Analysis of the urban thermal fingerprint of a small city in an alpine valley

The temperature contrasts typically marking the occurrence of Urban Heat Island (UHI) effects in Trento – a small city lying in the Adige Valley, in the Italian side of the Alps – are investigated evaluating the differences ΔT_u-r between an urban automated weather station placed over mean roof level (T_u) and five suburban/rural weather stations (T_r), located few kilometers outside the urban boundaries all around the city. The whole dataset obtained for the analysis covers the time period October 2002- December 2008. Three of the extra-urban weather stations analyzed are located in rural areas and on the valley floor, as is the urban site, while the other two are on the eastern slopes of the valley, one in the countryside and one in a suburban area. Therefore the five extra-urban weather stations are affected by different landscape and urbanization effects, reflecting the situation of a city located in a complex topography. As a result the average UHI intensity varies considerably (between 0.6°C and 1.5°C), when different extra-urban weather stations are taken into account. Compared to intensities typically found in larger cities, these values are quite small. However the average temperature difference with the urban site takes into account all weather conditions at every hour of the day, whereas UHI intensity is highly variable depending on these factors. For this reason the influence of these variables on UHI intensity is investigated in detail.

In order to analyze the diurnal variations of UHI intensity, data are split into a daytime and a nighttime subset (simply identified by the detection of incoming solar radiation at the urban weather station). UHI is found to be essentially a nocturnal phenomenon. In fact the analysis of daytime UHI intensity shows that during this temporal subset urbanization effects do not affect significantly temperatures. On the contrary the average nighttime temperature differences between the urban and the rural weather stations are appreciable, and range between 1.5°C and 2.5°C. Only the comparison between the urban site and the suburban weather station on the sidewall does not display strong temperature differences during nighttime (less than 1°C).

UHI intensity is also analyzed on a hourly basis, in order to better understand the characteristics of its diurnal cycle. During the central hours of the day, when solar radiation is stronger, the temperature inside the urban area is on average slightly lower than at the extra-urban weather stations. Thus during these hours an “urban cool island” is likely to occur. The UHI begins to develop in the afternoon and becomes increasingly stronger till the early hours of the night, whereas it is roughly constant between 23 LST and 06 LST. During these hours maximum diurnal UHI intensities are generally reached at the extra-urban stations on the valley floor, with average values ranging between 2°C and 4°C. Considering the extra-urban weather stations located on the slopes of the valley, the maximum diurnal UHI intensities are more frequently reached some hours before, probably because the drainage flow arrives earlier on the sidewall with respect to the valley floor.

The seasonal variations of UHI intensity are evaluated analyzing the temperature differences between the urban site and the extra-urban weather stations on a monthly basis. Considering the extra-urban weather stations located on the valley floor, the seasonal variations of UHI intensity are negligible over daytime. On the other hand in the nighttime UHI intensity is slightly stronger during dry months, when weather conditions are most favorable for radiative cooling. As to the extra-urban stations located on the sidewalls, the seasonal variations of UHI intensity seem mainly amenable to the different frequency of occurrence of thermal inversions rather than to urbanization effects. In fact both during daytime and nighttime UHI intensity is lower during winter months, when thermal inversions are more likely to occur.

The dependence of UHI intensity on wind speed and cloud amount is also analyzed. To this purpose wind speed data measured at the urban site are utilized, while cloud cover observations are taken from the dataset of another weather station, located ten kilometers north-west of Trento, on the top of a mountain at 2125 m MSL. Temperature differences between urban and extra-urban weather stations are grouped into six wind speed and five cloud cover classes. It is found that these two meteorological factors affect urbanization effects, making them weaker with stronger winds and cloudier skies.