Caratterizzazione della circolazione locale mediante analisi di dati di vento

The low level circulation is the result of non linear interaction between mesoscale and local circulations. The latter ones often prevail in the Mediterranean regions, especially in the warmer periods, and have important effects on agriculture and other forms of human activity. For example, it plays a major role in the processes of transport and diffusion of pollutants, in particular around and in the urban areas where these may affect the air quality and the health of the population. In this work, the local circulation in Rome and the surrounding area is studied, by using the statistical analysis of the wind field. This area, within the lower Tiber valley and surrounded by hills, is adjacent to the Tyrrhenian sea located to the southwest. The time series of wind data used in this study are from different sites and refer to different periods. Some of the data were recorded by the Doppler sodar systems operating in the area; the others are routine meteorological data. Moreover, meteorological information reported in the METAR (the international code to report routine, hourly weather conditions at air terminals) files, collected in the last ten years at Fiumicino Airport, were used to determine different meteorological conditions. Wind profiles recorded by 3 sodars simultaneously operating in the area were used to retrieve the vertical structure and depth of the air masses involved in the local circulation. The statistical analysis of the daily behaviour of the wind, on a monthly and seasonal basis, made the characteristics of the local wind (speed and direction) clearly emerging. The information of the direction and starting time of the wind at some locations was used to determine the origin of the circulation. The wind speed statistical analysis evidenced the relative intensity of local circulation components and the effect of their interaction. At Pratica di Mare the recording of gravity driven flows of different origin that are recorded alternatively nighttime were attributed to the effect of synoptic forcing that changes the flow trajectories and makes the one or the other prevailing at the site. The comparison among measurements at the different locations allowed to estimate the average inland propagation time of the sea breeze and to provide a reconstruction of the wind field evolution in the area. The statistical analysis of 10 years of data sampled every 5 minutes at the Fiumicino Airport provided a description of the seasonal and the interannual variability of the local circulation. In particular this analysis has shown that no relevant modification occurred in the local circulation from year to year during the period. The fog episodes, recorded at Fiumicino were correlated with the wind field. The role of the top-down convection in the nocturnal boundary layer on the fog episodes was highlighted. The annual behaviour of the occurrence of fog as well as the diurnal behaviour for different season were determined.