satellite remote sensing

Landsat 7 ETM+ Thermal Image of the City of BaselWith in situ measurements at surface stations the radiation balance can only be measured for point locations and not over the whole urban area. But measurements of net radiation and heat fluxes suffer from the extreme heterogeneity of the urban landscape. One solution is the application of satellite remote sensing in combination with numerical models. Especially the thermal remote sensing of surface temperatures and the combination of this with the in situ measured data is important.

The project BUBBLE-SARAH (Satellite Analysis of Radiation And Heat Fluxes ) is closely linked to the measurements carried out at the surface but it is designed to tackle the problems in the spatial domain in the City of Basel. Scientific objectives are to compute radiation and heat fluxes in urban areas under the conditions of spatial homogeneity/heterogeneity of urban land use.

Most studies of the urban climate by remotely sensed data are focused on the surface temperature distribution by using information of the thermal infrared channel of satellites. Urban surfaces are significantly warmer compared to their surroundings. This feature is well documented since decades and it is often interpreted as the UHI effect. During daytime this is hardly to be documented with air temperature measurements in spite of higher surface temperatures. A more complex approach than a simple surface temperature interpretation is needed to analyse the UHI. The UHI effect can only be investigated in the spatial domain when considering the whole energy balance. This means that the spatially distributed net radiation is a minimum requirement and the spatially distributed heat fluxes are a further prerequisite.

A first aim of BUBBLE-SARAH is to compute the spatially distributed net radiation as a key factor for heat flux studies. Shortwave reflection and longwave emission can be computed from multi-spectral satellite data. In the meantime there a several satellite platform available which offer a very promising spatial resolution like LANDSAT-7-ETM+ and ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer). Depending on the successful launch of the European ENVISAT satellite it is also planned to try to use ENVISAT data for the analysis. The most interesting sensors of ENVISAT for this study are MERIS (Medium resolution imaging Spectrometer) which measures radiation in 15 frequency bands, ASAR (Advanced synthetic aperture Radar) and AATSR (Advanced along track scanning radiometer).

In addition to these high resolution sensors it is planned also to analyse NOAA-AVHRR and MODIS data which have a poor spatial resolution of 1000 m only, but offer a fairly good temporal resolution of several orbits per day to cover the diurnal variations of surface temperature distribution.