David Grawe

Skill, correction, and downscaling of GCM-simulated precipitation

AbstractThe ability of general circulation models (GCMs) to correctly simulate precipitation is usually assessed by comparing simulated mean precipitation with observed climatologies. However, to what extent the skill in simulating average precipitation indicates how well the models represent temporal changes is unclear. A direct assessment of the latter is hampered by the fact that freely evolving climate simulations for past periods are not set up to reproduce the specific evolution of internal atmospheric variability. Therefore, model-to-real-world comparisons of time series of daily, monthly, or annual precipitation are not meaningful. Here, for the first time, the authors quantify GCM skill in simulating precipitation variability using simulations in which the temporal evolution of the large-scale atmospheric state closely matches that of the real world. This is achieved by nudging the atmospheric states in the ECHAM5 GCM, but crucially not the precipitation field itself, toward the 40-yr European Ce...

Influence of thermal effects on street canyon circulations

A numerical study has been carried out to investigate the influence of large-scale thermal effects and strong local-scale temperature gradients near the ground on the circulation inside a street canyon. The results show that the dynamical forcing dominates the circulation inside a street canyon. But this forcing is influenced by the large-scale thermal stability. Thus, atmospheric stability indirectly controls the street canyon circulation. Small temperature gradients inside the street-canyon are neutralised by the external dynamical forcing. Strong temperature gradients inside the street-canyon show an impact on the street canyon circulation. While stable stratification reduces the circulation for the building configuration investigated, convective stratification seems to intensify it.

A model system for the assessment of ambient air quality conforming to EC directives

EC framework directive 96/62/EC and its daughter directives allow the use of models to evaluate ambient-air quality. The model system M-SYS has been developed for this purpose. M-SYS employs a hierarchy of mesoscale and microscale models for both, meteorology and chemistry, to calculate air quality metrics corresponding to the limit values of the EC daughter directives. Corresponding to the directives the model system delivers maps showing concentration distributions in different spatial resolution for the different subjects of protection. M-SYS results show the resolution dependence of maximum values. Concentration fields show a large variability on spatial scales smaller than the average spacing of air quality stations. These heterogeneities cannot be resolved by the operational air quality network. Commonly used interpolation techniques can also not capture these spatial patterns as long as they lack information on the small-scale variability of wind, turbulence, and chemical conversion. At traffic-orientated sites in the urban obstacle layer, particularly large heterogeneities are found which require the use of three-dimensional obstacle resolving models.

Stadtklima in Hamburg

Weltweit lebt jeder zweite Mensch in einer Stadt; in Deutschland leben sogar etwa 76 % der Bevolkerung in stadtischen Gebieten (◉ Abb. 3.1). Schatzungen der kunftigen Bevolkerungsentwicklung zeigen eine noch starker zunehmende Urbanisierung (◉ Abb. 3.1) mit geschatzten 8 von 10 Burgern, die bis zur Mitte dieses Jahrhunderts in Deutschland in einem Stadtgebiet leben werden. Die Urbanisierung ist auch in der Metropolregion Hamburg (MRH) hoch und hoher als im weltweiten Durchschnitt. Von den etwa 4,3 Mio. Menschen (Metropolregion 2009) leben allein 55 % in den 20 grosten Stadten (mehr als 25.000 Einwohner). Daher ist es von groster Bedeutung, die Zusammenhange von Klima und Stadtklima zu verstehen und zu analysieren, wie sich beides in Zukunft entwickeln wird.

Relevance of Photolysis Frequencies Calculation Aspects to the Ozone Concentration Simulation

For the simulation of photochemically created pollutants like ozone it is essential to correctly consider reaction rates induced by short-wave radiation. In atmospheric chemistry transport models this is achieved by the use of either off- or online calculated photolysis frequencies. In this study the effect of different input parameters of a radiation model on the calculated photolysis frequencies have been investigated. In the second step an atmospheric chemistry transport model was used to assess the impact of changed photolysis frequencies on the simulation of ozone concentrations. The impact of changed radiation model input parameters on the calculated photolysis frequencies vary not only with regard to the changed parameter but also with regard to the to the species to be dissociated. Furthermore the impact of different sets of photolysis rates employed in a chemical transport simulation on the modelled concentrations is differed and likely to be less important than other aspects of the simulation like the resolution of the grid and the emissions used. Apart from major surface albedo changes (grass to snow) and extreme changes in total ozone column content for JO3 clouds are the dominating factor in modifying the photolysis frequencies especially as they feature a highly temporal and special variation. The results show that simulated maximum ozone concentrations in areas with clouds are reduced.