Impact of a new railway line embankment (Mediterranean TGV) on the frequency of spring frosts in a fruit‐growing area of the Durance Valley (south of France)

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AbstractThe new Mediterranean High Speed Railway (Paris–Marseille) passes through the fruit orchards of the Durance Valley in the south of France (Provence). With radiative weather conditions (clear sky, cool wind), spatial variability of nocturnal temperature is significant especially on rough terrains (slope, depression) and in areas with an obstacle. Thus, every obstacle, such as an embankment, can modify the cold airflow and induces an accumulation upstream. In this area, the average number of frost days in spring is 1.82 days (climate normals of 1961–1990 at the Istres weather station). The aim of this article is to present a methodology adapted to fine scales (measurements and cartography) to evaluate the impact of the embankment on the frost risk.Results show that the embankment modifies local night airflow. The embankment modifies the circulation of the down‐valley wind while permitting the local gravity winds to be established more intensely and to cause an increase of cooling in topographic depressions or upstream from an obstruction. Two areas can be identified on both sides of the embankment. The first area, to the northeast of the embankment, is influenced by the down‐valley wind. Conversely, in the second area southwest of the embankment, gravity winds are established. In this area, the minimum temperature at night can be colder by more than 2 °C and this increases the risk of frost damage. Cartography showed that nearly 100 ha of fruit trees were subjected to this additional frost risk. Copyright © 2008 Royal Meteorological Society