Investigation on the climatic effect of small sloping areas of open-country on the urban climate

Research on the genesis of cold air and katabatic winds and the compensation- effect of green patches on the urban climate has been undertaken for a long time. Interestingly, research methods have not changed to a great degree after the classical basic work of KING; most recently, models and computer simulations are being used to determine cold air flow on the one hand; on the other hand manual measurements are still being used to verify and test these models. As weak climatic effects (e.g. cold air flow on slopes) are frequently well below the limit of detection of traditional measurement instrumentation, proof of such effects has always been difficult if not impossible. Using sonic anemometers, however, yields better opportunities and results for this kind of studies as they are highly sensitive, have a high temporal resolution and measure more precisely.

During the summer of 1997, measurements were made on three small areas of open-country in Schwelm (a small town in western Germany); these were carried out during clear radiation nights and the influence of open-country areas on the inner-urban temperature and air flow was determined. The study areas are in the Schwelme Valley whereas their area is in the range of 0.1 to 1.7 km2. This latter has to be emphasised as it is generally agreed that such a difference in area has no climatic compensation effect whatsoever. The approach was divided into the following questions:

Is it possible to determine the generation of a local climatic system on the small areas? If this is the case, how far does this system reach into the urban areas? Do the small areas of open country reduce the thermal load?

The investigation showed that a weak, local, climatic system developed over all small areas and that this system is characterised by cold air flow. In addition to the extent of the area that is producing cold air flow, the degree to which such an area is susceptible to external influences is a major control which is also governing the intensity of the local climatic systems.

Proof of weak cold air flows in the urban areas had only little success: The attempt to correlate data of cold air flow with those recorded in the urban, densely built-up area at the same time, did not yield any clear results. Smoke which were used additionally, did proof the non-existence of such a cold air flow into the densely built-up areas once more.

The measurements showed that even very small areas of open country do generate a distinct system of cold air flow; they also showed that these systems are able to reduce the thermal load in the nearby densely built-up areas. As a direct proof of even cold air flows inside the densely built-up area was not possible, it can be interpreted as being due to a “diffuse” effect. Therefore, even smallest areas contribute to the reduction of the urban heat island.