## 5 Potential of the Data and First Results
### Surface Layer Observations
Figure 8 shows the time series of the stability parameters: (a) Monin-Obukhov (MO) stability parameter, \(\zeta=z/L\) with \(z\) being the measurement height and \(L\) the Obukhov length, defined as \(L=-(\theta_{v}\cdot u_{s}^{3})/(x\cdot g\cdot\overline{w^{\prime}\theta_{v}^{ \prime}})\); (b) the flux Richardson Number, \(Rf=(g\cdot\overline{w^{\prime}\theta_{v}^{\prime}})/(\theta_{v}\cdot\overline{ u^{\prime}w^{\prime}}\cdot\partial\overline{U}/\partial z)\); (c) the bulk Richardson Number, \(Ri_{B}=(g\cdot\Delta\theta_{v}\cdot\Delta z)/(\overline{\theta_{v}}\cdot( \Delta U)^{2})\); (d) the difference in potential temperature, \(\Delta\theta\), between the 4 m and 1 m levels, as observed over the sea ice; and (e) the atmospheric boundary layer (ABL) height (\(h_{ABL}\)), estimated from the SODAR observations. The gray vertical lines indicate events with \(U<0.5\,\mathrm{m\,s^{-1}}\), 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., \(\zeta\), \(Ri_{B}\), \(Rf\), becomes difficult. The dynamic stability, \(\zeta\), covers a wide range of different stabilities from weakly unstable (4%), \(\zeta<-0.1\), to stable or very stable (29%), \(\zeta\geq 0.05\), with most observations in the near-neutral range (66%), \(-0.1\leq\zeta<0.05\). Figure 8: Time series of the stability parameters: (**a**) MO stabilty, \(\zeta\); (**b**) flux Richardson Number, \(Rf\); (**c**) bulk Richardson Number, \(Ri_{B}\); (**d**) the difference in potential temperature between the 4 m and 1 m levels, observed over the sea ice; and (**e**) the ABL height, \(h_{ABL}\), estimated from the SODAR (Sound Detection and Ranging) observations. The quality of the underlying eddy covariance (EC) data for \(\zeta\) and \(Rf\) is indicated by blue and red markers. The horizontal dashed lines for \(\zeta=-0.1\) and \(\zeta=0.05\) in (**a**) and \(Ri_{cr}=0.2\) in (**c**) mark the thresholds for different stability classes. The gray vertical lines mark near-calm events with \(U<0.5\,\mathrm{m\,s^{-1}}\). However, a fair number of cases (27%) with very stable stratification (\(Ri_{B}>Ri_{cr}=0.2\)) were found. All such cases were related to weak wind conditions, when \(u_{*}\) or \(\overline{u^{\prime}w^{\prime}}\) approach zero, resulting in high stability values. During the observation period, 34 cases (30-min averages) of a near-calm SBL were observed, which frequently resulted in very sharp surface inversions with potential temperature differences between the 4 m and 1 m levels reaching up to 6 \({}^{\circ}\)C and greater. \(h_{ABL}\) was typically below 100 m during these cases and reached values as low as 20 m. It has to be noted that no absolutely reliable algorithm for determining \(h_{ABL}\) from SODAR observations exists and that our estimates are partially based on human judgment and therefore, are somewhat subjective. Furthermore, no reliable estimates could be provided when the data quality of the SODAR observations was too poor or when \(h_{ABL}\) exceeded the vertical range of the instrument, i.e., \(h_{ABL}>340\) m.