Cut‐off  low pressure systems (COLs) are defined as closed lows in the upper troposphere that have become completely detached from the main westerly current. These slow‐moving systems often affect the weather conditions at the earth's surface and also work as a mechanism of mass transfer between the stratosphere and the troposphere, playing a significant role in the net flow of tropospheric ozone. In the first part of this work we provide a comprehensive summary of results obtained in previous studies of COLs. Following this, we present three long‐term climatologies of COLs. The first two climatologies are based on the conceptual model of a COL, using European Centre for Medium‐range Weather Forecasts (ECMWF) analyses (1958–2002) and National Centers for Environmental Prediction–National Center for Atmospheric Research (1948–2006) reanalysis data sets. The third climatology uses a different method of detection, which is based on using potential vorticity as the physical parameter of diagnosis. This approach was applied only to the ECMWF reanalysis data. The final part of the paper is devoted to comparing results obtained by these different climatologies in terms of areas of preferential occurrence, life span, and seasonal cycle. Despite some key differences, the three climatologies agree in terms of the main areas of COL occurrence, namely (1) southwestern Europe, (2) the eastern north Pacific coast, and (3) the north China–Siberian region. However, it is also shown that the detection of these areas of main COL occurrence, as obtained using the potential vorticity approach, depends on the level of isentropic analysis used.

Identification and Climatology of Cut-off Lows near the Tropopause

This study explores the predictability of a heavy rainfall event that struck North Africa on 9 and 10 November 2001. This case is a paradigm of Mediterranean extreme events characterized by the presence of a deep upper-level trough associated with an intense cyclone which developed over the Western Mediterranean basin. Using the French non-hydrostatic mesoscale model MESO-NH, numerical experiments starting from various initial atmospheric states were conducted to assess the impact of initial condition uncertainties on the precipitation and cloud cover forecast. To generate a set of perturbed atmospheric states, a simple date-shifting initialization method was used. Two sets of simulations were run, using lateral boundaries and initial conditions derived from both the French operational global assimilation system Action de Recherche Petite Echelle Grande Echelle (ARPEGE) and the European Centre for Medium-Range Weather Forecasts (ECMWF) system. Initial perturbations applied to the upper-level trough propagated and intensified throughout the simulation, leading to some discrepancy in the forecast of the low-level cyclone. While it was found that the upper-level trough and the surface cyclone controlled the location of the overall precipitation pattern, the predictability of smaller-scale features such as localized heavy rainfall was directly related to specific mesoscale structures. The low-level jet associated with the surface cyclone and the location and the intensity of the surface-low both impact upon the triggering and the sustainment of the convective cells. In consequence, small-scale perturbations of these mesoscale features led to large errors in the precipitation forecast, especially in the Algiers area. Copyright © 2008 Royal Meteorological Society

Impact of initial condition uncertainties on the predictability of heavy rainfall in the Mediterranean : a case study.

. Extreme precipitation events (EPEs) frequently cause
flooding with dramatic socioeconomic impacts in many parts of the world.
Previous studies considered two synoptic-scale processes, Rossby wave
breaking and intense moisture transport, typically in isolation, and their
linkage to such EPEs in several regions. This study presents for the first
time a global and systematic climatological analysis of these two
synoptic-scale processes, in tandem and in isolation, for the occurrence of
EPEs. To this end, we use 40-year ERA-Interim reanalysis data (1979–2018)
and apply object-based identification methods for (i) daily EPEs, (ii)
stratospheric potential vorticity (PV) streamers as indicators of Rossby
wave breaking, and (iii) structures of high vertically integrated horizontal
water vapour transport (IVT). First, the importance of these two
synoptic-scale processes is demonstrated by case studies of previously
documented flood events that inflicted catastrophic impacts in different
parts of the world. Next, a climatological quantification shows that Rossby
wave breaking is associated with >90  % of EPEs over central
North America and the Mediterranean, whereas intense moisture transport is
linked to >95  % of EPEs over many coastal zones, consistent
with findings of atmospheric river-related studies. Combined Rossby wave
breaking and intense moisture transport contributes up to 70 % of EPEs in
several subtropical and extratropical regions, including (semi)arid desert
regions where tropical–extratropical interactions are of key importance for
(heavy) rainfall. Odds ratios of EPEs linked to the two synoptic-scale
processes suggest that intense moisture transport has a stronger association with the occurrence of EPEs than Rossby wave breaking. Furthermore, the
relationship between the PV and IVT characteristics and the precipitation
volumes shows that the depth of the wave breaking and moisture transport
intensity are intimately connected with the extreme precipitation severity.
Finally, composites reveal that subtropical and extratropical EPEs, linked
to Rossby wave breaking, go along with the formation of upper-level troughs
and cyclogenetic processes near the surface downstream, reduced static
stability beneath the upper-level forcing (only over water), and dynamical
lifting ahead (over water and land). This study concludes with a concept
that reconciles well-established meteorological principles with the
importance of Rossby wave breaking and intense moisture transport for the
formation of EPEs. Another conclusion with major implications is that
different combinations of Rossby wave breaking and intense moisture
transport can reflect a large range of EPE-related weather systems across
climate zones and can thus form the basis for a new classification of EPE
regimes. The findings of this study may contribute to an improved
understanding of the atmospheric processes that lead to EPEs and may find
application in climatic studies on extreme precipitation changes in a
warming climate.

/pdf/a-global-climatological-perspective-on-the-importance-of-4apcdtcrcr.pdf

A global climatological perspective on the importance of Rossby wave breaking and intense moisture transport for extreme precipitation events

Abstract. The physical understanding and timely prediction of extreme weather events are of enormous importance to society due to their associated impacts. In this article, we highlight several types of weather extremes occurring in Europe in connection with a particular atmospheric flow pattern, known as atmospheric blocking. This flow pattern effectively blocks the prevailing westerly large-scale atmospheric flow, resulting in changing flow anomalies in the vicinity of the blocking system and persistent conditions in the immediate region of its occurrence. Blocking systems are long-lasting, quasi-stationary and self-sustaining systems that occur frequently over certain regions. Their presence and characteristics have an impact on the predictability of weather extremes and can thus be used as potential indicators. The phasing between the surface and the upper-level blocking anomalies is of major importance for the development of the extreme event. In summer, heat waves and droughts form below the blocking anticyclone primarily via large-scale subsidence that leads to cloud-free skies and, thus, persistent shortwave radiative warming of the ground. In winter, cold waves that occur during atmospheric blocking are normally observed downstream or south of these systems. Here, meridional advection of cold air masses from higher latitudes plays a decisive role. Depending on their location, blocking systems also may lead to a shift in the storm track, which influences the occurrence of wind and precipitation anomalies. Due to these multifaceted linkages, compound events are often observed in conjunction with blocking conditions. In addition to the aforementioned relations, the predictability of extreme events associated with blocking and links to climate change are assessed. Finally, current knowledge gaps and pertinent research perspectives for the future are discussed. 

Atmospheric blocking and weather extremes over the Euro-Atlantic sector – a review

As lightning-detection records lengthen and the efficiency of severe weather reporting increases, more accurate climatologies of convective hazards can be constructed. In this study we aggregate flashes from the National Lightning Detection Network (NLDN) and Arrival Time Difference long-range lightning detection network (ATDnet) with severe weather reports from the European Severe Weather Database (ESWD) and Storm Prediction Center (SPC) Storm Data on a common grid of 0.25° and 1-h steps. Each year approximately 75–200 thunderstorm hours occur over the southwestern, central, and eastern United States, with a peak over Florida (200–250 h). The activity over the majority of Europe ranges from 15 to 100 h, with peaks over Italy and mountains (Pyrenees, Alps, Carpathians, Dinaric Alps; 100–150 h). The highest convective activity over continental Europe occurs during summer and over the Mediterranean during autumn. The United States peak for tornadoes and large hail reports is in spring, preceding the maximum of lightning and severe wind reports by 1–2 months. Convective hazards occur typically in the late afternoon, with the exception of the Midwest and Great Plains, where mesoscale convective systems shift the peak lightning threat to the night. The severe wind threat is delayed by 1–2 h compared to hail and tornadoes. The fraction of nocturnal lightning over land ranges from 15% to 30% with the lowest values observed over Florida and mountains (~10%). Wintertime lightning shares the highest fraction of severe weather. Compared to Europe, extreme events are considerably more frequent over the United States, with maximum activity over the Great Plains. However, the threat over Europe should not be underestimated, as severe weather outbreaks with damaging winds, very large hail, and significant tornadoes occasionally occur over densely populated areas.

/pdf/severe-convective-storms-across-europe-and-the-united-states-4bo0pgspc3.pdf

Severe Convective Storms across Europe and the United States. Part I: Climatology of Lightning, Large Hail, Severe Wind, and Tornadoes

. Mediterranean cyclogenesis is known to be frequently linked to ridge building over the North Atlantic and subsequent anticyclonic Rossby wave breaking over Europe. But understanding of how this linkage affects the medium-range forecast uncertainty of Mediterranean cyclones is limited, as previous predictability studies have mainly focused on the relatively rare cases of Mediterranean cyclogenesis preceded by upstream extratropical transition of tropical cyclones. This study exploits a European Centre for Medium-Range Weather Forecasts (ECMWF) operational ensemble forecast with an uncertain potential vorticity (PV) streamer position over the Mediterranean that, 3 d after initialization, resulted in an uncertain development of the Mediterranean tropical-like cyclone (Medicane) Zorbas in September 2018. Later initializations showed substantially lower forecast uncertainties over the Mediterranean. An ad hoc clustering of the ensemble members according to the PV streamer position in the Mediterranean is used to study the upstream evolution of the synoptic to mesoscale forecast uncertainties. Cluster differences show that forecast uncertainties were amplified on the stratospheric side of a jet streak over the North Atlantic during the first day of the ensemble prediction. Subsequently, they propagated downstream and were further amplified within a short-wave perturbation along the wave guide, superimposed onto the large-scale Rossby wave pattern. After 3 d, the uncertainties reached the Mediterranean, where they resulted in a large spread in the position of the PV streamer. These uncertainties further translated into uncertainties in the position and thermal structure of the Mediterranean cyclone. In particular, the eastward displacement of the PV streamer in more than a third of the ensemble members resulted in a very different cyclone scenario. In this scenario, cyclogenesis occurred earlier than in the other members in connection to a pre-existing surface trough over the Levantine Sea. These cyclones did not develop the deep warm core typical of medicanes. It is proposed that the eastward-shifted cyclogenesis resulted in reduced values of low-level equivalent potential temperature in the cyclogenesis area. As a result, latent heating was not intense and deep enough to erode the upper-level PV anomaly and allow the formation of a deep warm core. The westward displacement led to surface cyclones that were too weak, and a medicane formed in only half of the members. The central, i.e. correct, PV streamer position resulted in the most accurate forecasts with a strong medicane in most members. This study is the first that explicitly investigates the impact of PV streamer position uncertainty for medicane development. Overall, results extend current knowledge of the role of upstream uncertainties in the medium-range predictability and unsteady forecast behavior of Mediterranean cyclones including medicanes.

/pdf/how-an-uncertain-short-wave-perturbation-on-the-north-kwwo7sz816.pdf

How an uncertain short-wave perturbation on the North Atlantic wave guide affects the forecast of an intense Mediterranean cyclone (Medicane Zorbas)

. The aim of this study is to explore the nature of potential vorticity (PV)
cutoff life cycles. While climatological frequencies of such near-tropopause cyclonic vortices are well known, their life cycle and in particular their three-dimensional evolution is poorly understood. To address this gap, a novel method is introduced that uses isentropic air parcel trajectories to track PV cutoffs as three-dimensional objects. With this method, we can distinguish the two fundamentally different PV cutoff lysis scenarios on isentropic surfaces: complete diabatic decay vs. reabsorption by the stratospheric reservoir. This method is applied to the ERA-Interim dataset (1979–2018), and the first global climatology of PV cutoffs is presented that is independent of the selection of a vertical level and identifies and tracks PV cutoffs as three-dimensional
features. More than 150 000 PV cutoff life cycles are identified and
analyzed. The climatology confirms known frequency maxima of PV cutoffs and
identifies additional bands in subtropical areas in the summer hemispheres and a circumpolar band around Antarctica. The first climatological analysis of diabatic decay and reabsorption shows that both scenarios occur equally
frequently – in contrast to the prevailing opinion that diabatic decay
dominates. Then, PV cutoffs are classified according to their position
relative to jet streams (equatorward (Type I), between two jets (Type II), and poleward (Type III)). A composite analysis shows distinct dynamical scenarios for the genesis of the three types. Type I forms due to anticyclonic Rossby wave breaking above subtropical surface anticyclones and hardly results in precipitation. Type II results from anticyclonic Rossby wave breaking in mid-latitudes in regions with split-jet conditions and is frequently accompanied by surface cyclogenesis and substantial precipitation. Type III cutoffs preferentially form due to cyclonic Rossby wave breaking within extratropical cyclones in the storm track regions. We show that important track characteristics (speed, travel distance, frequency of decay and reabsorption, isentropic levels) differ between the categories, while lifetime is similar in all categories. Finally, 12 PV cutoff genesis regions in DJF and JJA are selected to study the regional characteristics of PV cutoff life cycles. As a particularly novel aspect, the vertical evolution of PV cutoffs along the life cycle is investigated. We find that, climatologically, PV cutoffs reach their maximum vertical extent about one day after genesis in most regions. However, while in some regions PV cutoffs rapidly disappear at lower levels by diabatic decay, they can grow downward in other regions. In addition, regional differences in lifetimes, the frequencies of diabatic decay and reabsorption, and the link to surface cyclones are identified that cannot be explained only by the preferred regional occurrence of the different cutoff types as defined above. Finally, we also show that in many regions PV cutoffs can be involved in surface cyclogenesis even after their formation. This study is an important step towards quantifying fundamental dynamical
characteristics and the surface impacts of PV cutoffs. The proposed
classification according to the jet-relative position provides a useful way to improve the conceptual understanding of PV cutoff life cycles in different regions of the globe. However, these life cycles can be substantially modified by specific regional conditions.

/pdf/the-three-dimensional-life-cycles-of-potential-vorticity-dfhxcrotu2.pdf

The three-dimensional life cycles of potential vorticity cutoffs: a global and selected regional climatologies in ERA-Interim (1979–2018)

. Over three weeks in May and June 2018, an exceptionally large number of thunderstorms hit vast parts of western and central Europe, causing precipitation of up to 80 mm and several flash floods. During this time, the large-scale atmospheric circulation, which was characterized by a blocking situation over northern Europe, influenced atmospheric conditions relevant for thunderstorm development. Initially, the southwesterly flow on the western flank of the blocking anticyclone induced the advection of warm, moist, and unstably stratified air masses. Due to a low-pressure gradient associated with the blocking anticyclone, these air masses were trapped in western and central Europe, remained almost stationary and prevented a significant air mass exchange. In addition, the low-pressure gradient led to weak flow conditions in the mid-troposphere and thus to low vertical wind shear that prevented thunderstorms from developing into severe organized systems. Most of the storms formed as local-scale, relatively slow-moving single cells. However, due to the related weak propagation speed, several thunderstorms were able to produce torrential heavy rain that affected local-scale areas and triggered several flash floods. Atmospheric blocking also increased the upper-level cut-off low frequency on its upstream regions, which was up to 10 times higher than the climatological mean. Together with filaments of positive potential vorticity (PV), the cut-offs served as trigger mechanisms for a majority of the thunderstorms. For the 22-day study period, we found that more than 50 % of lightning strikes can be linked to a nearby cut-off low or PV filament. The exceptional persistence of low stability combined with weak wind speed in the mid-troposphere over three weeks has not been observed during the past 30 years.

/pdf/the-role-of-large-scale-dynamics-in-an-exceptional-sequence-2tw91w1wb7.pdf

The role of large-scale dynamics in an exceptional sequence of severe thunderstorms in Europe May–June 2018

The complex life cycles of two long-lived potential vorticity cut-offs over Europe

Abstract Forests can store large amounts of carbon and provide essential ecosystem services. Massive tree planting is thus sometimes portrayed as a panacea to mitigate climate change and related impacts. Recent controversies about the potential benefits and drawbacks of forestation have centered on the carbon storage potential of forests and the local or global thermodynamic impacts. Here we discuss how global-scale forestation and deforestation change the Earth’s energy balance, thereby affect the global atmospheric circulation and even have profound effects on the ocean circulation. We perform multicentury coupled climate model simulations in which preindustrial vegetation cover is either completely forested or deforested and carbon dioxide mixing ratio is kept constant. We show that global-scale forestation leads to a weakening and poleward shift of the Northern mid-latitude circulation, slows-down the Atlantic meridional overturning circulation, and affects the strength of the Hadley cell, whereas deforestation leads to reversed changes. Consequently, both land surface changes substantially affect regional precipitation, temperature, clouds, and surface wind patterns across the globe. The design process of large-scale forestation projects thus needs to take into account global circulation adjustments and their influence on remote climate. 

/pdf/global-forestation-and-deforestation-affect-remote-climate-2spw2m2m.pdf

Raphael Portmann

Papers

How an uncertain short-wave perturbation on the North Atlantic wave guide affects the forecast of an intense Mediterranean cyclone (Medicane Zorbas)

The three-dimensional life cycles of potential vorticity cutoffs: a global and selected regional climatologies in ERA-Interim (1979–2018)

The role of large-scale dynamics in an exceptional sequence of severe thunderstorms in Europe May–June 2018

The complex life cycles of two long-lived potential vorticity cut-offs over Europe

Global forestation and deforestation affect remote climate via adjusted atmosphere and ocean circulation