## 5 Potential of the Data and First Results
### Surface Layer Observations  
The conditions in the SL, observed over the sea ice, are presented in Figure 7, from top to bottom as follows: (a) the turbulent friction velocity, \(u_{*}=(\overline{u^{\prime}w^{\prime}}^{2}+\overline{v^{\prime}w^{\prime}}^{2} )^{1/4}\); (b) the turbulent kinetic energy per unit mass, \(TKE=1/2\cdot(\overline{u^{\prime 2}+v^{\prime 2}}+\overline{w^{\prime 2}})\); (c) the turbulent sensible heat flux, \(H_{S}=c_{p}\cdot\rho\cdot\overline{w^{\prime}T^{\prime}}\); and (d) the turbulent latent heat flux, \(LE=\lambda\cdot\overline{w^{\prime}a^{\prime}}\). \(u_{*}\) and \(TKE\) were both highly correlated with  
Figure 6: Overview of the meteorological conditions: (**a**) temperature, \(T\), and surface pressure, \(p\); (**b**) relative, \(RH\), and specific humidity, \(q\); (**c**) wind speed, \(U\); and direction, \(Dir\); (**d**) short-wave, \(K\), long-wave, \(I\), and net radiation balance, \(Net\); (**e**) total cloud fraction and cloud base height. Gray shading indicates times of UAV operation. Note that the wind measurements over land were performed at 29 m above ground. the horizontal wind speed, ranging from values close to zero up to roughly \(0.75\,\mathrm{m}\,\mathrm{s}^{-1}\) and \(3\,\mathrm{m}^{2}\,\mathrm{s}^{-2}\), respectively. Both parameters did not show any obvious dependency on the wind direction (see Figure 6). However, when directional aspects were considered, it has to be taken into account that the sonic anemometer was facing off-shore and that a fair amount of data with flow over the island was flagged by the post-processing software due to potential flow distortion errors from the mast [92]. \(H_{S}\) was mostly negative, ranging from \(-73.6\,\mathrm{W}\,\mathrm{m}^{-2}\) to \(27.5\,\mathrm{W}\,\mathrm{m}^{-2}\). The strongest negative values of \(H_{S}\), associated with rapid cooling of the ABL, were reached under conditions with strong negative radiation balance (dominated by the outgoing long-wave radiation, \(I\), see also Figure 6d), resulting in moderate values of \(u_{*}\) or \(TKE\). Such situations are typically associated with large positive temperature gradients (not shown in detail here). However, the turbulent flux of the latent heat, \(LE\), showed very different values, ranging from \(-16.3\,\mathrm{W}\,\mathrm{m}^{-2}\) to \(37.0\,\mathrm{W}\,\mathrm{m}^{-2}\). More than half of the observed values of \(LE\) were positive. This is not surprising, as sea ice and snow are saturated surfaces. Hence, if the air relative humidity is below saturation, an upward latent heat flux may occur simultaneously with a downward sensible heat flux. Over Polar oceans, the air relative humidity is at, or very close to, saturation [43], and dry air masses are often advected over the sea ice, allowing sublimation (upward latent heat flux) even if the sensible heat flux is directed downwards. For example, during a Foehn event over the Bothnian Bay in March 2004, [96] observed a relative humidity of \(40\,\mathrm{\char 37}\) with an upward latent heat flux simultaneous to a downward sensible heat flux. In our case, the largest upward latent heat flux was observed on 17 to 18 February 2017 (Figure 7), when the relative humidity was \(70\%\) to \(80\%\) and wind was coming from the west (Figure 6). Calculation of a three-day backward trajectory applying the Meteorological Data Explorer [97] indeed suggested a Foehn event with adiabatic subsidence heating when the air mass descended down the mountain slopes in Northern Sweden. Figure 7: Time series of (**a**) 30-min averaged friction velocity, \(u_{*}\); (**b**) turbulent kinetic energy per unit mass, \(TKE\); (**c**) turbulent sensible heat flux, \(H_{S}\); and (**d**) latent heat flux, \(LE\), observed with the EC system, \(2.7\,\mathrm{m}\) above the sea ice. The quality of the data is indicated by blue and red markers for high and intermediate quality, respectively. Poor quality data is not shown. Gray shading indicates times of UAV operation.