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

Abstract

Wind patterns in main and tributary valley systems undergo diurnal variations that are very dependent on the valley's dimensions. Characterizing the down‐valley winds is important for pollutant dispersion studies. It is also a challenge for numerical simulations. This paper uses 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 sizes located in the pre‐alpine region of southeast France. The Durance Valley and its tributary valley, the Cadarache Valley, are different in depth (200 and 100 m depth, respectively), slope (0.2 and 1.2°), width (5–8 and 1–2 km) and length (>50 and 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 of up to 1–4 m s−1 with its jet nose at around 30 m in height above ground. The flow at that height initiates 2 h after sensible heat flux turnover, a time‐scale which relates to a linear wave solution for along‐valley winds. The Durance down‐valley wind involves larger scales –for both terrain features and large‐scale circulation –and is related to a mountain‐to‐plain circulation. Therefore its onset occurs with a delay of ∼6 h after sunset. It reaches speeds of 4–8 m s−1 and a depth of 175–225 m above ground. Its highest occurrence is just after sunrise. Between the two valley winds, a shear layer is observed, the origin of which could be a combination of a Cadarache valley flow return current and a Durance down‐valley wind stream redirected by orography.

Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley

“…The evening transition is close to the local sunset, which occurs at 3:50 p.m. UTC. Those delays are consistent with those observed in the Cadarache valley by Duine et al [27]. It is interesting to note that the daytime and nightime maximum wind intensities are similar.…”

Fog in heterogeneous environments: the relative importance of local and non‐local processes on radiative‐advective fog formation

“…Results over the LANFEX area are in agreement with previous studies focusing on more pronounced topography, suggesting that they are not specific to the Shropshire hills and indicate a general applicability. Indeed, the fact that narrow valleys are more exposed to drainage flows than large valleys is a known feature over complex terrain (Duine et al, 2017). Such valley circulations are impacted by the local valley geometry; the "near-basin" configuration of the Pentre valley is similar to the configuration studied by Arduini et al (2017), where a stronger cold pool developed in the upstream basin.…”

“…Drainage flows are affected by valley width; lateral constrictions are favourable to flow acceleration and enhance turbulent mixing (Zängl, 2004). This is mainly true for narrower valleys which are subject to thermally driven circulations while larger valleys are more influenced by large-scale (LS) winds (Duine et al, 2017). In this way, LS winds and pressure gradient can interact differently with valley winds (Whiteman and Doran, 1993;Banta et al, 2004).…”

Semi‐idealized simulations of wintertime flows and pollutant transport in an Alpine valley. Part II: Passive tracer tracking

“…Lehner and Gohm, 2010;Lang et al, 2015;Leukauf et al 2016;Quimbayo-Duarte et al, 2019) and real-case (e.g., Martínez et al, 2010;Jiménez and Cuxart, 2014;Jiménez et al, 2019) studies as we consider a realistic orography and a simplified initialization of the atmosphere. The use of a realistic orography is necessary as it greatly modulates local thermally driven flows (Rucker et al, 2008;Duine et al, 2017). The simplified atmospheric state (initial zero wind profile, no large-scale tendency) has the advantage of focusing on tracer dispersion by local circulations without considering large-scale coupling.…”