A model-aided investigation of winter thermally driven circulation on the Italian Tyrrhenian coast: A case study

Data collected during a campaign carried out in and around Rome in February 1996 using sodars, tethered balloon, and surface data suggest the development of a winter sea breeze that could not be detected by conventional data and analyses. A model-aided study is performed to verify the development of thermally driven local circulation and to prove the onset of a winter sea breeze regime, propagating inland and interacting with the urban heat island (UHI). The area of interest for the study is the central part of Italy (latitude 41degrees-43degreesN, longitude 10degrees-16degreesE). The MM5 mesoscale model was used at high resolution: three nested domains with up to 3 km of grid resolution were used in the area where the sea breeze was observed. To highlight the effect of the sea breeze, a simulation with a thick cloud layer in the innermost domain was performed. By removing the cloud layer the diurnal evolution of the horizontal temperature gradient led to the development of the sea breeze and the UHI circulation, in agreement with the data. Sensitivity tests were carried out to evaluate the effect of land use resolution and of satellite-retrieved/climatological sea surface temperature on the simulation results. Doppler-sodar data recorded at three sites, tethersonde profiles, and surface data were used to verify the results. As expected, using high-resolution land use and the daily sea surface temperature retrieved by satellite observations led to improvements in the local circulation produced by the model. However, the model falls short in reproducing some of the sea breeze characteristics. It finds that the sea breeze starts too early and lasts for a shorter time than the observed one does, and the wind direction shows a too strong northerly component. Also, it slightly underestimates the daily temperature pattern. The interaction of the urban heat islands of Rome and Ostia with the sea breeze flow is analyzed to explain the causes of these discrepancies.