Characteristics of flow circulation in complex terrain and its implications on pollutant dispersion: results from “the Biferno Valley (Italy) experiment”

Flow circulation over complex terrain has been the subject of many experimental and theoretical investigations. Thermally-driven flows develop and are of practical significance under low synpotic forcing. Amongst them, thermally-driven up and down-slope flows in the presence of a breeze are among the least investigated. Understanding of main turbulence characteristics of such flows is highly relevant in dispersion modelling and in particular for the prediction of the transport and dillution of pollutants in enclosed or semi-enclosed valleys. The “Biferno Valley Project” aimed to investigate the relations between local flow circulation in a valley (the Biferno Valley, Termoli, Italy) that develops from the sea toward the mountains in central Italy and the dispersion of locally produced pollutants from a large industrial site. This paper reports on results from the first experimental campaign carried out from 28th March to 6th April 2009. A suite of four fast-response flow sensors, an aerosol lidar, and a ceilometer were employed during the experiment. Main pollutant averaged concentrations were also available from routine measurements in the valley. Data analysis showed that during the campaign two different flow regimes occur: one dominated by high synoptic forcing and one characterised by low winds and thermally-driven circulation. Under those conditions the observation of evening and morning flow transitions was possible. During these periods, intense local mixing events are perceived to take place. These mixing processes are complex; during the evening transition, up-slope stagnates and reverses and during morning transition, a number of transition scenarios appear based on local conditions. Wind speed and direction, temperature, velocity and temperature variances, and momentum and heat fluxes at or in the proximity of the morning transition were analyzed to educe the dynamics of these flow transition events. Of particular relevance is the observation of breeze circulation which adds to the typical up-slope flow circulation. The analyses allowed development of parameterizations for these types of flow to be used in flow and dispersion models.