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The surface–atmosphere turbulent exchange fluxes are experimentally determined using the eddy-covariance method. Their estimation using profiles of the variables of interest is a less costly alternative, although restricted to certain ranges of stability and assumed to hold for relatively flat and homogeneous terrain. It relays usually on the prescription of the roughness lengths for momentum, heat and matter, the latter two being adjustable parameters with unclear physical significance. The relations are derived with data from screen level to a few tens of metres upward. The application of these expressions using data only at one level in the surface layer implies assuming zero wind speed and the land surface temperature at their respective roughness lengths. The latter is a quantity that experimentally can only be determined radiatively with a substantial uncertainty. In this work the flux-profile relationships for momentum and sensible heat are assessed over a flat site in moderately inhomogeneous complex terrain in the southern pre-Pyrenees, using data between 2 m and the surface. The main findings are that (i) the classical expressions hold in the daytime for most of the dataset, (ii) the iterative estimations using the Obukhov length and the direct ones using the bulk Richardson number provide very similar results, (iii) using a second observation of temperature avoids a radiometric measure of land surface temperature and the prescription of a thermal roughness length value, (iv) the estimations over wet terrain with high irradiance depart largely from observations. [ABSTRACT FROM AUTHOR]

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. 50 m‐wind vectors at 0400 UTC in the region covering the Aura Valley and the Pyrenees foothills where Lannemezan is located. [ABSTRACT FROM AUTHOR]

To study the evolution of the nocturnal atmospheric boundary layer in a nearly flat region at the foothills of a major mountain range, observations at the foothills of the Pyrenees during the Boundary-Layer Late Afternoon and Sunset Turbulence (BLLAST) experimental field campaign are taken together with high-resolution mesoscale simulations. The main site of the BLLAST campaign is Lannemezan, located on a nearly flat plateau at about 10 km from the exit of the Aura valley. This 40-km long valley is oriented to the north, with a width between mountain peaks of about 5 km in the middle of the valley, diminishing to 2 km at the end. Results show that under clear-sky conditions the evolution of the nocturnal boundary-layer in Lannemezan differs from what can be expected over an isolated flat terrain region due to the presence of thermally-driven winds. Locally-generated downslope winds are observed close to sunset over the plateau and as night progresses the thermal gradient between the plain and the mountains induce mountain-plain circulations. The organization of the flow in the Aura valley generates a valley exit jet close to midnight that travels through the foothills enhanced by the thermal gradient between mountain and plain already established. Once the valley exit jet propagates through the foothills, it interacts with the locally-generated circulations and decreases its speed and height. As a result, in Lannemezan a maximum of wind speed of about 5-10 m/s from the southern sector is found between 50 m and 200 m above the ground during nighttime. Results show that mesoscale winds can enhance or diminish the intensity of the valley exit jet and even advance or delay the arrival of the valley exit jet in Lannemezan.The temporal and spatial evolutions of the valley exit jet are further analyzed with observations taken during summer 2018 in the Aura valley. Three meteorological stations along the valley axes and a Doppler scanning lidar at the valley exit are used to characterize the organization of the flow at lower levels in the valley and the formation of the valley exit jet. Data from the Atmospheric Research Center (CRA) at Lannemezan are also used together with several radio-soundings launched close to sunrise (the exit valley jet is completely developed in the simulations) to better characterize the vertical evolution of the jet when it propagates through the foothills. [ABSTRACT FROM AUTHOR]