Abstract
Abstract. The interactions among several stable-boundary-layer (SBL) processes occurring just after the evening transition of 2 July 2011 have been analysed using data from instruments deployed over the area of Lannemezan (France) during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. The near-calm situation of the afternoon was followed by the formation of local shallow drainage flows (SDFs) of less than 10 m depth at different locations. The SDF stage ended with the arrival of a stronger wind over a deeper layer more associated with the mountain-plain circulation, which caused mixing and destruction of the SDFs. Several gravity-wave-related oscillations were also observed on different time series. Wavelet analyses and wave parameters were calculated from high resolution and accurate surface pressure data of an array of microbarometers. These waves propagated relatively long distances within the SBL. The effects of these phenomena on turbulent parameters (friction velocity and kinematic heat flux) have been studied through multi-resolution flux decomposition methods performed on high frequency data from sonic anemometers deployed at different heights and locations. With this method, we were able to detect the different time-scales involved in each turbulent parameter and separate them from wave contributions, which becomes very important when choosing averaging-windows for surface flux computations using eddy covariance methods. The extensive instrumentation allowed us to highlight in detail the peculiarities of the surface turbulent parameters in the SBL, where several of the noted processes were interacting and producing important variations in turbulence with height and between sites along the sloping terrain.
Highlights
A theoretical understanding of stable boundary layers (SBLs) is still an important and unachieved challenge (Mahrt, 2014), especially for numerical weather prediction (NWP) purposes (Van de Wiel et al, 2003; Baklanov et al, 2011; Seaman et al, 2012; Holtslag et al, 2013; Davy and Esau, 2014; Fernando et al, 2015)
The wind speed decreased close to the surface around 18:55 UTC, with values below 0.5 m s−1 at the divergence site (Fig. 2a, near-calm stage). This site will be the reference site for the shallow drainage flows (SDFs) analysis due to the availability of six sonic anemometers from 0.8 to 8 m above ground level (a.g.l.). This situation of near-calm is propitious for the appearance of surface drainage flows (SDFs) with a markedly SSE–SE component in the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) area, which is the direction of most of the local slopes where the instrumentation of the field campaign were deployed
Several stable-boundary-layer processes occurring along the afternoon and evening transition during the 2 July 2011 (IOP 10) of the BLLAST field campaign have been analysed in detail taking advantage of the large amount of accurate and high frequency instrumentation deployed over the area of Lannemezan (France)
Summary
A theoretical understanding of stable boundary layers (SBLs) is still an important and unachieved challenge (Mahrt, 2014), especially for numerical weather prediction (NWP) purposes (Van de Wiel et al, 2003; Baklanov et al, 2011; Seaman et al, 2012; Holtslag et al, 2013; Davy and Esau, 2014; Fernando et al, 2015). Among the reasons for these difficulties is the existence of the so-called submeso or submesoscale motions (Mahrt, 2009) that coexist with weak or very weak surface fluxes conditions (Mahrt et al, 2012). These motions (which include wave-like motions in the SBL) do not belong to the mesoscale nor to turbulent or micrometeorogical scales. They are usually defined as submeso motions (Mahrt, 2014), comprising scales of less than 2 km, this limit can be quite subjective.
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