(2019) Atmospheric Chemistry and Physics. 19, 17, p. 11143-11158 Abstract
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 μm. Ice nucleation experiments were conducted in the Weizmann Supercooled Droplets Observation on a Microarray (WISDOM) 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 (ns) 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 including information on particle size in order to increase the accuracy of ice formation modeling and thus weather and climate predictions.
Climatology and dynamics of the link between dry intrusions and cold fronts during winter, Part II: Front-centred perspective(2019) Climate Dynamics. 53, 3-4, p. 1893-1909 Abstract
The conceptual picture of an extratropical cyclone typically includes a cold front and a dry intrusion (DI) behind it. By objectively identifying fronts and DIs in ECMWF ERA-Interim data for 1979–2014, Part I quantified the climatological relationship between cold fronts and DIs. Driven by the finding that front intensity and frontal precipitation are enhanced in the presence of DIs, here we employ a front-centred perspective to focus on the dynamical and thermodynamical environment of cold fronts with and without DIs in the Northern Hemisphere winter. Distinguishing between trailing fronts (that connect to a parent cyclone) and isolated fronts, examples of DIs behind each type illustrate the baroclinic environment of the trailing front, and the lack of strong temperature gradients across the isolated front. Composite analyses of North Atlantic and North Pacific fronts outline the major differences in the presence of DIs, compared to similar fronts but without DIs in their vicinity. The magnitude and spatial structure of the modification by DIs depends on the front intensity. Yet, generally with DIs, trailing fronts occur with stronger SLP dipole, deeper upper-tropospheric trough, stronger 10-m wind gusts, enhanced ocean sensible and latent heat fluxes in the cyclone cold sector and heavier precipitation. Isolated weak fronts exhibit similar behaviour, with different spatial structure. This study highlights the central role of DIs for shaping the variability of fronts and their associated environment and impact.
Climatology and dynamics of the link between dry intrusions and cold fronts during winter. Part I: global climatology(2019) Climate Dynamics. 53, 3-4, p. 1873-1892 Abstract
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.
Investigation of ozone deposition to vegetation under warm and dry conditions near the Eastern Mediterranean coast(2019) Science of the Total Environment. 658, p. 1316-1333 Abstract[All authors]
Dry deposition of ozone (O-3) to vegetation is an important removal pathway for tropospheric O-3, while O-3 uptake through plant stomata negatively affects vegetation and leads to climate change. Both processes are controlled by vegetation characteristics and ambient conditions via complex mechanisms. Recent studies have revealed that these processes can be fundamentally impacted by coastal effects, and by dry and warm conditions in ways that have nut been fully characterized, largely due to lack of measurements under such conditions. Hence, we hypothesized that measuring dry deposition of O-3 to vegetation along a sharp spatial climate gradient, and at different distances from the coast, can offer new insights into the characterization of these effects on O-3 deposition to vegetation and stomatal uptake, providing important information for afforestation management and for climate and air-quality model improvement. To address these hypotheses, several measurement campaigns were performed at different sites, including pine, oak, and mixed Mediterranean forests, at distances of 20-59 km from the Eastern Mediterranean coast, under semiarid, Mediterranean and humid Mediterranean climate conditions. The eddy covariance technique was used to quantify vertical O-3 flux (F-tot) and its partitioning to stomatal flux (F-st) and non-stomatal flux (F-ns). Whereas F-st tended to peak around noon under humid Mediterranean and Mediterranean conditions in summer, it was strongly limited by drought under semiarid conditions from spring to early winter, with minimum average F-st/F-tot, of 8-11% during the summer. F-ns in the area was predominantly controlled by relative humidity (RH), whereas increasing F-ns with RH for RH 70% indicated enhancement of F-ns by aerosols, via surface wetness stimulation. At night, efficient turbulence clue to sea and land breezes, together with increased RH, resulted in strong enhancement of F-tot Extreme dry surface events, some induced by dry intrusion from the upper troposphere, resulted in positive F-ns events. (C) 2018 Elsevier B.V. All rights reserved.
(2018) Meteorologische Zeitschrift. 27, 1, p. 67-80 Abstract
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 cutoff 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.
(2017) Journal of Climate. 30, 17, p. 6661-6682 Abstract
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 degrees-50 degrees latitude, DIs typically reach the lower troposphere (with maximum frequencies of similar to 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 3K 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.
(2017) Atmospheric Science Letters. 18, 5, p. 215-221 Abstract
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.
Large-scale wind and precipitation extremes in the Mediterranean: dynamical aspects of five selected cyclone events(2016) Quarterly Journal of the Royal Meteorological Society. 142, 701, p. 3097-3114 Abstract
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
(2015) Water Resources Research. 51, 9, p. 7702-7722 Abstract
Nickel migration measured in laboratory-scale, natural soil column experiments is shown to display anomalous (non-Fickian) transport, nonequilibrium adsorption and desorption patterns, and precipitation/dissolution. Similar experiments using a conservative tracer also exhibit anomalous behavior. The occurrence of ion exchange of nickel, mainly with calcium (but also with other soil components), is measured in both batch and flow-through column experiments; adsorption and desorption isotherms demonstrate hysteresis. Strong retention of nickel during transport in soil columns leads to delayed initial breakthrough (?40 pore volumes), slow increase in concentration, and extended concentration tailing at long times. We describe the mechanisms of transport and retention in terms of a continuous time random walk (CTRW) model, and use a particle tracking formulation to simulate nickel migration in the column. This approach allows us to capture the non-Fickian transport and the subtle local effects of adsorption/desorption and precipitation/dissolution. Consideration also of preferential pathways accounts for the evolution of the measured breakthrough curve and measured spatial concentration profiles. The model uses non-Fickian transport parameters estimated from the conservative tracer and, as a starting point, adsorption/desorption parameters based on batch experiments and a precipitation parameter based on Ksp values. The batch parameters are found to underestimate the actual amount of adsorption. We suggest that the sorption and precipitation/dissolution dynamics, and resulting breakthrough curves, are influenced strongly by preferential pathways; such pathways significantly alter the availability of sorption sites and ion availability for precipitation. Analysis of these results provides further understanding of the interaction and dynamics among transport, precipitation, and sorption mechanisms in natural soil. 2015. American Geophysical Union. All Rights Reserved
(2015) Climate Dynamics. 44, 9-10, p. 2411-2427 Abstract
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 20 km 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.
Large-scale wind and precipitation extremes in the Mediterranean: a climatological analysis for 1979-2012(2015) Quarterly Journal of the Royal Meteorological Society. 141, 691, p. 2404-2417 Abstract
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.
(2012) Journal of Contaminant Hydrology. 132, p. 28-36 Abstract
We present experimental breakthrough curve (BTC) data and a modeling investigation of conservative and sorbing tracer transport in natural soils. By analyzing the data using the continuous time random walk (CTRW) model, we probe the traditional approach of using conservative tracer model parameters as a basis for quantifying the transport of sorbing solutes in the same domain when non-Fickian transport is present. Many known contaminants in groundwater are sorbed to the host solid porous medium, to varying extents, while being transported: this enhances the long tailing of BTCs which often already occurs because of the inherent non-Fickian nature of the transport. The CTRW framework has been shown to account very well for non-Fickian conservative (nonsorbing) transport. Here, we examine two BTC data sets in laboratory columns packed with natural soils; the first (previously analyzed by Mao and Ren (2004)) comprises transport of (conservative) bromide and (sorbing) atrazine tracers, while the second presents new data with bromide and tribromoneopentyl alcohol (TBNPA), a key flame retardant, as a sorbing solute. TBNPA has received little attention in the past, and is shown to be sorbed onto Bet Dagan soil in a nonlinear manner. We find that the transport behavior of bromide is non-Fickian in all cases, which is caused by the heterogeneity of the soil. Comparative model analysis of the non-Fickian BTCs of the conservative, and sorbing tracers and examination of the fitting parameters, exemplify the coupling between transport and adsorption/desorption processes. The difference in transport parameters used to match the conservative and sorbing data sets shows that conservative tracer parameters (average velocity and dispersion coefficient) are not valid for the transport of reactive tracers. (C) 2012 Elsevier B.V. All rights reserved.
(2011) Vadose Zone Journal. 10, 3, p. 843-857 Abstract
We collected and analyzed Br- breakthrough curve (BTC) data to identify the parameters controlling transport from a series of soil cores and a field-scale tracer test at the Shale Hills Critical Zone Observatory (SH-CZO) in central Pennsylvania. The soil cores were retrieved from a continuous hole that extended through the soil profile to quantify also how solute transport behavior changes with depth and weathering. Additionally, we performed a field-scale doublet tracer test to determine transport behavior in the weathered shale bedrock. Hydraulic conductivity and porosity were as low as 10(-15) m s(-1) and 0.035, respectively, in the shale bedrock and upward of 10(-5) m s(-1) and 0.45, respectively, in the shallow soils. Bromide BTCs demonstrated significant tailing in soil cores and field tracer experiments, which does not fit classical advection-dispersion processes. To quantify the behavior, numerical simulation of solute transport was performed with both a mobile-immobile (MIM) model and a continuous-time random walk (CTRW) approach. One-dimensional MIM modeling results yielded low mass transfer rates (
(2011) Journal of Contaminant Hydrology. 120-21, p. 213-221 Abstract
Tracer tailing in breakthrough curves in porous media with two distinct porosities is analyzed in terms of the dynamic responses of experimental fixed bed columns filled either with solid or porous beads. The flow is fast in the column interstitial space between beads (for both solid and porous beads) but slow within the porous beads that act as controlled 'traps' constituting an immobile zone. The transport is quantified using a Continuous Time Random Walk (CTRW) framework, which accounts for domains with controlled structural and flow heterogeneity associated with two distinct spatial and time spectra. We first demonstrate that breakthrough curves for a column containing solid glass beads exhibit non-Fickian transport, quantifiable both in fitting and validation mode by a CTRW based on a power law transition time distribution. We then examine breakthrough curves in the porous bead case, obtaining fits with a two-scale CTRW model that accounts explicitly for the two time spectra. Because the porous beads are uniform, tracer trapping within them is described by a simple first-order approximation trap model, with relatively weak capture and relatively faster release rates. The extent of tailing apparent in the porous bead breakthrough curves, due to the traps, can be quantitatively distinguished from the contribution to tailing due to mobile zone non-Fickian transport. A parameter study of the two-scale CTRW adds further insight into the dynamics of the process, showing the interaction between the advective non-Fickian transport and the mass exchange to immobile regions. (C) 2010 Elsevier B.V. All rights reserved.
(2007) Geophysical Research Letters. 34, 23, Abstract
Considerations of the temporal changes in angular momentum are employed to diagnose air trajectories over large scale distances calculated from the publicly available global data base of NCEP/NCAR. It is shown that outside the Tropics both the total angular momentum and its Lagrangian (material) time-derivative are dominated by the planetary term, regardless of the direction of the transport, whereas the contribution of the non-linear term is always negligible. In the Tropics the planetary term and the acceleration terms alternately dominated the angular momentum evolution. These Lagrangian results are reinforced by Eulerian calculations of instantaneous maps of the relative contributions of the various terms in the evolution equation of the angular momentum. The simple equation of angular momentum evolution (compared to the more complex zonal momentum equation) is accurately satisfied by NCEP/NCAR re-analyzed fields even over Africa, where the data for the reanalysis is scant. The only exceptions to the general dominance of the planetary term over the angular momentum evolution in the extra-Tropics are along trough-/ridge-lines (where the flow changes direction sharply) and at the entrance/exit regions of straight jets, where the flow accelerates/decelerates.
(2007) Monthly Weather Review. 135, 12, p. 4135-4148 Abstract
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 degrees-15 degrees N, 5 degrees W-15 degrees 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.
A consistent theory for linear waves of the shallow-water equations on a rotating plane in midlatitudes(2007) Journal of Physical Oceanography. 37, 1, p. 115-128 Abstract
The present study provides a consistent and unified theory for the three types of linear waves of the shallow-water equations (SWE) in a zonal channel on the beta plane: Kelvin, inertia-gravity (Poincare), and planetary (Rossby). The new theory is formulated from the linearized SWE as an eigenvalue problem that is a variant of the classical Schrodinger equation. The results of the new theory show that Kelvin waves exist on the beta plane with vanishing meridional velocity, as is the case on the f plane, without any change in the dispersion relation, while the meridional structure of their height amplitude is trivially modified from exponential on the f plane to a one-sided Gaussian on the beta plane. Similarly, inertia-gravity waves are only slightly modified in the new theory in comparison with their characteristics on the f plane. For planetary waves (which exist only on the beta plane) the new theory yields a similar dispersion relation to the classical theory only for large gravity wave phase speed, such as those encountered in a barotropic ocean or an equivalent barotropic atmosphere. In contrast, for low gravity wave phase speed, for example, those in an equivalent barotropic ocean where the relative density jump at the interface is 10(-3), the phase speed of planetary waves in the new theory is 2 times those of the classical theory. The ratio between the phase speeds in the two theories increases with channel width. This faster phase propagation is consistent with recent observation of the westward propagation of crests and troughs of sea surface height made by the altimeter aboard the Ocean Topography Experiment (TOPEX)/Poseidon satellite. The new theory also admits inertial waves, that is, waves that oscillate at the local inertial frequency, as a genuine solution of the eigenvalue problem.