A High-Resolution and Computationally Efficient Forecasting System for Urban Temperatures: Validation against Intensive Field Campaigns in two Cities in the Italian Alps

A forecasting system composed of the Weather Research and Forecasting (WRF) model coupled with a single-layer urban canopy parameterization scheme (Giovannini et al. 2013) is implemented to perform high-resolution forecasts of the thermal field inside the urban areas of Trento and Rovereto, in the Italian Alps. Simulations with the WRF model are routinely performed at 1-km resolution over the Province of Trento, to provide suitable meteorological upper boundary conditions to the urban canopy model. The single-layer urban parameterization scheme is applied offline to downscale the WRF forecasts inside the urban area, taking into account the local characteristics of the city morphology, down to a final horizontal resolution of 100 m. Gridded maps of urban canopy parameters (UCPs) and anthropogenic heat emissions are utilized as input for the urban canopy model. UCPs are obtained with GIS techniques from LIDAR maps with a horizontal resolution of 1 m, while anthropogenic heat emissions are evaluated from detailed energy consumption and vehicular traffic data.

Results from the modeling system are validated against measurements performed during intensive field campaigns in the two cities during the summer 2016: a very dense network of portable thermohygrometers was deployed in the urban areas to evaluate intra-urban differences in the microclimatic conditions. The selection of the measurement sites was based on Local Climate Zone (LCZ) maps developed for both cities, in order to comprehensively characterize the dependence of the temperature field on urban morphology and land use.

Preliminary results show that the modelling system is able to capture microclimatic conditions peculiar of the urban canopy layer, as for example the development of an urban cool island in the morning and the presence of lower diurnal temperatures in the historical centers of the cities due to overshadowing. Moreover, both observations and model results highlight that the nighttime urban heat island strongly depends on the local urban morphology, with higher intensities where the urban morphology is more compact and smaller values in less intensively urbanized areas and inside urban parks.

This forecasting system is available on a dedicated website and will be utilized to predict comfort indexes in the urban areas. Moreover, the forecasts are expected to be used by the meteorological and public health departments as a crucial tool to support decision-making and to provide reports on the comfort conditions inside the urban areas, especially in critical situations, such as during summer heat waves, in order to prevent heat-related health risks for urban residents.

Giovannini L., D. Zardi, M. de Franceschi, 2013: Characterization of the thermal structure inside an urban canyon: field measurements and validation of a simple model. Journal of Applied Meteorology and Climatology, 52, 64-81.