Avoiding urban heat islands - UHI's

Initial situation

Their origins and possible effects have become a source of controversial discussions in recent years. This fact makes objective analysis of the current situation, and sober assessment proposed measures more necessary than ever. In particular, urban environments are prone to the so-called Urban Heat Islands (UHIs) phenomenon. Hot air enclosed between buildings and sealed surfaces cannot escape and cool down even during nights. With Urban population rapidly increasing, studies on UHIs are important and challenging since effective, reliable cooperation between interdisciplinary scientists, industrial facilities, and local councils is indispensable.

Analysis of requirements

Urban heat islands degrade the quality of life in many urban centers. This is why local councils of fast-growing megapoli work hand in hand with scientific communities and research facilities to identify and reduce the man-made sources of UHI by imposing forestation, employing sustainable and heat-emissive materials and strengthening public transport. Obviously, most of these measures require not only high financial burdens and interdisciplinary coordination, but also their effect may only be assessed as sensible in a long term. However, large-scale prompt measurements and progress assessments are desirable.

Simulation approaches retrieving unknown surface temperatures using mathematical me­thods, have many advantages. The costs are predictable and relatively low, even for monitoring large scenes, three-dimensional environments are more easily taken into account, and arbitrary weather events and climatic conditions can be simulated. Simulation even allows an insight into the future, not only enabling reproducing the phenomenon of UHIs but also providing guidelines for their mitigation. With this information, a city planner or a local council are able to save many resources and dispose of a real-time assessment of the measures. The essential requisite is the 4D thermal digital twin representing both the three-dimensional scene and its temporal development. The time axis reflecting growing trees, modified buildings, etc. makes up the fourth dimension.

Simulation approaches retrieving unknown surface temperatures using mathematical me­thods, have many advantages. The costs are predictable and relatively low, even for monitoring large scenes, three-dimensional environments are more easily taken into account, and arbitrary weather events and climatic conditions can be simulated. Simulation even allows an insight into the future, not only enabling reproducing the phenomenon of UHIs but also providing guidelines for their mitigation. With this information, a city planner or a local council are able to save many resources and dispose of a real-time assessment of the measures. The essential requisite is the 4D thermal digital twin representing both the three-dimensional scene and its temporal development. The time axis reflecting growing trees, modified buildings, etc. makes up the fourth dimension.

Solution

The methods developed at department Scene Analysis contribute to the solution of UHI detection and mitigation problem in two different ways: The first is three-dimensional semantic reconstruction of the scene from airborne sensor data. Landcover and material classification of the underlying data with machine learning methods for remote sensing data processing, reconstruction of buildings, and detection of individual trees are the most important subtasks we provide state-of-the-art solutions. Hereby, we strongly benefit from fruitful cooperation with architects, meteorologists, and urban planners. Small patches initially represent the 3D digital twin of the scene. Each patch is assigned a class, from which several important mathematical and physical properties are derived. Using a weather server, we retrieve atmospheric conditions, such as air temperature, cloud coverage, relative humidity. Our second contribution is the framework for modeling the surface temperature by considering conductive, convective, and radiative heat transfer. This enables the final 4D digital twin and yields the temperature value of each patch at each given time. Examining the peak and the average values of the temperature allows to draw conclusions about UHI formation. With our accurate and fast simulation, automatized assessment of future scene designs aiming UHI mitigation is possible.