Obtaining accurate and above all representative meteorological measurements inside cities is not an easy task, from a scientific point of view mainly because of the extreme heterogeneity of the terrain, but also because of logistics. The WMO has established a number of different guidelines for urban meteorological measurements, however many cities do lack meteorological measurements inside the urban environment. This often makes model validation quite challenging. Precisely for this reason, our group has built up some expertise in experimental monitoring campaigns.
To get an accurate reading for air temperatures, it is essential that sensor elements be shielded from solar radiation which would cause them to heat up and thus measure the wrong temperature. Different types of radiation shields have been developed, ranging from the traditional weather huts to widely employed gill-type radiation shields. Given low-wind speed conditions in urban environments, often these types of radiation shields are not adequate enough to have an accurate reading for the air temperature, the differences may easily go up to 1.5 °C under intense solar radiation. Such errors are detrimental when accurately measuring an urban - rural temperature difference of a few degrees. For this reason we are using actively ventilated radiation shields for in-situ monitoring campaings.
Using professional Campbell Scientific monitoring equipment, we are currently operationally monitoring the urban heat island intensity in the Belgian city of Antwerp. All monitoring sites are equipped with air temperature and relative humidity probes, 2D sonic wind meters and pyranometers to measure the shortwave downwelling solar radiation. In order to reduce errors from radiation loading on the air temperature measuremens, our group uses Young aspirated radiation shields. The picture above shows our meteo stations in test setup behind the office buildings in Mol.
In addition to the fixed monitoring stations, our group posesses a number of easy-deployable autonome temperature/humidity loggers. These Onset HOBO data loggers can be used to temporarily (e.g. during a summer period or in dedicated projects) monitor the urban climate dynamics in a city. The are also housed in actively ventilated radiation shields as well for encreased accuracy and comparability between different sampling locations. On the left hand side you can see a picture of such a setup deployed during the summer of 2013 at the Flanders environmental agency air quality monitoring site in Borgerhout in the city of Antwerp.
In-situ measurements provide only a very localized (depending on the footprint) assessement of urban climatic parameters. One way to generate more spatially explicit information is via mobile measurements. During such measurement campaigns, sensors are mounted on some mobile platform (car, bike, bus...) and a number of transsects are repeately driven through the city using a GPS logger. During two experimental campaigns in the cities of Gent and Antwerp, we drove a number of transsects through the city during and after a hot day in 2012 and 2013. A resulting transsect is visualized on the left for the city of Ghent.
Where mobile measurements are a very communicative means of informing the broad public about the urban climate, care must be taking regarding several aspects. Even though active ventilation is not as crucial anymore w.r.t. in-situ measurements (afterall, the moving mobile platform provides a natural ventilation), sensors will only respond with a delay to changes in the ambient temperature. In other words : response time corrections can be important to investigate when interpreting the mobile measurement results.