Which climate for each urban context? A preliminary comparative study on urban climate prediction and measurement in different districts in Rome and Barcelona

The research progress in building energy modeling and simulation has led to the widespread diffusion of more and more sophisticated software. By contrast, there aren't such effective results when it comes to the urban-scale climate modeling, whose geometric, morphological, material and anthropogenic features clearly distinguishes it from a rural context. The clearest effect of these differences is the Heat Island phenomenon. Despite that, the weather files currently used for energy simulations practice refer to measurements gauged in out-of-town weather stations, as like airports, causing the results to be unreliable and inaccurate. The aim of this study is to prove that, despite referring to the same region, latitude or city, quite different microclimates may occur depending on urban local features. To this purpose, temperature data available in urban weather stations located in Rome and Barcelona are inspected and compared to the rural reference station, in order to evaluate the difference of Heat Island intensity in different urban context. The experimental verification is then used to evaluate a recently developed method for generating urban weather files from a rural station, the Urban Weather Generator (UWG). The experimental verification shows a maximum intensity of Urban heat island in Barcelona in July, with a Dt of 4.7° C at 18:00 local time. In the case of Rome there is a maximum Dt of 5° C in August at 17:00 local time. The comparison between measured data and calculate data show that the reliability of the UWG calculation strongly depends on the location of the urban site within the city and on its features. The temperature discrepancies decrease when the urban site is located in a fairly central position and in a rather homogeneous urban fabric. There is a systematic error during the central hours of the day which suggests an underestimation of the effect of radiation and radiative trapping.