Sachsen gehört zu den gewitter- und blitzreichsten Regionen Deutschlands. Durchschnittlich werden hier etwa 3,5 Blitze pro km² und Jahr gemessen (in Thüringen etwa 1,5 Blitze). Gewitter und Blitzaktivitäten bergen hohe Risiken für Umwelt und Gesellschaft. Bedeutend sind dabei die Stromstärke, die Anzahl der Blitze und Begleiterscheinungen wie Hagel, Windböen oder Starkregen. Die Studie umfasste eine grundlegenden Analyse der Blitzaktivitäten in Sachsen. Die Beobachtungsdaten seit 1999 belegen Trends zur Zunahme der Blitzhäufigkeit pro Tag und den Einfluss westlicher, südwestlicher und südlicher Anströmungen auf die Blitzaktivität. Für den Beobachtungszeitraum werden hohe jährliche Variabilitäten aufgezeigt. Offenkundige Zusammenhänge zwischen Blitzaktivität und Landnutzung bzw. Klimaparametern sind bisher nicht erkennbar. Für die Zukunft ist aber die Beeinflussung der Gewitterhäufigkeit durch die Erwärmung der Atmosphäre nicht auszuschließen.

Für die zukünftige klimatische Entwicklung in Mitteldeutschland wurden die Ergebnisse von zwölf globalen Klimamodellen ausgewertet. Im Laufe des 21. Jahrhunderts ergeben sich aus den untersuchten Simulationen für alle Jahreszeiten Temperaturzunahmen. Die Zunahmen der Jahresmitteltemperaturen bewegen sich je nach Modell und Emissionsszenario zwischen 0,5 K und 4,8 K. Während die Zunahmen im Frühjahr vergleichsweise moderat ausfallen, unterliegen die (Spät)sommer und Wintermonate einem besonders starken Erwärmungstrend. Hinsichtlich der Niederschlagsentwicklung ergeben sich insbesondere in den Sommermonaten signifikante Abnahmetrends. Besonders betroffen sind hierbei die Monate Juli und August. Deutlich zunehmende Winterniederschläge führen zu nur vergleichsweise unauffälligen Veränderungen der Jahresniederschlagssummen (Niederschlagsumverteilung). Insbesondere die kombinierte Wirkung von erhöhten Temperaturen und deutlich abnehmenden Niederschlägen sowie der damit einhergehende erhöhte Verdunstungsanspruch während der Sommermonate birgt die Gefahr angespannter Wasserhaushaltssituationen.

This study aimed to determine whether rainfall regime has driven differentiation in the Australian perennial grass, Austrodanthonia caespitosa, resulting in local ecotypes possessing characters, such as deep rootedness or summer activity, that may be particularly useful in reducing deep drainage for salinity mitigation, or whether the species shows a plastic response in root growth to soil water distribution. Rainfall regime varies within a given annual rainfall because size and ditribution of rainfall event vary. This can have an important effect on soil water distribution, both spatially and temporally. This study investigates the relationship between rainfall regime and the structure of root systems in local populations of Austrodanthonia caespitosa (Gaudich.), Firstly, it examined a number of indices useful in quantifying variation in small-scale rainfall regime, including seasonal bias, event size, event frequency, and the clustering of events, as well as how rainfall event size may be changing over time across Australia. The variation in soil water distribution that results from different rainfall regimes is expected to interact with root distribution in plants, either acting as a selective force and driving genotypic differentiation in response to soil water availability, or through plasticity in root placement. The relationship between rainfall regime and root depth distribution was examined in Austrodanthonia caespitosa (Gaudich.), or white-top wallaby grass, a perennial grass common across southern Australia. Growth and reproductive traits of plants grown from seeds collected from across the range of this species under a single rainfall regime were compared and correlated with the rainfall indices and soil type in order to establish possible abiotic explanations for trait variability. Phenological characters were found to be particularly variable between ecotypes, but high local variation between ecotypes suggested factors operating on a spatial scale smaller than the rainfall gradients are responsible for population differentiation. In order to investigate the interaction between rainfall event size and root depth, an experiment was conducted to investigate plant response to watering pulse size and frequency, with plants grown under a range of controlled watering regimes, and root depth distribution compared. The primary response in root growth was plastic, with shallow roots being developed under small, frequent events, and deep roots developed under large, infrequent waterings. Differences between ecotypes were less important, and there was no interaction between ecotype and watering treatment, indicating the same degree of plasticity in all ecotypes. Plants from a range of populations were grown under a controlled climate, first under winter conditions, then under summer conditions, with summer water withheld from half the plants, in order to determine the response to summer watering and summer drought. Plants that were watered over summer showed a strong growth response, increasing shoot biomass significantly. This effect was particularly strong in South Australian populations, which was unexpected as they originate from a region with low, unpredictable summer rainfall. Root depth was not strongly influenced by summer watering treatment. Finally, an evolutionary algorithm model was constructed in order to examine optimal plant traits under a variety of rainfall regimes. The model highlighted the importance of the interaction between rainfall regime and soil type in determining optimal root placement. Variable root cost with depth was also found to be an important trade-off to be considered, with high root loss in the surface soil layers, due to high temperatures, making a shallow rooted strategy less efficient than if root costs were equal throughout the root system. Overall, no ecotypes of A.caespitosa could be identified that had characters particularly suited to deep drainage reduction, as the drought tolerant nature of the species, and the dormancy during times of drought, may lead to low overall water use. However, it may be a useful native component in pasture systems, due to its strong growth response to summer rainfall, a characteristic found to be particularly strong in a number of South Australian ecotypes. / Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2008

In boreal summer, satellite measurements show that the central Sahara is the dustiest region of the planet. However, ground-based observations of the central Sahara have been limited to its outer edges, leaving a void in observations approximately 1 million km² in area. The Fennec Project has been the first campaign to instrument this remote but climatologically important region. This thesis uses these new observations to detect and explain the atmospheric mechanisms that make the central Sahara the summer global dust maximum. Four atmospheric mechanisms are found to cause dust storms in the central Sahara in June 2011 and June 2012. These are cold pool outflows, low-level jets (LLJs), monsoon surges and dry convective plumes. Dust may be emitted locally by these phenomena, or be advected, principally by cold pools. In both field seasons, dust emission by cold pool outflows is the most important dust mechanism, causing roughly half of the total dust loadings at the Fennec supersite of Bordj-Badji Mokhtar (BBM), the closest station to the dust maximum. The second most important mechanism is dust advection by cold pools (roughly 30% dust at BBM), followed by dust emission by monsoon surges, LLJs and finally dry convective plumes (only 2% dust at BBM). Although June 2012 was significantly more dusty than June 2011, the relative importance of the different atmospheric dust mechanisms at BBM did not change. At the automatic weather stations (AWSs) across the remote desert, cold pools and LLJs are by far the most frequently detected atmospheric dust mechanisms. LLJs are particularly common in the Atlantic Inflow in western Mauritania and in the north-easterly Harmattan in western Algeria. Cold pools are much more frequent at BBM, the station under the greatest moist monsoon influence, than at the AWSs to the north. Detection of advected dust is a particular difficulty without dedicated dust-detection instrumentation or human observers (e.g. at the AWSs). Detection of dust emission mechanisms can be very successful with only routine ground observations and satellite measurements, but quantifying the associated dust burden without dedicated dust instruments is problematic. The choice of instrumentation for dust measurement is crucial. Because cold pool outflows - the most important dust mechanism - frequently occur at night or under cloud, sun photometers miss about half of cold pool dust. Lidars have the advantage of providing height resolved dust profiles, but they suffer from attenuation in thick dust. The nephelometer proved to be the most reliable dust instrument. Although LLJs occurred on 21/28 mornings at BBM in June 2011, only five of these jets led to dust emission. Calculations of momentum exchanges through the atmospheric column show that momentum mix-down from the jet core is the cause of dust emission on these occasions, but that the LLJ has to be particularly strong (≥ 16 m s^-1) to result in dust emission at the surface. Met Office Africa-LAM underestimates monsoon LLJ wind profiles and ERA-Interim reanalysis underestimates both monsoon and Harmattan LLJ wind profiles. At the surface, the Met Office Africa-LAM and GLOBAL models significantly underpredict the frequency of observed wind speeds &gt6 m s^-1. This will cause them to significantly underestimate dust emission, as emission is a threshold process proportional to the cube of wind speed. A particularly interesting implication of the research presented here is that the central Sahara is likely much more dusty than previously thought. This is because almost all of the techniques currently used to study dust in the region are systematically biased to result in underestimates of dust burden. Cold pools are the most important dust mechanism but, since they rarely occur during the daytime or in cloud-free conditions they are often missed by sun photometers. Many will be missed by satellites that cannot retrieve below cloud and satellites that pass over the Sahara in daylight hours (e.g. the A-train). A commonly-used satellite dust detection algorithm often misses dust under moist (i.e. cold pool) conditions. Cold pools cannot be simulated by models without explicit convection, which requires very high spatial resolution. Finally, the numerical models assessed here significantly underpredict the frequency of wind speeds over the dust emission threshold. The Sahara is probably much dustier than current estimates suggest.