Stefan Petri

Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes

In recent years, the Northern Hemisphere has suffered several devastating regional summer weather extremes, such as the European heat wave in 2003, the Russian heat wave and the Indus river flood in Pakistan in 2010, and the heat wave in the United States in 2011. Here, we propose a common mechanism for the generation of persistent longitudinal planetary-scale high-amplitude patterns of the atmospheric circulation in the Northern Hemisphere midlatitudes. Those patterns—with zonal wave numbers m = 6, 7, or 8—are characteristic of the above extremes. We show that these patterns might result from trapping within midlatitude waveguides of free synoptic waves with zonal wave numbers k ≈ m. Usually, the quasistationary dynamical response with the above wave numbers m to climatological mean thermal and orographic forcing is weak. Such midlatitude waveguides, however, may favor a strong magnification of that response through quasiresonance.

Quasi-resonant circulation regimes and hemispheric synchronization of extreme weather in boreal summer

Significance The recent decade has seen an exceptional number of boreal summer weather extremes, some causing massive damage to society. There is a strong scientific debate about the underlying causes of these events. We show that high-amplitude quasi-stationary Rossby waves, associated with resonance circulation regimes, lead to persistent surface weather conditions and therefore to midlatitude synchronization of extreme heat and rainfall events. Since the onset of rapid Arctic amplification around 2000, a cluster of resonance circulation regimes is observed involving wave numbers 7 and 8. This has resulted in a statistically significant increase in the frequency of high-amplitude quasi-stationary waves with these wave numbers. Our findings provide important insights regarding the link between Arctic changes and midlatitude extremes. The recent decade has seen an exceptional number of high-impact summer extremes in the Northern Hemisphere midlatitudes. Many of these events were associated with anomalous jet stream circulation patterns characterized by persistent high-amplitude quasi-stationary Rossby waves. Two mechanisms have recently been proposed that could provoke such patterns: (i) a weakening of the zonal mean jets and (ii) an amplification of quasi-stationary waves by resonance between free and forced waves in midlatitude waveguides. Based upon spectral analysis of the midtroposphere wind field, we show that the persistent jet stream patterns were, in the first place, due to an amplification of quasi-stationary waves with zonal wave numbers 6–8. However, we also detect a weakening of the zonal mean jet during these events; thus both mechanisms appear to be important. Furthermore, we demonstrate that the anomalous circulation regimes lead to persistent surface weather conditions and therefore to midlatitude synchronization of extreme heat and rainfall events on monthly timescales. The recent cluster of resonance events has resulted in a statistically significant increase in the frequency of high-amplitude quasi-stationary waves of wave numbers 7 and 8 in July and August. We show that this is a robust finding that holds for different pressure levels and reanalysis products. We argue that recent rapid warming in the Arctic and associated changes in the zonal mean zonal wind have created favorable conditions for double jet formation in the extratropics, which promotes the development of resonant flow regimes.

Load balancing and fault tolerance in workstation clusters migrating groups of communicating processes

In the past, several process migration facilities for distributed systems have been developed. Due to the complex nature of the subject, all those facilities have limitations that make them usable for only limited classes of applications and environments. We discuss some of the usual limitations and possible solutions. Specifically, we focus on migration of groups of collaborating processes between Unix systems without kernel modifications, and from this we derive the design for a migration system. First experiences with our implementation show that we reach performance figures for the migration that are close to those of real distributed operating system.

Role of quasiresonant planetary wave dynamics in recent boreal spring-to-autumn extreme events

Significance Weather extremes are becoming more frequent and severe in many regions of the world. The physical mechanisms have not been fully identified yet, but there is growing evidence that there are connections to planetary wave dynamics. Our study shows that, in boreal spring-to-autumn 2012 and 2013, a majority of the weather extremes in the Northern Hemisphere midlatitudes were accompanied by highly magnified planetary waves with zonal wave numbers m = 6, 7, and 8. A substantial part of those waves was probably forced by subseasonal variability in the extratropical midtroposphere circulation via the mechanism of quasiresonant amplification (QRA). The results presented here support the overall hypothesis that QRA is an important mechanism driving many of the recent exceptional extreme weather events. In boreal spring-to-autumn (May-to-September) 2012 and 2013, the Northern Hemisphere (NH) has experienced a large number of severe midlatitude regional weather extremes. Here we show that a considerable part of these extremes were accompanied by highly magnified quasistationary midlatitude planetary waves with zonal wave numbers m = 6, 7, and 8. We further show that resonance conditions for these planetary waves were, in many cases, present before the onset of high-amplitude wave events, with a lead time up to 2 wk, suggesting that quasiresonant amplification (QRA) of these waves had occurred. Our results support earlier findings of an important role of the QRA mechanism in amplifying planetary waves, favoring recent NH weather extremes.

Record Balkan floods of 2014 linked to planetary wave resonance

Resonating atmospheric waves slowed down a cyclone’s movement, leading to four successive days of heavy rainfall over the Balkans. In May 2014, the Balkans were hit by a Vb-type cyclone that brought disastrous flooding and severe damage to Bosnia and Herzegovina, Serbia, and Croatia. Vb cyclones migrate from the Mediterranean, where they absorb warm and moist air, to the north, often causing flooding in central/eastern Europe. Extreme rainfall events are increasing on a global scale, and both thermodynamic and dynamical mechanisms play a role. Where thermodynamic aspects are generally well understood, there is large uncertainty associated with current and future changes in dynamics. We study the climatic and meteorological factors that influenced the catastrophic flooding in the Balkans, where we focus on large-scale circulation. We show that the Vb cyclone was unusually stationary, bringing extreme rainfall for several consecutive days, and that this situation was likely linked to a quasi-stationary circumglobal Rossby wave train. We provide evidence that this quasi-stationary wave was amplified by wave resonance. Statistical analysis of daily spring rainfall over the Balkan region reveals significant upward trends over 1950–2014, especially in the high quantiles relevant for flooding events. These changes cannot be explained by simple thermodynamic arguments, and we thus argue that dynamical processes likely played a role in increasing flood risks over the Balkans.

Baby, it's cold outside: Climate model simulations of the effects of the asteroid impact at the end of the Cretaceous

Sixty-six million years ago, the end-Cretaceous mass extinction ended the reign of the dinosaurs. Flood basalt eruptions and an asteroid impact are widely discussed causes, yet their contributions remain debated. Modeling the environmental changes after the Chicxulub impact can shed light on this question. Existing studies, however, focused on the effect of dust or used one-dimensional, noncoupled atmosphere models. Here we explore the longer-lasting cooling due to sulfate aerosols using a coupled climate model. Depending on aerosol stratospheric residence time, global annual mean surface air temperature decreased by at least 26°C, with 3 to 16 years subfreezing temperatures and a recovery time larger than 30 years. The surface cooling triggered vigorous ocean mixing which could have resulted in a plankton bloom due to upwelling of nutrients. These dramatic environmental changes suggest a pivotal role of the impact in the end-Cretaceous extinction.

Evidence for wave resonance as a key mechanism for generating high-amplitude quasi-stationary waves in boreal summer

Several recent northern hemisphere summer extremes have been linked to persistent high-amplitude wave patterns (e.g. heat waves in Europe 2003, Russia 2010 and in the US 2011, Floods in Pakistan 2010 and Europe 2013). Recently quasi-resonant amplification (QRA) was proposed as a mechanism that, when certain dynamical conditions are fulfilled, can lead to such high-amplitude wave events. Based on these resonance conditions a detection scheme to scan reanalysis data for QRA events in boreal summer months was implemented. With this objective detection scheme we analyzed the occurrence and duration of QRA events and the associated atmospheric flow patterns in 1979–2015 reanalysis data. We detect a total number of 178 events for wave 6, 7 and 8 and find that during roughly one-third of all high amplitude events QRA conditions were met for respective waves. Our analysis reveals a significant shift for quasi-stationary waves 6 and 7 towards high amplitudes during QRA events, lagging first QRA-detection by typically one week. The results provide further evidence for the validity of the QRA hypothesis and its important role in generating high amplitude waves in boreal summer.

Summertime Planetary Wave Resonance in the Northern and Southern Hemispheres

AbstractSlow-moving planetary waves of high amplitudes are often associated with persistent surface weather conditions. This persistence can lead to extreme weather events with potentially serious implications for society and nature. Quasi-resonant amplification (QRA) of planetary waves has been proposed as a mechanism to generate high-amplitude hemisphere-wide patterns of wavenumbers 6–8 in the Northern Hemisphere (NH) summer. Here this mechanism is studied in both hemispheres. Analyzing 1979–2015 reanalysis data, evidence for QRA in the Southern Hemisphere (SH) is found for wavenumbers 4 and 5. It is shown that the difference in resonating wavenumbers between hemispheres stems from the different magnitude and latitudinal shape of the respective zonal-mean zonal winds. During resonance events a strong and narrow jet is observed in both hemispheres, whereas the emergence of a second zonal mean jet at high latitudes (i.e., a “double jet”) is seen in the NH only. Strong and narrow jets can form stable, zona...

Architecture and implementation of the socket interface on top of GAMMA

GAMMA (the Genoa active message machine) is a lightweight messaging system for fast and Gigabit Ethernet. It is based on an active message-like paradigm and provides a performing, cost-effective alternative to proprietary high-speed networks, e.g. Myrinet, with the combination of low end-to-end latency and high throughput. GAMMA supports the important class of MPI based parallel applications via the MPI/GAMMA interface [G. Ciaccio], but up to now support for the also important class of socket based cluster applications is still missing. This paper describes two different approaches to how the socket interface can be adapted to GAMMA: The first is transparent for both application and GAMMA layer, the second is only transparent for the application. First performance results with the so-called GAMMA sockets are given. They show that the second approach performs almost as good as native GAMMA communication.

Scalability of multicast based synchronization methods

Consistent maintenance of distributed data is important in application areas like groupware and for runtime support for parallel computing. We examine the performance of different multicast based methods for maintaining the consistency of distributed data depending on the network topology and concurrency. Our prototype software implements ordered, reliable multicasts on top of the unreliable IP broad- or multicast with three different methods (Master-Slave, Token Exchange on Demand, Totem Single Ring). This paper shows measurement results for the efficiency and scalability of the three methods in different topologies. The measurements confirm earlier analytical results. Totem behaves well in large networks with many concurrent senders. The overhead of Token on Demand and of the Master-Slave algorithm is almost the same. Also we could not find an indication for the often-read opinion that the Master-Slave approach scales worse because of the central bottleneck.