Mesoscale modeling of the urban boundary layer in a coastal city. The case of Valencia.

Extreme weather events, such as heat waves, are increasingly common during summers in the
Mediterranean area. The effect of urban overheating -known as the increase of the average
temperature within the urban canopy caused by the heat storage on their surfaces thanks to their
optical and thermal characteristics, in addition to the heat emissions due to anthropogenic activities-
can cause the thermal stress of the population in the city to increase significantly during these
episodes, leading to serious health consequences. The study of this type of impact must be
quantified by means of thermal comfort indices that establish a relationship between the
meteorological conditions observed or predicted by a model and the physiological response of
human beings, such as the Universal Thermal Climate Index (UTCI). In this work, we study the
spatial-temporal behaviour of the UTCI in the city of Valencia (east coast of Spain) during a heat
wave (HW) and non-heat wave (NHW) period in August 2023 using the mesoscale meteorological
model (WRF, Weather Research and Forecasting) and in situ observations. For this purpose, a
comparison of atmospheric conditions in the planetary boundary layer (PBL) is performed, as well as
a study of the influence of temperature, shortwave and longwave radiation (from the mean radiant
temperature, TMR), wind speed and relative humidity on the behaviour of the UTCI. The main
results show a similar spatial distribution and temporal evolution of the UTCI for both periods,
differing in the magnitude of the UTCI. The positive (negative) temperature anomaly with regards to
the rest of the month is mainly the factor that causes a greater increase (decrease) in UTCI during
the HW (NHW). There is also a less developed PBL during the HW, as a consequence of the lower
intensity of the coastal breeze in the city during this period, which also has a significant effect on the
increase in UTCI during the HW.