Field study of the dynamics and modelling of subgrid-scale turbulence in a stable atmospheric surface layer over a glacier

Abstract

A field experiment -the Snow Horizontal Array Turbulence Study (SnoHATS) -has been performed over an extensive glacier in Switzerland in order to study smallscale turbulence in the stable atmospheric surface layer, and to investigate the role, dynamics and modelling of the subgrid scales (SGSs) in the context of large-eddy simulations. The a priori data analysis aims at comparing the role and behaviour of the SGSs under stable conditions with previous studies under neutral or unstable conditions. It is found that the SGSs in a stable surface layer remain an important sink of temperature variance and turbulent kinetic energy from the resolved scales and carry a significant portion of the fluxes when the filter scale is larger than the distance to the wall. The fraction of SGS fluxes (out of the total fluxes) is found to be independent of stability. In addition, the stress-strain alignment is similar to the alignment under neutral and unstable conditions. The model coefficients vary considerably with stability but in a manner consistent with previous findings, which also showed that scale-dependent dynamic models can capture this variation. Furthermore, the variation of the coefficients for both momentum and heat SGS fluxes can be shown to be better explained by stability parameters based on vertical gradients, rather than vertical fluxes. These findings suggest that small-scale turbulence dynamics and SGS modelling under stable conditions share many important properties with neutral and convective conditions, and that a unified approach is thus possible. This paper concludes with a discussion of some other challenges for stable boundary-layer simulations that are not encountered in the neutral or unstable cases.

A Simple Model for the Afternoon and Early Evening Decay of Convective Turbulence Over Different Land Surfaces

“…The typical nighttime boundary layer is very different. Stable stratification associated with surface inversions tends to suppress vertical motions generated by mechanical turbulence, thus leading to much lower turbulence levels than typically observed during daytime (Fernando 2002;Cheng et al 2005;Bou-Zeid et al 2010).…”

The Near-Calm Stable Boundary Layer

“…4. Gravity waves with periods much larger than the largest turbulent time scales can lead to modulation of the turbulence as a result of clear separation between turbulence and wave scales (Finnigan et al 1984;Sun et al 2004;Meillier et al 2008;Nappo et al 2008;Tjernström et al 2009;Fritts et al 2009;Viana et al 2009Viana et al , 2010Bou-Zeid et al 2010). However, partitioning the fluctuations between turbulence, the smallest waves and other non-turbulent motions may be problematic for some situations, suggesting that these phenomena should be studied in concert (Galperin et al 2007).…”

The Effect of Scale on the Applicability of Taylor’s Frozen Turbulence Hypothesis in the Atmospheric Boundary Layer

“…For field scale measurements, one must still invoke a frozen turbulence assumption, for example in recent analyses of subgrid-scale physics for large-eddy simulation (LES) (Tong et al 1999;Porté-Agel et al 1998, 2000a,b, 2001aHiggins et al 2003Higgins et al , 2004Higgins et al , 2009Kleissl et al 2003Kleissl et al , 2004Kelly et al 2009;Bou-Zeid et al 2010;Patton et al 2011). Due to its widespread use, Taylor's hypothesis has been studied theoretically (Lumley 1965;Wyngaard and Clifford 1977;Hill 1996), with simulations (Horst et al 2004;Dosio et al 2005;Bahraminasab et al 2008;Del Alamo and Jimenez 2009;Moin 2009), with laboratory experiments (Willis and Deardorff 1976;Dahm and Southerland 1997), and consequences of Taylor's hypothesis have been inferred from field measurements (Tong 1996; Thomas 2011) but to our knowledge there has been no test of Taylor's hypothesis across the wide range of scales relevant to field measurements.…”