## 5 Potential of the Data and First Results
### Case Study on Very Stable Conditions--26 to 27 February
During the evening and throughout the night until about 2:00 UTC, the general wind direction at 29 m asl was from north (Figure 11c). During the rest of the night and the morning, the direction shifted to northeast (from about 3:00 to 4:00) and finally, to southwest (from 06:00). Over the sea ice, the wind direction deviated by a few degrees toward the east in the beginning, which might have partially been caused by a small error in the azimuthal sensor alignment. Due to the weak wind speeds below the detection range of our wind vanes, i.e., \(0.6\,\mathrm{m}\,\mathrm{s}^{-1}\), a fair amount of wind direction observations over the sea ice had to be neglected during the calm and cold periods. The available data from these events revealed frequent direction shifts of more than \(90\,\mathrm{\SIUnitSymbolCelsius}\) to the east and southeast, with relatively large deviations between the three observation levels. The relative humidity (Figure 11d) and specific humidity (not shown) closely followed the pattern of the temperatures at the corresponding levels, observed at the AWS-ice. The vertical velocity variance, observed with our EC system at 2.7 m asl (lowermost panel in Figure 11), indicated very weak vertical turbulent motion in the order of \(\overline{w^{\prime 2}}=0.001\,\mathrm{m}^{2}\,\mathrm{s}^{-2}\) during cold episodes. The values were about one order of magnitude higher during the warmer phases. This supports the argument that vertical mixing, or its absence, is causing the observed oscillations. The values of \(\overline{w^{\prime 2}}\) were typically up to two orders of magnitude higher before the first event. The last multicopter profile from this night taken with the AMOR originated from 1:40 to 1:43 UTC during one of the cold and calm events. The corresponding temperature profile is shown in Figure 12a) from ground level to 200 m, together with the AWS-ice data. The AMOR's downward facing infra-red sensor confirmed the low temperatures of the ice-covered surface (\(T_{IR}=-23\,\mathrm{\SIUnitSymbolCelsius}\)). The temperature gradient within the lowermost 20 m was extremely strong with a total gradient of \(\Delta T=10\,\mathrm{\SIUnitSymbolCelsius}\). Right above, in the layer from roughly 20 m agl to 60 m agl, there was a remarkably strong variation in temperature, with a superadiabatic lapse rate from about 20 m agl to 40 m agl. The air parcel at about 30 m agl to 40 m agl had the potential to penetrate further down to a level of approximately 5 m agl to 10 m agl, assuming a dry adiabatic descent. This can be interpreted as the signature of a strong, most likely, Kelvin-Helmholtz instability, causing local mixing, which then penetrated further down, causing the SL to switch back from the cold and calm state. The time series of the attenuated backscatter from the LATAN-3M SODAR (Figure 12b) at the moment of the multicopter profile acquisition was characterized by two echoing layers: one layer within the lowest 20 m and the second one within 60 m to 100 m. The layers nicely correspond to the temperature inversion observed by the multicopter. The evolution of the attenuated backscatter profile clearly shows variability in the vertical structure of the ABL and allows for the estimation of temporal validity frame for multicopter profiles. Furthermore, the elevated inversion layer oscillated with a period of 1 h to 2 h, which could be an indication of gravity-wave activity during this night.