Projected changes in atmospheric circulation types inducing high intensity of the urban heat island in Iasi city

Iași, the largest urban center in northeastern Romania, with approximatively 400000 inhabitants  living in the city and its vicinity, experiences a recent development that raises concerns on the increasing summer heat stress as a result of the combined effect of urban heat island and the increase in air temperature indicated by climate scenarios. Trying to tackle this concern, in our study we wanted (1) to estimate  if  the intervals with intense UHI will be more/less frequent in the future and (2) to assess how much the UHI will exacerbate the expected increase in air temperature.

In the study we used (1) air temperature data, collected from the monitoring network of the urban climate conditions with hourly resolution, to define the intensity of UHI, (2) daily sea level pressure data for 1980-2021 from ERA-5 Land, as input data for historical GWT classification over the study region, (3) daily sea level pressure data for 2023-2100 from EUROCORDEX, as input data for projected GWT classification for RCP4.5 and RCP8.5 climate scenarios and also (4) daily minimum, mean and maximum air temperature data for 2023-2100 from RoClib dataset, to describe the projected changes in air temperature in the study region.     

As resulted from urban climate monitoring Iași city was assessed to have an urban heat island (UHI) with a mean annual intensity approaching 1°C. Beyond this annual mean intensity of UHI, the hourly UHI intensity surrpasses 3°C during 10% of the interval between 2013 and 2021, most of these intervals with high UHI intensity being concentrated during summer nights. Especially during these intervals of high intensity, the UHI will exacerbate the expected increase in air temperature towards the end of the century.

Therefore, our proposed study is designed firstly to identify, through the use of Grosswetter Typen (GWT) classification derived from COST-733 software, those atmospheric circulation types commonly associated to the maximum intensity of UHI of Iași in the past (2013-2021). Secondly, the GWT was conceived using EUROCORDEX data for 2023-2100, aiming to identify the projected variability/trends in those types leading to intense UHI nowadays. After that, using climate scenarios projections regarding air temperature, the air temperature conditions associated with weather patterns characterized by intense UHI are described.

The results indicate that in the future, on one hand the changes in the frequency of those weather patterns that are historicaly associated with intense UHI and summer night heat stress will be marginal, but on the other hand the thresholds of intense UHI and heat stress will be overpassed by other weather patterns not known before to generate heat stress.