Influence of a valley exit jet on the nocturnal atmospheric boundary layer at the foothills of the Pyrenees

Abstract

The evolution of the nocturnal atmospheric boundary layer at the foothills of a major mountain range is studied, exploring the influence of thermally driven winds. Observations made in Lannemezan (in the foothills of the Pyrenees, about 10 km from the exit of the Aura Valley) during the Boundary‐Layer Late Afternoon and Sunset Turbulence (BLLAST) experimental field campaign are taken together with high‐resolution mesoscale simulations of some selected intensive observation periods. We find that the nocturnal boundary layer features in Lannemezan are different from those reported in flat terrain regions due to the influence of thermally driven winds. Close to sunset, a thermal gradient between the plain and the mountains favours the initiation of mountain–plain circulations, while at the mountain slopes locally generated downslope winds are formed. The organization of the flow in the Aura Valley generates a valley exit jet close to midnight, which propagates through the foothills while its speed and height decrease. As a result, a maximum wind speed of about 5–10 m/s from the southern sector is found in Lannemezan between 50 and 200 m above the ground, depending on the intensity and direction of the mesoscale winds. Turbulence (above and below the jet nose) and the jet features affect the relative importance of the terms of the surface energy balance and the temperature evolution at lower levels. The organization of the flow at lower levels in the Aura Valley and the thermal gradient between the valley and the foothills are the most important factors in the formation and maintenance of the valley exit jet, strongly modulated by the intensity and direction of the mesoscale winds.

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

“…The present study falls between idealized (e.g. 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).…”

Nocturnal boundary layer turbulence regimes analysis during the BLLAST campaign

“…3). At this site, they are usually generated after the near-calm conditions of the evening transition, first as shallow drainage flows, and then interrupted by the arrival of a larger and deeper mountain-plain flow with greater values of V (Román-Cascón et al, 2015;Jiménez et al, 2019). The transition between them can cause wind speed oscillations between regime 1 and 2 and shallow wind oscillations within regime 1, leading to turbulence intermittency of category A and B, respectively.…”

“…At higher levels, z 30m -z 60m , wind roses show a large fraction of winds coming from the south-east quadrant (35 % of the data), which are associated with the larger-scale mountainplain flows (MPFs). In this case the location of the maximum wind is higher, at 80 m as stated Román-Cascón et al (2015), or between 50 and 200 m corresponding to the Aura Valley exit flow (Jiménez et al, 2019). To document the SDFs and the MPFs, we present an example shown in Román-Cascón et al (2015) between noon on the 2 of July and the morning of the 3 July 2011, including nIOP09.…”

Potential and Limitations in Estimating Sensible-Heat-Flux Profiles from Consecutive Temperature Profiles Using Remotely-Piloted Aircraft Systems

“…The simulations extend over 30 h, starting at 0600 UTC of the corresponding IOP and finishing at 1200 UTC the next day. Further details of the model set-up and the simulations are presented in Jiménez and Cuxart (2014) and Jiménez et al (2019), respectively.…”