Lorenzo Giovannini

Analysis of the Urban Thermal Fingerprint of the City of Trento in the Alps

AbstractThe temperature contrasts typically marking urban heat island (UHI) effects in the city of Trento, Italy, located in an Alpine valley and inhabited in its inner urban area by a population of about 56 000, are investigated. Time series of air temperature data, collected at an urban weather station, in the city center, and at five extraurban stations are compared. The latter are representative of rural and suburban areas, both on the valley floor and on the valley sidewalls. It is found that the extraurban weather stations, being affected by different local-scale climatic conditions, display different temperature contrasts with the urban site. However, the diurnal cycle of the UHI is characterized by similar patterns of behavior at all of the extraurban weather stations: the UHI intensity is stronger at night, whereas during the central hours of the day an “urban cool island” is likely to occur. The diurnal maximum UHI intensity turns out to be typically of order 3°C, but under particularly favorabl...

Characterization of the Thermal Structure inside an Urban Canyon: Field Measurements and Validation of a Simple Model

AbstractThe results of measurement campaigns are analyzed to investigate the thermal structure in an urban canyon and to validate a simplified model simulating the air and surface temperatures from surface energy budgets. Starting from measurements at roof-top level, the model provides time series of air and surface temperatures, as well as surface fluxes. Two campaigns were carried out in summer 2007 and in winter 2008/09 in a street of the city of Trento (Italy). Temperature sensors were placed at various levels near the walls flanking the canyon and on a traffic light in the street center. Furthermore, the atmosphere above the mean roof-top level was monitored by a weather station on top of a tower located nearby. Air temperatures near the walls, being strongly influenced by direct solar radiation, display considerable contrasts between the opposite sides of the canyon. On the other hand, when solar radiation is weak or absent, the temperature field remains mostly homogeneous. Moreover, air temperature...

Optimization of Noah and Noah_MP WRF Land Surface Schemes in Snow-Melting Conditions over Complex Terrain

AbstractThe paper presents the results of high-resolution simulations performed with the WRF Model, coupled with two different land surface schemes, Noah and Noah_MP, with the aim of accurately reproducing winter season meteorological conditions in a typical Alpine valley. Accordingly, model results are compared against data collected during an intensive field campaign performed in the Adige Valley, in the eastern Italian Alps. In particular, the ability of the model in reproducing the time evolution of 2-m temperature and of incoming and outgoing shortwave and longwave radiation is examined. The validation of model results highlights that, in this context, WRF reproduces rather poorly near-surface temperature over snow-covered terrain, with an evident underestimation, during both daytime and nighttime. Furthermore it fails to capture specific atmospheric processes, such as the temporal evolution of the ground-based thermal inversion. The main cause of these errors lies in the miscalculation of the mean g...

Challenges in the application of a WRF/Urban-TRNSYS model chain for estimating the cooling demand of buildings: A case study in Bolzano (Italy)

In the present study, a WRF/Urban-TRNSYS model chain is proposed to evaluate the cooling demand of buildings located in an urban area. A case study is proposed to show the applicability of the method for a hypothetical residential building located in the city of Bolzano (Italy) on a clear-sky hot day in summer. WRF/Urban results were first validated against measurements from permanent weather stations located both in the urban area and in the surrounding countryside. Then, several TRNSYS simulations were performed, in order to assess the impact of the gridded input from WRF/Urban against both measurements from a weather station located close to the sample building and to standard data from the Test Reference Year (TRY). Compared with estimates using input data from the weather station, the daily cooling demand of the sample building estimated by WRF/Urban-TRNSYS differed by only 6% to 8%, while differences of 60% were found when using standard TRY data. Moreover, results show that energy estimates obtained by means of WRF/Urban-TRNSYS model chain satisfy the standard requirement suggested by Ashrae Guidelines 14–2002, suggesting that this model chain is a useful tool for the estimation of real buildings energy consumption.

A Refinement of the McMillen (1988) Recursive Digital Filter for the Analysis of Atmospheric Turbulence

We present a refinement of the recursive digital filter proposed by McMillen (Boundary-Layer Meteorol 43:231–245, 1988), for separating surface-layer turbulence from low-frequency fluctuations affecting the mean flow, especially over complex terrain. In fact, a straightforward application of the filter causes both an amplitude attenuation and a forward phase shift in the filtered signal. As a consequence turbulence fluctuations, evaluated as the difference between the original series and the filtered one, as well as higher-order moments calculated from them, may be affected by serious inaccuracies. The new algorithm (i) produces a rigorous zero-phase filter, (ii) restores the amplitude of the low-frequency signal, and (iii) corrects all filter-induced signal distortions.

Large eddy simulation (LES) of wind-driven circulation in a peri-alpine lake: Detection of turbulent structures and implications of a complex surrounding orography

We investigate wind-driven circulation in a peri-alpine lake (Lake Ledro - Italy) using LES-COAST. Lake Ledro is interesting because its own dimensions are suited for LES and it is surrounded by complex orography, affecting wind distribution. We consider the winter condition when stratification is nearly absent. Two types of time-varying wind stress are used: spatially homogeneous and spatially inhomogeneous respectively. The analysis of the eddy viscosities shows substantial differences with respect to the ocean case characterized by absence of coastal boundaries and homogeneous, steady wind. The quantities exhibit a noticeable inhomogeneous behavior: the horizontal eddy viscosity is larger in the water body far from the boundaries, whereas the vertical one is larger close to the lateral boundaries due to the presence of a boundary layer. The energetic bottom boundary layer, typically occurring in lakes, is not present. This because of the intrinsic unsteadiness of the thermal wind blowing over the lake and due to the absence of large amplitude internal waves, the latter present only in case of stable stratification. In the inhomogeneous wind case, up-welling and down-welling areas are not confined along the shoreline only, but are also generated in the water body due to substantial horizontal velocity divergence, and turbulent mixing, quantified by eddy viscosities, TKE and its dissipation rate, appears enhanced with respect to the homogeneous wind case. Finally, downwelling/upwelling areas along the windward/leeward coastline respectively were observed, whose quantitative estimation may give explanation for the bloom of cyanobacteria at the lake surface observed in winter.

Dispersion Modeling Over Complex Terrain in the Bolzano Basin (IT): Preliminary Results from a WRF-CALPUFF Modeling System

This chapter presents preliminary results obtained from a WRF-CALPUFF modeling system applied at a local scale, over complex terrain, in order to reproduce the dispersion of a tracer gas released from an incinerator stack.