Observations of Wind-Direction Variability in the Nocturnal Boundary Layer

Statistical Investigation of Flow Structures in Different Regimes of the Stable Boundary Layer

“…In the latter study, the strongest wind-direction shifts were shown to occur often with a sharp decrease of temperature (a cold microfront). This was also found by Lang et al (2017) over a flat site in Australia. Moreover, Mahrt et al (2012b) attributed an observed increase of wind directional shear at the FLOSSII site for increasing stratification to advection of cold air flow due to a cold pool forming upwind of the site.…”

“…The example of F4 has a sharp change of wind direction which is simultaneous to the sharp change of temperature (8h). This is the typical signature of a microfront which is commonly found with weak winds and thin stable boundary layers (Mahrt, 2010;Lang et al, 2017). For both examples, the dynamical evolution of σ w is highly non-stationary, partly affected by the evolution of T and U (Fig.…”

Small-Scale Variability in the Nocturnal Boundary Layer

“…The difference between the wind direction and the apparent direction of the warm microfront propagation could be at least partly related to warming at the surface by the relatively "strong" downward mixing of warmer air behind the warm microfront, in which case the temperature changes are not completely controlled by horizontal advection. Large misalignment between the microfront propagation and the wind direction is common in the observational study of Lang et al (2018).…”

“…However, Monahan et al (2015) finds that traditional stability parameters alone are inadequate for partitioning the stable boundary layer. The causes of the variability of the turbulence in the more stable regime are not precisely known, although some of the variability is attributed to non-turbulent submeso motions (Acevedo et al 2014) that include microfronts and wind direction shifts (Lang et al 2018), internal gravity waves (Sun et al 2015b), nearly horizontal two-dimensional modes (Mortarini et al 2016), locally generated large-scale structures (Ansorge and Mellado 2014), and more complex modes. These motions collectively perturb the local flow (e.g., Monti et al 2002;Sun et al 2015a;Vercauteren et al 2016;Cava et al 2019a Intermittent bursts of turbulence and associated warming are common and occur on a variety of time scales in the stable boundary layer (e.g., Nappo 1991;Ohya et al 2008;Tampieri et al 2015;Burman et al 2018), although their relation to the non-turbulent motions is not always clear.…”

“…The very stable boundary layer also includes short periods of particularly low turbulence intensity and lower temperatures that may last minutes (Zeeman et al 2015) or tens of minutes (Mahrt 2017c). These micro cold events are sometimes well defined with sharp edges and arrive as microfronts (Zeeman et al 2015;Kang et al 2015;Lang et al 2018;Grudzielanek and Cermak 2018). Cold air can originate over colder surfaces related to even weak heterogeneity of the soil and vegetation (Van de Wiel et al 2002) or may be generated in cloud-free areas embedded within a general cloud cover and then advect over adjacent surfaces.…”