Showing papers by "Heini Wernli published in 2015"

The LAGRANTO Lagrangian analysis tool – version 2.0

Abstract: . Lagrangian trajectories are widely used in the atmospheric sciences, for instance to identify flow structures in extratropical cyclones (e.g., warm conveyor belts) and long-range transport pathways of moisture and trace substances. Here a new version of the Lagrangian analysis tool LAGRANTO (Wernli and Davies, 1997) is introduced, which offers considerably enhanced functionalities. Trajectory starting positions can be defined easily and flexibly based on different geometrical and/or meteorological conditions, e.g., equidistantly spaced within a prescribed region and on a stack of pressure (or isentropic) levels. After the computation of the trajectories, a versatile selection of trajectories is offered based on single or combined criteria. These criteria are passed to LAGRANTO with a simple command language (e.g., "GT:PV:2" readily translates into a selection of all trajectories with potential vorticity, PV, greater than 2 PVU; 1 PVU = 10−6 K m2 kg−1 s−1). Full versions of this new version of LAGRANTO are available for global ECMWF and regional COSMO data, and core functionality is provided for the regional WRF and MetUM models and the global 20th Century Reanalysis data set. The paper first presents the intuitive application of LAGRANTO for the identification of a warm conveyor belt in the North Atlantic. A further case study then shows how LAGRANTO can be used to quasi-operationally diagnose stratosphere–troposphere exchange events. Whereas these examples rely on the ECMWF version, the COSMO version and input fields with 7 km horizontal resolution serve to resolve the rather complex flow structure associated with orographic blocking due to the Alps, as shown in a third example. A final example illustrates the tool's application in source–receptor analysis studies. The new distribution of LAGRANTO is publicly available and includes auxiliary tools, e.g., to visualize trajectories. A detailed user guide describes all LAGRANTO capabilities.

Importance of latent heat release in ascending air streams for atmospheric blocking

Abstract: Atmospheric blocking can contribute to extreme weather events. A Lagrangian approach applied to reanalysis data shows that a large fraction of air masses are heated before entering a blocking system, pointing to a role for latent heating.

Large‐scale wind and precipitation extremes in the Mediterranean: a climatological analysis for 1979–2012

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.

A Lagrangian investigation of hot and cold temperature extremes in Europe

Abstract: Since temperature extremes have a strong impact on environment and society, it is crucial to understand their underlying mechanisms. While their relationship to the large-scale atmospheric circulation has been comprehensively investigated, the role of physical processes in the formation of air masses with extreme temperatures is less well understood. This study presents a Lagrangian analysis of the 0.1% most extreme six-hourly hot and cold events in three European regions (UK, Central Europe, Balkans) for the time period 1989–2009. The results provide insight into typical transport patterns and physical processes (adiabatic compression, radiation, surface heat fluxes) occurring along the trajectories of extreme temperature air masses. Cold events in Europe are most frequently induced by advection of cold air masses from the Arctic and Russia. The transport to the target region is characterized by a temperature increase caused by adiabatic compression and, in the maritime setting of the UK, also by diabatic heating due to surface sensible heat fluxes. Despite the warming along the trajectories, the extremeness of the associated 2 m temperature increases, due to the dislocation of the air mass to regions with a milder climate. Hot events are generally associated with weaker horizontal transport, but strong adiabatic warming and local temperature increase caused by enhanced radiation and surface heat fluxes. This in situ warming is particularly strong in Central Europe. Evaluating the temperature evolution along the trajectories reveals that hot and cold extremes develop on a similar time-scale of 2–3 days. This time-scale is mostly set by physical processes for hot extremes and controlled by advective transport for cold extremes. The diagnostics applied in this study lead to an improved process understanding that can provide a basis for more accurate predictions of temperature extremes.

Tropopause folds in ERA‐Interim: Global climatology and relation to extreme weather events

Abstract: Tropopause folds are intimately linked to upper level frontogenesis and jet stream dynamics. They play an important role for stratosphere-troposphere exchange, the dynamical coupling of upper and lower tropospheric levels, and for generating severe weather events. This study presents a global climatology of tropopause folds using ERA-Interim reanalysis data from 1979 to 2012 and a refined version of a previously developed 3-D labeling and fold identification algorithm. This algorithm objectively separates stratospheric and tropospheric air in complex situations, e.g., in regions with strong low-level inversions, and in extratropical cyclones where diabatically generated potential vorticity anomalies typically occur. Three classes of tropopause folds are defined (shallow, medium, and deep), and their geographical distribution, vertical extent, and seasonal cycle are investigated. Most shallow folds occur along the subtropical jet stream, in agreement with previous studies. Hot spots of medium and deep tropopause folds are found west of Australia and along the coast of Antarctica in the Southern Hemisphere and around the east coast of North America in the Northern Hemisphere. Seasonal cycles show maxima in winter for all fold classes. Medium and deep folds are frequently associated with surface wind gust and precipitation extremes, as quantified for folds over the southern Indian Ocean. Wind gust extremes occur mainly in an elongated band upstream and equatorward of folds, whereas precipitation extremes occur mainly east and poleward of folds. Overall, in the considered region, about 20% of medium folds and 33% of deep folds are associated with surface wind or precipitation extremes in the vicinity of the fold.

A Climatology of Cold Air Outbreaks and Their Impact on Air–Sea Heat Fluxes in the High-Latitude South Pacific

Abstract: A climatology of cold air outbreaks (CAOs) in the high latitudes of the South Pacific and an analysis of the dynamical mechanisms leading to their formation are presented. Two major and distinct regions with frequent CAOs from autumn to spring are identified: one in the Ross Sea and another in the Amundsen and Bellingshausen Seas. Using an objective method to attribute CAOs to extratropical cyclones, it is shown that about 80% of the CAOs occur in association with the cyclonic flow induced by the passage of extratropical cyclones. Based on kinematic backward trajectories it is quantified that more than 40% of the air masses leading to CAOs originate from Antarctica and descend substantially, with the Ross Ice Shelf corridor as the major pathway. CAO trajectories descending from Antarctica differ from those originating over sea ice by a much lower specific humidity, stronger diabatic cooling, and much more intense adiabatic warming, while potential vorticity evolves similarly in both categories. In...

Isotope meteorology of cold front passages: A case study combining observations and modeling

Abstract: This study investigates the role of below-cloud evaporation and evapotranspiration for the short-term variability of stable isotopes in near-surface water vapor and precipitation associated with central European cold fronts. To this end, a combination of observations with high temporal resolution and numerical sensitivity experiments with the isotope-enabled regional weather prediction model COSMOiso is used. The representation of the interaction between rain droplets and ambient vapor below the cloud is fundamental for adequately simulating precipitation isotopes ((p)) and total rainfall amount. Neglecting these effects leads to depletion biases of 20-40 in delta H-2(p) and 5-10% in delta O-18(p) and to an increase of 74% in rainfall amount. Isotope fractionation during soil evaporation is of primary importance for correctly simulating the variability of continental low-level vapor delta H-2(v) and delta O-18(v) and particularly of the secondary isotope parameter deuterium excess (d(v)). (Less)

The dynamical structure of intense Mediterranean cyclones

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.

The transatlantic dust transport from North Africa to the Americas : Its characteristics and source regions

Abstract: Transport of Saharan dust over the Atlantic to the Americas is a relevant process since dust is a nutrient for marine and terrestrial ecosystems. It is therefore important to better quantify the frequency and amount of transatlantic dust transport, its preferred altitude and duration, and the regions of dust origin. This study uses a novel combination of Eulerian and Lagrangian diagnostics, applied to a previously validated 5 year simulation of the fifth generation European Centre for Medium Range Weather Forecast-Hamburg-model (ECHAM5)/Modular Earth Submodel System (MEESy) Atmospheric Chemistry model, to quantify these dust transport characteristics and their seasonal variations. Results confirm the previously found preferred transatlantic dust pathways: in boreal winter and spring, African dust is mainly transported below 800 hPa toward South America, whereas in summer and autumn the preferred pathway is to the Caribbean and occurs in a layer up to 500 hPa. The averaged transport duration from dust emission to deposition is 10 days in winter for deposition in the Amazon region and almost 12 days in summer for deposition in the Caribbean. These estimates were obtained by combining correlation analyses of Eulerian dust fluxes and trajectory calculations. The latter were also essential to identify the main source regions of transatlantic dust transport, which were found in all seasons in northwestern Africa (Algeria, Mali, and Mauritania) but not farther east, e.g., in the Bodele Depression. A specific Lagrangian analysis for this dust emission hot spot suggests that wet deposition associated with the Intertropical Convergence Zone in winter and the African monsoon in summer inhibits Bodele dust to leave the African continent.

Climatology of potential vorticity streamers and associated isentropic transport pathways across PV gradient barriers

Abstract: The European Centre for Medium-Range Weather Forecasts reanalysis product ERA-Interim from 1979 to 2011 is used to investigate potential vorticity (PV) streamers as indicators of Rossby wave breaking (RWB) and the exchange of air masses between the tropics and the extratropics on isentropes between 350 and 500 K. The concept of the strongest isentropic gradients of PV and wind is applied to mark a dynamically relevant PV boundary on every isentrope. An Eulerian algorithm is then used to identify streamers of this PV contour and a trajectory-based technique to study the exchange across this contour. The climatology of PV streamers reveals a seasonal cycle with a higher frequency in summer than in winter in both hemispheres on all isentropes. During winter, PV streamers are suppressed above 400 K in line with the occurrence of polar vortices. Globally, the highest frequency of PV streamers is found between 380 and 420 K over the eastern North Pacific in summer. Exchange trajectories associated with these PV streamers constitute important atmospheric transport pathways between the tropics and extratropics. In both hemispheres, PV streamers and their associated air mass exchange are controlled by the presence of monsoon anticyclones in summer and by RWB in the vicinity of jet splittings in winter. Cross-hemispheric transport is observed between 380 and 400 K in the westerly ducts over the Pacific and Atlantic. Equatorward PV streamers are connected with a more frequent exchange than poleward PV streamers, indicating a more reversible poleward and irreversible equatorward RWB. The latter is often connected with a frequent breaking into PV cutoffs.

Stratosphere-troposphere exchange (STE) in the vicinity of North Atlantic cyclones

Abstract: . It is well known that the storm tracks are a preferred region of stratosphere–troposphere exchange (STE), but a systematic and climatological investigation of the connection between cyclones and STE has not yet been performed. We use two ERA-Interim climatologies of STE and cyclones for the years 1979–2011 to quantify the amount of STE that occurs during the life cycle of North Atlantic cyclones. A Lagrangian method serves to identify individual STE events, and a sophisticated cyclone identification tool detects cyclones, their shape and size from the sea-level pressure (SLP) field and from geopotential height anomalies at 300–700 hPa. Combining the two data sets reveals that roughly 50–60 % of the total STE in the North Atlantic occurs in the vicinity of cyclones and that both downward and upward fluxes of mass across the tropopause (STT and TST, respectively) are more intense in deeper cyclones (lower minimum SLP) compared to less intense cyclones. In summer, STT and TST in the vicinity of cyclones are almost equal; in the other seasons, STT is larger by 25–60 %. Compared to climatology, cross-tropopause mass fluxes are enhanced by a factor of about 1.29 and 1.06 for STT and TST, respectively, when a cyclone is present. On average, STE is strongest during the mature phase of cyclones, i.e., in a 24 h time window around the time of maximum intensity. Systematic patterns of exchange locations relative to the cyclone centre are identified via composite analysis and shed light on the different characteristics of STT and TST. During cyclone intensification and in the mature stage, TST is mainly confined to the cyclone centre, whereas STT occurs mainly in a region further southwest. During the decay of the cyclones, both STT and TST are most frequent close to the cyclone centre, in a region with a fairly low tropopause.

Mechanisms underlying temperature extremes in Iberia: a Lagrangian perspective

Abstract: The mechanisms underlying the occurrence of temperature extremes in Iberia are analysed considering a Lagrangian perspective of the atmospheric flow, using 6-hourly ERA-Interim reanalysis data for the years 1979–2012. Daily 2-m minimum temperatures below the 1st percentile and 2-m maximum temperatures above the 99th percentile at each grid point over Iberia are selected separately for winter and summer. Four categories of extremes are analysed using 10-d backward trajectories initialized at the extreme temperature grid points close to the surface: winter cold (WCE) and warm extremes (WWE), and summer cold (SCE) and warm extremes (SWE). Air masses leading to temperature extremes are first transported from the North Atlantic towards Europe for all categories. While there is a clear relation to large-scale circulation patterns in winter, the Iberian thermal low is important in summer. Along the trajectories, air mass characteristics are significantly modified through adiabatic warming (air parcel descent), upper-air radiative cooling and near-surface warming (surface heat fluxes and radiation). High residence times over continental areas, such as over northern-central Europe for WCE and, to a lesser extent, over Iberia for SWE, significantly enhance these air mass modifications. Near-surface diabatic warming is particularly striking for SWE. WCE and SWE are responsible for the most extreme conditions in a given year. For WWE and SCE, strong temperature advection associated with important meridional air mass transports are the main driving mechanisms, accompanied by comparatively minor changes in the air mass properties. These results permit a better understanding of mechanisms leading to temperature extremes in Iberia. Keywords: temperature extremes, cold episodes, warm episodes, Lagrangian trajectories, large-scale circulation, surface fluxes, Iberian Peninsula, ERA-Interim (Published: 22 April 2015) Citation: Tellus A 2015, 67, 26032, http://dx.doi.org/10.3402/tellusa.v67.26032

Verification of North Atlantic warm conveyor belt outflowsin ECMWF forecasts

Abstract: A feature-based approach is introduced to assess the quality of the representation of warm conveyor belts (WCBs) in high-resolution forecasts of the European Centre for Medium-range Weather Forecasts (ECMWF). WCBs are moist ascending airstreams in extratropical cyclones, which transport boundary-layer air within 1–2 days poleward to the upper troposphere. The WCB outflow is typically characterised by low potential vorticity (PV), amplifying upper-level ridges and in turn modifying the jet stream and downstream Rossby wave evolution. Therefore the correct representation of WCBs can be essential for medium-range weather prediction. A three-component verification measure PAL is introduced, measuring errors in the WCB outflow's PV anomaly (P component), the amplitude of the WCB (A component), and the location of the outflow (L component). The PAL approach is applied to North Atlantic WCBs in ECMWF forecasts during three winters between 2002/2003 and 2010/2011. For the latest winter, the PAL results are also compared to the anomaly correlation coefficient (ACC) of upper-level PV. It is shown that (i) the representation of WCBs improves for forecasts with a shorter lead time, (ii) PAL values are on average better for more recent forecasts, (iii) recent model versions show no systematic over- or underestimation of WCB intensity, (iv) individual medium-range forecasts can be associated with large errors in particular in the amplitude of WCBs, and (v) the comparison of PAL and the ACC indicates that several poor forecasts in terms of ACC are indeed associated with significant errors in the representation of WCBs. Limitations of this study are also discussed.

A scaling relation for warm‐phase orographic precipitation: a Lagrangian analysis for 2D mountains

Abstract: The precipitation produced by orographic clouds is the result of the interaction between nonlinear dynamical and microphysical processes. Focusing on warm-phase stable clouds, we propose that the precipitation efficiency is essentially controlled by the bulk Damkohler number of the system, i.e. the ratio of the characteristic advective to the microphysical time-scale of the cloud. These time-scales are investigated thoroughly for single air parcels along temporally highly resolved trajectories from quasi-two-dimensional numerical simulations with vertical Froude numbers between 0.38 and 2.31. Based on these results, analytical formulations for the Damkohler number of single parcels and for a bulk Damkohler number for the entire cloud are developed, which depend only on the far upstream flow properties and the cloud droplet number density. Relating the bulk Damkohler numbers to the precipitation efficiencies from the numerical simulations results in a unique scaling relation between the two non-dimensional numbers: no precipitation is observed for very small Damkohler numbers, while for large Damkohler numbers the precipitation efficiency asymptotically approaches a maximum value of about 0.8. For intermediate Damkohler numbers, the precipitation efficiency depends strongly on the microphysical properties of the clouds.

Diabatic Rossby waves in the Southern Hemisphere

Abstract: Diabatic Rossby waves (DRWs) are a special type of low-level cyclone with a self-maintaining mechanism given an environment of moderate or strong baroclinicity and abundant moisture. Under certain conditions, these shallow cyclones associated with a cyclonic low-level PV anomaly can serve as precursors for explosive cyclone intensification. The present study is the first to investigate DRWs in the Southern Hemisphere (SH). Two typical DRW case-studies are presented and a sophisticated algorithm is used to detect DRWs in operational analysis data from 2001 to 2012. DRWs occur in all ocean basins and seasons with a maximum in summer. On average, about four DRWs are found per month and 12% intensify explosively. Thereby, they contribute 6% to the total set of explosively deepening extratropical cyclones in the SH. DRW tracks originate slightly closer to the Equator than normal extratropical cyclones, and they follow the convergence zones eastward and poleward. This results in a climatological DRW track density similar to a spiral, from the western South Pacific ending in the eastern South Indian Ocean, broken only by the Andes. Typical synoptic situations associated with DRW genesis are identified. The categories include (i) low-level PV generation by upper-level induced lifting, (ii) low-level jet-induced diabatic PV generation, and (iii) transformation of the low-tropospheric PV anomaly from another system.

IWAL–An Interactive Weather Analysis Laboratory

Abstract: Complementary key elements of meteorological education are the provision of a thorough theoretical understanding of the physical laws governing atmospheric motions, and the hands-on investigation and visualization of specific weather systems. However, the latter task is technically challenging, because specific skills must be acquired for flexibly handling meteorological data. Some examples are superimposing satellite pictures and reanalysis fields, producing an isentropic potential vorticity (PV) map, and visualizing a vertical section across a flow feature of interest. Although learning these technical issues has its own merits, it can distract students from investigating the complexities of meteorology. This experience from teaching beginner classes in synoptic meteorology at ETH Zurich and the University of Mainz was the main motivation for developing the educational software tool IWAL, the Interactive Weather Analysis Laboratory. IWAL is designed as a web application for easy, fast, and inter...

Dynamics and predictability of middle latitude weather systems and their higher and lower latitude interactions

Abstract: This theme addresses all aspects of the dynamics and predictability of midlatitude weather systems including Rossby wave trains, high-pressure systems and blocking, extratropical cyclones and fronts, and also embedded mesoscale phenomena. A particular focus is on studies investigating interactions of these midlatitude systems with the (sub)tropics and polar regions, and on high-impact weather events. Also included are novel results from recent field experiments, theoretical and idealised studies, numerical modelling case studies, and ensemble and long-term evaluations of the forecasting performance for specific midlatitude weather systems. The article provides a selective overview of key research accomplishments during the last decade and highlights important open science questions for the coming years - some of them requiring continued international collaboration and increased cooperation between operational weather centres and academic research institutions.

DYNAMICAL METEOROLOGY | Quasigeostrophic Theory

Abstract: Synoptic- and larger-scale atmospheric flow in the extratropics is almost geostrophic in character, indicating an approximate balance between the Coriolis and pressure gradient forces. The concept of quasigeostrophy (QG) encapsulates elegantly and simply the dynamics whereby the flow retains this character, although both the maintenance of geostrophy and the existence of other key flow features require a nongeostrophic flow component.