A study of the hourly variability of the urban heat island effect in the Greater Athens Area during summer

摘要

Measurements of air temperature and humidity in the urban canopy layer during July 2009 in 26 sites in Athens, Greece, allowed for the mapping of the hourly spatiotemporal evolution of the urban heat island (UHI) effect. City districts neighboring to the mountains to the east were the hottest during the afternoon, while being among the coolest during the early morning hours. While during the early morning some coastal sites were the hottest, the warm air plume slowly moved to the densely urbanized center of the city until 14:00-15:00, moving then further west, to the Elefsis industrial area in the afternoon. Results from the UrbClim model agree fairly well with the observations. Satellite-derived land surface temperature (LST) data from AATSR, ASTER, AVHRR and MODIS, for pixels corresponding to ground stations measuring T_(air), showed that LST can be up to 5 K lower than the respective T_(air) during nighttime, while it can be up to 15 K higher during the rest of the day. Generally, LST during late afternoon as acquired from AATSR is very near to T_(air) for all stations and all days, i.e., the AATSR LST afternoon retrieval can be used as a very good approximation of T_(air). The hourly evolution of the spatial T_(air) distribution was almost the same during days with NE Etesian flow as in days with sea breeze circulation, indicating that the mean wind flow was not the main factor controlling the diurnal UHI evolution, although it influenced the temperatures attained. No unambiguous observation of the urban moisture excess (UME) phenomenon could be made.