## 1 Introduction
Additional processes, such as inertial oscillations and gravity waves [26] can then contribute significantly to the turbulent kinetic energy (TKE) budget. [PERSON] and [PERSON] [15] and [PERSON] [27] presented an attempt for a non-local theory for the SBL, taking into account the effect of internal gravity waves in the free atmosphere. In addition, other small-scale processes and phenomena, such as drainage flow, radiation divergence [1; 6; 28], fog, and close interactions with the surface as well as potential snow feedback [29] further increase the complexity of the SBL. The effects of all those phenomena are neither well understood, nor sufficiently captured by MOST or its extensions [15; 30; 31; 32]. SBL conditions also impose challenges with respect to observations, as the typically weak turbulent fluxes close to the surface become difficult to measure precisely under very stable conditions. Gradient-based scaling schemes, as proposed by [20; 33; 34] and formally equivalent to the MOST approach, might overcome some of the observational issues of weak turbulent fluxes, since the vertical gradients within the SBL are usually strong and relatively easy to measure. From a modeling point of view, recent high-resolution large-eddy simulation (LES) studies have shown a lack of grid convergence under stable conditions [35; 36; 37] which might be attributed to the fact that MOST is usually applied between the surface and the first grid level in the atmosphere (i.e., typically at heights between \(1\,\mathrm{m}\) to \(10\,\mathrm{m}\)). This might violate basic assumptions for MOST, e.g., that the measurement level or the first grid level in LES cases must lie inside the inertial sublayer, in which the flow is spatially homogeneous and dissipation follows [PERSON]'s 5/3 law. Errors can be induced by the fact that turbulence is not properly resolved at the first couple of grid points adjacent to the surface. In such cases, turbulence is not fully resolved and the flow is dominated by the subgrid-scale model in use. It is often observed that this general deficiency of LES models to resolve turbulence near the surface leads to near-surface gradients that are too strong and inherently lead to an underestimation of the surface friction [38]. Field campaigns addressing the SBL generally face logistical challenges in taking measurements at remote sites that are difficult to reach and are often characterized by harsh weather conditions, especially in regard to low temperatures. In particular, observations over sea ice involve additional risks for equipment and people, e.g., due to sea ice motion and melt. Major campaigns with focus on the SBL over sea ice have included the Weddell ice station in the Austral autumn and winter of 1992 [39; 40; 41; 42], the Surface Heat Budget over the Arctic Ocean (SHEBA) in the Beaufort Sea in 1997-1998 [23; 43; 44], the drifting ice station, Tara, in the central Arctic in the spring and summer of 2007 [45; 46; 47], and the drifting station, N-ICE2015, north of Svulbard in the winter and spring of 2015 [48]. Other land-based campaigns, e.g., ARTIST [49], CASES-99 [50], GABLS [51; 52], FLOSS-II [53], the measurements at Summit Station in central Greenland [54], and recently, MATERHORN [55] have also contributed considerably to the current state of knowledge on SBLs. The typical observation methods applied in such campaigns are profile measurements using weather masts, tethersondes, and radiosondes, as well as eddy covariance (EC) measurements at one or multiple levels. Several SBL studies have also been based on manned research aircraft observations, mainly over sea ice in the Arctic [56; 57; 58; 59; 60] and the Baltic Sea [61; 62]. Manned research aircraft may also release dropsondes and apply airborne LIDARs [63]. Over the last decade, the use of Unmanned Aerial Vehicles (UAVs) has also rapidly increased in the field of atmospheric research [64; 65] and corresponding systems have been applied in ABL campaigns, both in the Arctic [66; 67; 68] and Antarctic [69; 70; 71; 72; 73].