Evaluation of a Novel City Climate Model – Evaluation of PALM-4U for big German Cities against Data from intensive Observation Periods

With the growing economical importance of cities, the people’s “natural” environment for working, accommodation and recreation is an urban settlement. A growing city population is associated with a high energy demand, concentrated consumption of resources, increased air pollution, and reduced natural areas. In combination with these changes and the growing importance of urban areas, problems associated with the urban climate, like the urban heat island or reduced air quality in urban regions, become more and more important for the world’s population. Such problems can dramatically influence human health and comfort. It is therefore necessary to include these aspects in sustainable and future-oriented city planning. This becomes even more important considering future impacts of climate change leading to increased threats of heat waves and declining air quality.
A perfect tool to estimate effects of the city morphology (such as building density, degree of soil sealing, facade greening, etc.) on air quality and thermal/wind comfort for urban residents, are urban climate models (UCMs). Within the joint research project MOSAIK, as part of the three-year nation-wide program Urban Climate Under Change ([UC]2) funded by the German Federal Ministry of Education and Research, a new modern UCM of unprecedented spatial resolution and computational performance has been developed. The new model PALM-4U (reads “PALM for you”  or “PALM for Urban applications”) is based on the research model PALM, a highly parallelized and optimized turbulence-resolving simulation model for atmospheric science. PALM-4U is applicable on massively-parallel computers as well as on city planners’ local PCs and workstations.
PALM-4U is able to capture influences of
•    buildings (shape, roof/wall materials, waste heat),
•    various different landcover types (e.g., cropland, gras-covered surfaces, rivers, pavements, sand, roads, etc.),
•    single trees of different shape and species or entire forests,
•    regional climate and weather influences calculated from weather-prediction models,
•    3-dimensional solar radiation and shadow casting,
•    air pollution through a sophisticated built-in air-chemistry model.
PALM-4U is the first UCM capable to explicitly simulate the effect of atmospheric turbulence (see Fig. 1), allowing for a direct analysis of, e.g., wind gusts or peak concentrations of air pollutants. PALM-4U provides maps of urban climate and bio-climate analysis including, e.g., physiological equivalent temperature (PET, see Fig. 2). The built-in multi-agent model allows to study environmental effects on city dwellers by directly simulating individual people walking through the streets experiencing the simulated climate. This will help city planners to identify areas with high heat-stress potential. Mitigation strategies can then be planned more efficiently for these identified hot-spots. Embedded in the planning process, PALM-4U can even help to prevent the creation of new hot-spots by simulating future building scenarios. 
In order to gain strong confidence in the results of PALM-4U, extensive evaluation is required. This is the main goal of this project. Broad measurement campaigns were conducted within the cities of Berlin, Stuttgart and Hamburg to gain reference data. In total, four large simulations were conducted at the HLRN super-computing centres. First evaluation results already show high agreement between PALM-4U data and observations. Evaluation runs for complete cities are typically carried out on 10.000 cores and require several days wallclock time to simulate a daily cycle of 24 hours.