Characterisation of valley flows within two confluent valleys under stable conditions observations from the KASCADE field experiment

International audienceWind patterns in main and tributary valley systems undergo diurnal variation that are very dependent on the valleys dimensions. Characterising the down-valley winds is important for pollutant dispersion studies and a challenge for numerical simulations. This paper uses the KASCADE (KAtabatic winds and Stability over CAdarache for Dispersion of Effluents) observations to describe down-valley flow development and characteristics in two intersecting, shallow valleys of different size located in the prealpine region of southeastern France. The Durance Valley and its tributary valley, the Cadarache Valley, are different in depth (200 vs. 100 m depth, respectively), slope (0.2 vs. 1.2 ), width (5 to 8 vs. 1 to 2 km) and length (>50 vs. 6 km). Both down-valley flows were dominant features for the period of measurements. The distinct valley scales lead to different characteristics the smaller Cadarache down-valley wind is primarily thermally driven and attains wind speeds up to 1 - 4 m s$^{-1}$ with its jet nose at around 30 m height a.g.l. The flow at that height initiates 2 hours after sensible heat flux turnover, a timescale that relates to linear wave solution for along-valley winds. The Durance down-valley wind involves larger scales - for both terrain features and large-scale circulation -, is related to a mountain-to-plain circulation and therefore its onset occurs with a delay of 6 hours after sunset. It reaches speeds of 4 - 8 m s$^{-1}$ and a depth of 175 - 225 m a.g.l. Its highest occurrence is just after sunrise. In between the two valley winds, a shear layer is observed whose origin could be a combination of a Cadarache valley flow return current and a Durance down-valley wind stream re-directed by orography