## 5 Potential of the Data and First Results
### Case Study on Very Stable Conditions--26 to 27 February  
During the last night of the campaign, 26 to 27 February, we observed a very stable case, which was characterized by strong, rapid temperature changes observed at AWS-ice. Almost the entire night was cloud-free without any indications of fog or other significant weather, according to the official weather observations from AWS-FMI. The radiation balance was strongly negative, especially until 0:00 UTC, and radiative cooling was the dominant term in the surface energy balance (compare Figure 6). Figure 11 shows the corresponding time series of (a) \(T\); (b) \(U\); (c) _Dir;_ (d) \(RH\); and (e) \(\overline{w^{\prime 2}}\) from the two locations over land (except for \(\overline{w^{\prime 2}}\)) and sea ice for the period between 16:00 and 8:00 UTC. All data are based on a 1-min averaging period, except for the data from the land-based AWS-FMI, which was only available at a resolution of 10 min. Figure 11: Time series of temperature, wind speed, wind direction, relative humidity, and vertical velocity variance (from top to bottom) during the night of 26 to 27 February. The displayed data represents 10-min and 1-min averaged data of (a) \(T\); (b) \(U\); (c) _Dir;_ (d) \(q\); and (e) \(\overline{w^{\prime 2}}\) from the permanent AWS-FMI on land and the three levels and EC of the AWS-ice, respectively. Gray shading indicates times of UAV operation. Until around 23:00 UTC, the conditions were relatively stationary, with temperatures between \(-15\,\mathrm{\SIUnitSymbolCelsius}\) to \(-12\,\mathrm{\SIUnitSymbolCelsius}\) and wind speeds between \(1\,\mathrm{m}\,\mathrm{s}^{-1}\) to \(3\,\mathrm{m}\,\mathrm{s}^{-1}\) close to the ice surface and \(5\,\mathrm{m}\,\mathrm{s}^{-1}\) to \(7\,\mathrm{m}\,\mathrm{s}^{-1}\) at \(29\,\mathrm{m}\,\mathrm{s}\mathrm{l}\). The vertical gradients and local differences between land and sea ice were generally small. The 29-m wind speed decreased from \(6.5\,\mathrm{m}\,\mathrm{s}^{-1}\) to \(1.5\,\mathrm{m}\,\mathrm{s}^{-1}\), before it started increasing again at about 4:20 UTC. At about 23:10, a drop in temperature from approximately \(-13\,\mathrm{\SIUnitSymbolCelsius}\) to \(-18\,\mathrm{\SIUnitSymbolCelsius}\) was observed at the 1 m level over the sea ice (Figure 11a), accompanied by a calming of the near surface winds over the ice (Figure 11b). This initial drop happened within 1 min to 2 min, but the cooling continued and temperatures of \(-20\,\mathrm{\SIUnitSymbolCelsius}\) were reached. The same kind of changes, albeit slightly weaker and slower, were observed at the 2 m and 4 m levels, whereas the observations over the slightly elevated land remained fairly constant. The near-surface temperature and wind speed stayed at low values for about 20 min and returned to their previous states at a slower rate within approximately 5 min, starting at the top and penetrating further down. The following warmer phase with a weak flow also lasted for about 20 min. During this first cold episode, the static stability in the SL was much stronger compared to the conditions before and after the episode, with temperature differences of up to \(5\,\mathrm{\SIUnitSymbolCelsius}\) and roughly \(0.5\,\mathrm{\SIUnitSymbolCelsius}\) between the 4 m and 1 m levels, respectively. The vertical gradient of \(U\) occasionally became negative during the near-calm events, indicating a decoupling of the near-surface layers. After the first cycle of rapid temperature changes, several similar events followed, which were, however, not as clearly structured as the first one, since the 1 m level and partially, the 2 m level remained at low temperatures with very weak or calm winds. Furthermore, these following events were significantly shorter and occurred with a higher frequency. At about 6:00 UTC--just after sunrise--temperatures at all observation levels started to rise again; the vertical temperature gradient decreased and the oscillations in temperature and wind became much weaker.