The Near-Calm Stable Boundary Layer

Abstract

For the near-calm stable boundary layer, nominally 2-m mean wind speed <0.5 m s −1 , the time-average turbulent flux is dominated by infrequent mixing events. These events are related to accelerations associated with wave-like motions and other more complex small-scale motions. In this regime, the relationship between the fluxes and the weak mean flow breaks down. Such near-calm conditions are common at some sites. For very weak winds and strong stratification, the characteristics of the fluctuating quantities change slowly with increasing scale and the separation between the turbulence and non-turbulent motions can become ambiguous. Therefore, a new analysis strategy is developed based on the scale dependence of selected flow characteristics, such as the ratio of the fluctuating potential energy to the kinetic energy. In contrast to more developed turbulence, correlations between fluctuating quantities are small, and a significant heat flux is sometimes carried by very weak vertical motions with large temperature fluctuations. The relation of the flux events to small-scale increases of wind speed is examined. Large remaining uncertainties are noted.

Turbulence in the nocturnal boundary layer with light and variable winds

“…These studies reveal the uncertain relationship between fluxes and gradients in conditions of weak wind and strong stratification. Mahrt (2011) found that the turbulence within the near-calm regime (nominally winds less than 0.5 m s −1 ) is not closely related to the weak mean wind, but is instead related to short-term accelerations associated with submesoscale motions. The current study extends this analysis to different features of the turbulence and employs a much larger dataset that includes more extremely stable conditions as well as a significant number of nearneutral near-calm conditions.…”

“…Generation of turbulence in very weak-wind stable conditions is poorly understood and does not satisfy existing similarity theory (Grachev et al, 2005;Basu et al, 2006;Mahrt and Vickers, 2006;Sorbjan, 2010;Mahrt, 2011). These conditions generally correspond to very small averaged turbulent heat flux, as in the radiative regime of Van de Wiel et al (2003).…”

“…For significant stratification and weak winds, Sun et al (2011) and Mahrt (2011) inferred that weak fine-scale diffusion is carried by small eddies that do not interact directly with the ground, even as low as a few metres above the ground. Significant flux is also carried by larger eddies during infrequent mixing events.…”

“…Little is known about the adjustment and decay time-scales of such turbulence except in a more idealized direct numerical simulation setting (Flores and Riley, 2011). Mahrt (2011) inferred that the characteristics of the fluctuations with weak winds and strong stratification vary gradually with scale, suggesting that a sharp separation between turbulence and non-turbulent motions is not possible. This distinction is avoided in Galperin and Sukoriansky (2010), who model the fluctuations as a mix of turbulence and waves that merge in an intermediate buoyancy subrange (Sukorianski and Galperin, 2011).…”

Innovative Strategies for Observations in the Arctic Atmospheric Boundary Layer (ISOBAR)—The Hailuoto 2017 Campaign

“…The gray vertical lines indicate events with U < 0.5 m s −1 , the threshold for the near-calm stable boundary layer (SBL), when, according to [98], the relationship between the fluxes and the weak mean flow breaks down and the use of the traditional stability parameters, e.g., ζ, Ri B , R f , becomes difficult. The dynamic stability, ζ, covers a wide range of different stabilities from weakly unstable (4%), ζ < −0.1, to stable or very stable (29%), ζ ≥ 0.05, with most observations in the near-neutral range (66%), −0.1 ≤ ζ < 0.05.…”