Characteristics of the Urban Heat Island in Barcelona (NE Spain) Along the 2006-2017 Period

Abstract

The main goal of this study is the characterization of the intensity of the urban heat island (UHI) in Barcelona city center along the October 2006–October 2017 period. The UHI intensity is evaluated by computing the difference in daily minimum temperatures, on a certain moment, between the city center and the peripheral or non-urban neighboring areas. It is the simplest indicator to quantify the warming of the city, in relation to the surrounding environment, especially at night. In order for the temperature differences can be attributed to the influence of urbanization, the sites to be compared must be nearby enough to have the same regional climate and their local geographical factors (altitude and distance from sea) be very similar. In this study, we compute the differences in daily minimum temperatures recorded at the meteorological station El Raval, in Barcelona downtown, and those recorded at the fix observatory of Barcelona Airport (El Prat), located in the metropolitan area. Although the airport is not placed in a rural environment, it is a broadly open area, with a high sky view factor and, consequently, with a marked night cooling.

The recording period spans 11 years, thus providing more than 4000 daily data of minimum temperature differences (ΔT_min). The frequency distribution of daily ΔT_min and their maximum and average values are investigated at annual, half-annual and seasonal scales. It is worth remarking that 94.5% of nights are warmer at the urban site (El Raval), this percentage increasing until 98.4% in winter. The maximum temperature difference is 7.1ºC and, on average, winter nights are 3.0ºC warmer at El Raval. The power spectrum shows that UHI intensity has annual and half-annual periodicities. The distribution of ΔT_min values along the calendar days permits exploring the linkages of UHI intensity to cold and hot periods of the year, especially, for the cases of extreme ΔT_min values, and for the scarce colder nights at El Raval. Finally, the influence of wind velocity recorded at the urban site in average and maximum ΔT_min is explored for different wind speed classes, at 0.2 m·s^-1 intervals. A clear decrease of average ΔT_min with increasing wind speed is observed. With respect to maximum ΔT_min values, the results indicate that a certain critical wind speed value (around 2.5 m·s^-1) has to be exceeded for an increase of wind velocity starts weakening the most intense UHI episodes.