTs, referred to as ‘wet RSTs’ ...(WRST), using data from 1958 to 2019. A WRST archive was established, based on rain data from a study region covering the arid part of Israel, the region where this system is most active. The study also aimed to find an atmospheric proxy for rainfall, to replace precipitation estimates obtained from climate models. A WRST day was defined as a day when the RST was identified and rainfall of ≥0.1 mm was observed by at least two of the 39 rain stations used. The annual occurrence of WRSTs is 16.4 days, ~16% of all RSTs, peaking in the winter. The relative contribution of WRSTs to the annual rainfall ranges from 16% in the northern part of the study region to 50% in the south. The tropical air intrusion was quantified through trajectory analysis and the results revealed that in most WRST days, 72%, the fraction of air originating from the tropics was less than 10% and only in 3.7% of the days did it exceed 50%. Attempts to extract an atmospheric proxy which identifies a WRST did not yield significant separation between dry and wet RSTs, because of significant differences among several WRST types. Therefore, identification of future changes in WRSTs will still require the use of rainfall data offered by climate models. We automatically classified the WRST days into four types. Two types are associated with 850 hPa negative temperature anomalies and the other two with higher‐than‐normal temperatures, resembling ‘tropical‐like’ RSTs, which are known as ‘active RSTs’ and tropical plumes. A predictive equation was extracted to quantify the tropical air intrusion and replace the use of trajectory analysis in future studies.
Backward air trajectories for three illustrative WRST cases using L0 = 20°N, with different TF values and belonging to different types. The −120 hr long trajectories are according to their pressure level (hPa), the black dots mark their location at −72 hr and at the target (i.e. time 0 hr). The type, date of arrival at the target and TF value appear on each panel. Maps of SLP and 500 hPa GPH are appended at the bottom of each panel.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Dry-air intrusions (DIs) are dry, deeply descending airstreams from the upper troposphere toward the planetary boundary layer (PBL). The significance of DIs spans a variety of aspects, including the ...interaction with convection, extratropical cyclones and fronts, the PBL, and extreme surface weather. Here, a Lagrangian definition for DI trajectories is used and applied to ECMWF interim reanalysis (ERA-Interim) data. Based on the criterion of a minimum descent of 400 hPa during 48 h, a first global Lagrangian climatology of DI trajectories is compiled for the years 1979–2014, allowing quantitative understanding of the occurrence and variability of DIs, as well as the dynamical and thermodynamical interactions that determine their impact. DIs occur mainly in winter. While traveling equatorward from 40°–50° latitude, DIs typically reach the lower troposphere (with maximum frequencies of ∼10% in winter) in the storm-track regions, as well as over the Mediterranean Sea, Arabian Sea, and eastern North Pacific, off the western coast of South America, South Africa, and Australia, and across the Antarctic coast. The DI descent is nearly adiabatic, with a mean potential temperature decrease of 3 K in two days. Relative humidity drops strongly during the first descent day and increases in the second day, because of mixing into the moist PBL. Significant destabilization of the lower levels occurs beneath DIs, accompanied by increased 10-m wind gusts, intense surface heat and moisture fluxes, and elevated PBL heights. Interestingly, only 1.2% of all DIs are found to originate from the stratosphere.
Full text
Available for:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Mid‐latitude cyclones are complex weather systems that are tightly related to surface weather impacts. Coherent air streams are known to be associated with such systems, in particular dry intrusions ...(DIs) in which dry air masses descend slantwise from the vicinity of the tropopause equatorward towards the surface. Often, DIs are associated with severe surface winds, heavy precipitation and frontogenesis. Currently, DIs can only be identified in hindsight by costly Lagrangian calculations using high resolution wind field data. Here, we use a novel method aiming to simplify the detection procedure of DI origins to allow their future identification in climate datasets, previously inaccessible for such diagnostic studies. A novel adaptation of a segmentation‐oriented neural network model is hereby presented as a successful tool to identify DI origins based solely on three ERA‐Interim reanalysis geopotential height fields, representing the state of the atmosphere. The model prediction skill is tested by calculating both the grid‐point and DI object based Matthews correlation coefficient. We find the model highly skilful in both reconstructing accurately the climatological distribution and predicting the vast majority of the individual DI origin objects. The skill decreases for relatively small objects and for objects occurring at locations where such cases are relatively less frequent. This indicates that geopotential height variability is related to the dynamic mechanisms involved in DI initiations. The results serve as a proof of concept for predicting DIs and other coherent air mass trajectories even when high resolution wind field data are not available, such as for model output for future climate projections.
The study of coherent air mass trajectories associated with various weather systems requires costly calculations using high resolution data. An adaptation of a segmentation‐oriented neural network can identify the origins of such trajectories based solely on three instantaneous geopotential height fields, applied for identifying dry intrusion (DI) origins. The model successfully predicts the majority of individual DI origin objects and their climatological spatial distribution, serving as a proof of concept for predicting other coherent air mass trajectories when high resolution data are unavailable.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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 m2s−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.
Plain Language Summary
Deep convection occurs periodically in the Gulf of Lion (located in the northwestern Mediterranean Sea), when water at the surface of the ocean is cooled enough to mix freely with the deeper water below, sometimes reaching the sea floor. It's an important part of the overall circulation of the Mediterranean Sea that leads to an explosion in the phytoplankton population in the following spring. In the gulf, the surface cooling is caused by the atmospheric transition from summer to winter and the Mistral winds. The latter is a cool, dry northerly wind that flows through the Rhône Valley out over the gulf. In our study, we ran ocean simulations that included and excluded the non‐seasonal effects of the Mistral to determine the importance of the seasonal and Mistral forcing on deep convection. The seasonal forcing was found to have a larger role, and contained a low frequency part of the Mistral, elevating the average wind speeds during the winter. Changes in the seasonal forcing and ocean water composition will need to be studied to understand the evolution of deep convection in the Gulf of Lion and its consequences on the Mediterranean Sea dynamics and biology in a changing climate.
Key Points
The seasonal atmospheric change is the main driver of destratification
Winters with deep convection have below average levels of stratification that the atmospheric forcing has to overcome
The Mistral winds have a low frequency signature that elevates the seasonal wind speeds in the winter
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
PDF
