NYTEFOX-The NY-Ålesund TurbulencE Fiber Optic eXperiment investigating the Arctic boundary layer, Svalbard

The NY-Ålesund TurbulencE Fiber Optic eXperiment, NYTEFOX, was a field experiment at the Arctic site NyÅlesund (11.9 ◦ E, 78.9 ◦ N) and yielded a unique meteorological data set. These data describe the distribution of heat, airflows, and exchange in the Arctic boundary layer for a period of 14 days from 26 February to 10 March 2020. NYTEFOX is the first field experiment to investigate the heterogeneity of airflow and its transport in temperatures, wind, and kinetic energy in the Arctic environment using the Fiber-Optic Distributed Sensing (FODS) technique for horizontal and vertical observations. 5 FODS air temperature and wind speed were observed at a spatial resolution of 0.127m and 9s in time along a horizontal array of 700m at 1m height above ground level (agl) and along three 7m vertical profiles. Ancillary data were collected from three sonic anemometers and an acoustic profiler (miniSodar, SOund Detection And Ranging) yielding turbulent flow statistics and vertical profiles in the lowest 300 m agl, respectively. The observations from this field campaign are publicly available on Zenodo (DOI: 10.5281/zenodo.4335461) and supplement the data set operationally collected by the Basic Surface Radiation 10 Network (BSRN) meteorological data set at Ny-Ålesund, Svalbard.

The climate of Ny-Ålesund is strongly influenced by polar night and day, lasting from 24 October to 18 February and from 18 April to 24 August, respectively (Maturilli et al., 2013). Due to the low solar elevation angle in spring, the mountain ridge 75 south of Ny-Ålesund cast a shadow on the experimental area during the whole field campaign except for very short periods of direct solar radiation in the last days.
The ABL over Ny-Ålesund is determined by the presence of land-sea contrast, channeling effects induced by the fjord and the topography, and katabatic airflows from mountains and glaciers in the vicinity of the village. The local wind field is driven by orography resulting in three main wind sectors: The year-round predominant wind directions are south-east and north-west 80 corresponding to the fjord axis with the full range of wind speeds (Maturilli et al., 2013;Jocher et al., 2012;Esau and Repina, 2012, Fig. 1). High wind speeds along this axis result from strong synoptical forcing. The third main wind direction is southwest with wind speeds typically less than 5 m s −1 (Maturilli et al., 2013). Southwesterly winds are associated with katabatic flows down the Zeppelin mountain and the Brøgger glacier and orographic channeling of the flow by the Brøgger massif (Schulz, 2017). In wintertime, southwesterly winds are often accompanied by stable stratification and gravity waves excited at 85 low wind speeds (Jocher et al., 2012).

Setup
The NYTEFOX experiment was conducted at the southern perimeter of the science station Ny-Ålesund. A picture of the field installation and the settlement taken from the Zeppelin station (474 m above sea level (asl)) and the schematic setup is shown in Figure 2. The setup consisted of six main components, which are displayed in Figure 3, and combined three different sampling 90 techniques: 1. Fiber-optic distributed sensing including a horizontal array, and vertical low-and high-resolution profiles yielding air and snow temperature and wind speed (Fig. 3 1.A, 1.B, 1.C), 2. ultrasonic anemometers enabling the computation of atmospheric flux densities using the eddy covariance technique and other flow statistics (Fig. 3 2.), and 3. acoustic ground-based remote sensing (miniSodar, SOund Detection And Ranging) yielding profile measurements of wind speed and direction and turbulent mixing strength (Fig. 3 3.). The operational parameters for all sampling systems are listed in Table 1. The horizontal 95 trapezoidal fiber-optic array had a perimeter of approximately 700 m, whose corners were marked by three 10 m-tall towers and the AWIPEV Balloon House. One tower was located near the Balloon House and the AWIPEV observatory (referred to as "tower Obse" marked 'd' in Fig. 3), the second tower was located in close proximity to the AWI eddy covariance station (referred to as "tower Eddy" marked 'e' in Fig. 3), and the third south of the AWI meteorological tower at the BSRN field (referred to as "tower BSRN" marked 'i' in Fig. 3).

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Ancillary atmospheric observations of the AWIPEV station complement the above mentioned experiment-specific observational systems. These data include meteorological tower measurements from the Baseline Surface Radiation Network (BSRN) site (Maturilli, 2020a), as well as balloon-borne meteorological data from radiosondes (Maturilli, 2020b).

Fibre-Optic Distributed Sensing measurements
The Fibre-Optic Distributed Sensing (Thomas and Selker, 2021) technique can be utilized to measure the spatial and temporal  ultrasonic anemometer: flux densities, wind direction and wind speed measurements at the towers, here at the BSRN tower, 3. acoustic profiler (miniSodar): wind measurements located near the Eddy tower. and longer submeso-scale motions in space and time (Peltola et al., 2020;Thomas et al., 2012;Pfister et al., 2019;Zeeman et al., 2015). A main advantage of FODS is that it does not require assumptions of spatial homogeneity and ergodicity as it explicitely resolves thermal and dynamic structures in space and time (Mahrt et al., 2020;Pfister et al., 2021a, b;Zeeman et al., 2015). Therefore, it is a key technology for investigating spatiotemporal phenomena which cannot be observed by traditional 110 meteorological point measurements or their relatively sparse networks. ( 2006)). The frequency-shifted backscatter of a near-infrared laser pulse emitted into a fiber-optic glass core is analyzed for two spectra bands known as Stokes (red-shifted) and Anti-Stokes (blue-shifted). The ratio of their backscatter intensities is proportional to the temperature of the light-scattering portion of the fiber-optic cable, which is why this technique is more 115 commonly referred to as distributed temperature sensing (DTS).
In our setup the fiber-optic cable was assumed to be in thermal equilibrium with the air and snow temperatures, which is a reasonable assumption for the low solar-intensity environment of the polar night which helps minimizing the radiative error . Distance along the fiber-optic cable is resolved by range-gating knowing the speed of light and the length and geometry of the fiber-optic cable. This yields a resolution of 0.127 m along the cable (Tab. 1) using the highest-120 resolution DTS device currently on the market (Model Ultima DTS 5km variant, Silixa, London, United Kingdom).

FODS reference baths
Since DTS devices yield only relative temperature measurements, portions of the fiber-optic cables are guided through known and stabilized temperature environments, so-called reference calibration sections, to convert the raw Stokes/Anti-stokes ratios Peltier elements to within ± 0.06 K and observed with two independent high-accuracy platinum resistance (PT-100) thermometers embedded within the copper body next to the fiber-optic cables. The walls of the internal groove housing the fiber-optic cables were painted with a high-emmissivity paint ( = 0.95) to enhance the radiative transfer between the adjacent solid state reference parts to eliminate thermal differences. Each solid state reference bath was contained in an insulated portable case to minimize temperature fluctuations in time and across the copper core. One solid state reference bath was cooled (referred to as 135 'cold bath') while the other was heated (referred to as 'warm bath') to span the range of environmental temperatures observed within the fiber-optic array.
Additionally, an ambient (non temperature-controlled) reference bath was deployed at the Eddy tower using an insulated plastic case, whose temperature was measured by a high-precision and accuracy resistance thermometer (RBRsolo 3 T, RBR, Ottawa, ON, Canada). This bath served as an additional reference section at the far end of the PVC-coated fiber (high-resolution Artifacts were visible as spatial perturbations in the mean temperature where the fiber was in contact to solid structures, like the fiber holders, due to different heating or cooling from radiation and/or convection. Additionally, these structures subdue the variability in air temperature and hence diminish the standard deviation of temperature for adjoining fibers. In an iterative process, the section margins were manually adjusted, discarding as little FODS data as possible. Third, all unheated and heated fiber sections were spatially aligned by finding the maximum spatial cross correlation when no heating was applied.

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The necessity arises from the wind speed derivation requiring the paired fibers to be spatially aligned. Due to strong vertical gradients, even small mismatches in the vertical coordinate on the order of a single LAF bin strongly reduces data quality.
Fourth, the aligned temperatures were mapped to physical geographic coordinates (UTM, with the z-coordinate being height agl) by interpolating the values obtained for the start and end points of each fiber-optic section. As a last step, data were temporally resampled to an evenly spaced time step of nine seconds to eliminate small deviations in integration times by the 215 internal signal processing in the DTS device.

Corrections
Due to deployment-specific technical difficulties in the cold solid state calibration bath, its temperature slowly drifted over time, rendering FODS observations implausible whenever the temperature differences between warm and cold baths were small or even reversed. Hence, a criterion for quality control was established: Two-minute temporal averages of the sonic temperature 220 were compared to the most closely collocated FODS section in the three vertical low-resolution profiles. Sonic temperatures were converted into dry bulb temperatures, using slow response humidity data from the Baseline Surface Radiation Network (Maturilli et al., 2013). The fiber temperature was spatially averaged over 1 m centered at the ultrasonic anemometer mounting heights for each profile for the ascending and descending branches of the unheated fiber, resulting in a spatial average over 14 bins. Next, the first approximate derivative of the temperature difference (the change of difference between each two minute 225 interval) between the FODS and sonic temperatures was calculated and averaged across all three towers. All data exceeding |0.61| K per 2 min, defining the upper 99 th percentile, were rejected. To avoid small data snippets, data between the resulting gaps were rejected if they were shorter than 1h 17min, which was minimum duration needed for scientific interpretation in subsequent data analysis. A total of 20h 50min of data were excluded from the fiber-optic data set for both the metal-encased and PVC-coated fiber).

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The bottom of the high-resolution profile (column, PVC-coated fiber) was immersed in snow with varying density because of uneven snow drift and compaction, resulting in substantial horizontal heterogeneity across the cross section of the column.
The heterogeneity manifested itself as systematic alternating stripes of warmer and colder temperatures across each coil.
To eliminate this artifact in snow temperatures, only the side of the column was retained whose signal was most strongly  The varying maximum measuring height (missing data displayed in grey) was caused by low clouds, snowfall, and wind noise around the acoustic enclosure during the strong-wind period.

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The NYTEFOX field campaign yielded a unique near-surface and boundary-layer meteorological data set for the Arctic polar night above land, which for the first time combines observations from fiber-optic distributed temperature and wind sensing, sonic anemometry and acoustic profiling in this environment. This combination allowed for unprecedented detail in observing the horizontal and vertical thermal and dynamic structure across the land-snow-air continuum. These data can be used to