The Innovative Strategies for Observations in the Arctic Atmospheric Boundary Layer Project (ISOBAR) Unique Finescale Observations under Stable and Very Stable Conditions

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T he stably stratified atmospheric boundary layer (SBL; see Table A1 in the appendix for a list of abbreviations and expansions) is common in the Arctic, where the absence of solar radiation during winter causes a negative net radiation at the surface.Even during daylight seasons, the high surface albedo of snow and ice favors SBL formation (Persson et al. 2002).The SBL is of particular interest for our understanding of the Arctic climate system (e.g., Bintanja et al. 2012;Lesins et al. 2012;Davy and Esau 2016), which experiences a significantly stronger warming than the rest of the globe, commonly referred to as Arctic amplification (Serreze et al. 2009;Serreze and Barry 2011;Pithan and Mauritsen 2014;Dai et al. 2019).The state of and the processes in the ABL affect the turbulent and radiative heat fluxes from the atmosphere to Earth's surface and, accordingly, the surface mass balance of sea ice, ice sheets, glaciers, and terrestrial snow.Hence, the correct understanding and parameterization of the SBL and its coupling to the underlying snow, ice, or land surface is crucial for the reliability of climate model projections in polar regions.Another strong indication for the importance of the SBL is the fact that the observed global warming trend over the last decades is most pronounced at nighttime and in polar regions, both when SBLs prevail (McNider et al. 2010).
Climate and numerical weather prediction (NWP) models suffer from insufficient atmospheric boundary layer (ABL) parameterizations and have a strong need for an improved representation of the SBL, in particular in very stable boundary layer (VSBL) conditions.This is demonstrated by large errors under VSBL conditions, where 2-m air temperature errors (∆T 2m ) on the order of 10 K are common even in short-term (24-h) NWP products (Atlaskin and Vihma 2012).In atmospheric reanalyses, broadly applied in diagnostics of climate variability and change, the monthly/seasonal means of ∆T 2m in the Arctic (Jakobson et al. 2012;Graham et al. 2019) and Antarctic (Jonassen et al. 2019) typically show values of a few kelvins, and can even reach 20 K, strongly depending on the VSBL parameterization applied (Uppala et al. 2005).The common positive temperature biases are typically related to excessively large downward sensible heat flux (Cuxart et al. 2005;Tjernström et al. 2005), whereas large negative biases may be generated via thermal decoupling between the atmosphere and the snow/ice surface (Mahrt 2003;Uppala et al. 2005).In addition to problems in the turbulence parameterization, most climate models use a too coarse vertical resolution for an appropriate representation of the VSBL (Byrkjedal et al. 2007).

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The numerical models used for weather prediction and climate scenarios rely on turbulence closure and surface-layer exchange schemes based on Monin-Obukhov similarity theory (MOST; Monin and Obukhov 1954), which relates the nondimensional vertical gradients of wind, temperature, and humidity to their respective surface fluxes.MOST is, however, theoretically only valid for stationary, homogeneous flow fields in the atmospheric surface layer, where variations of the turbulent fluxes with height can be neglected.Because the SBL rarely satisfies these conditions, there is substantial need for improvement in the description, characterization, and parameterization of the relevant SBL processes.Moreover, empirical studies evaluating MOST commonly indicate an inability to differentiate between near-neutral and very stable regimes (Foken 2006;Sorbjan and Grachev 2010;Sorbjan 2010;Grachev et al. 2013), which is largely related to the very weak turbulent heat fluxes present in both situations.
The motivation of the Innovative Strategies for Observations in the Arctic Atmospheric Boundary Layer (ISOBAR) project is to improve our understanding of the SBL by applying new observation techniques and numerical modeling experiments, based on the collected data (see the sidebar "SBL model simulations" for an illustration of current SBL issues based on three different types of numerical models).In combination with well-established ground-based micrometeorological instrumentation and boundary layer remote sensing, we utilize multiple unmanned aircraft systems (UAS)-designed for boundary layer observations-to intensively sample the SBL over sea ice.Through this endeavor, we aim to advance our understanding of the myriad of different processes relevant under very stable stratification.The potential of such observational approaches has been emphasized in a number of SBL review articles (e.g., Fernando and Weil 2010;Mahrt 2014).In particular, we investigate the role of wave-turbulence interaction, the formation and variability of low-level jets (LLJ), intermittency, the spatiotemporal evolution of the SBL structure, and interaction between the SBL and the free atmosphere.
The ABL is in general characterized by turbulence generated by wind shear that is either enhanced or suppressed by buoyancy effects, with surface friction and surface heating or cooling as the main drivers.SBL formation is favored by clear sky and weak wind conditions, typically associated with high pressure synoptic situations characterized by large-scale subsidence and weak pressure gradients.Warm air advection may also contribute to the formation or strengthening of an SBL.In SBL research, it is common to distinguish between the weakly stable boundary layer (WSBL), where turbulence is still the dominating process, and the VSBL, in which turbulence is weak or intermittent.Transitions between WSBL and VSBL take place under clear skies when the net radiative heat loss at the surface becomes larger than the maximum turbulent heat flux that can be maintained by wind shear (Van de Wiel et al. 2017).As turbulence in the VSBL is typically weak, other processes-such as radiation divergence, surface coupling, wave phenomena, and fog-may become more important.If present, the turbulence is often intermittent.Hoch et al. (2007), Steeneveld et al. (2010), andGentine et al. (2018) address the substantial role of radiation divergence on the temperature budget under these conditions.Moreover, the lack of turbulent drag in the VSBL coincides with the emergence of LLJ.Bosveld et al. (2014) showed that even for a relatively straightforward LLJ event at Cabauw (The Netherlands), different single-column models (SCM) represent this event rather differently and with considerable biases compared to observations.In addition, gravity waves might propagate under stratified conditions and transport momentum vertically (Nappo 2012;Lapworth and Osborne 2019).The sheer number of involved processes, and their often local nature, results in a rather poor understanding of the SBL in general (Mahrt 2014).An improved understanding of the SBL archetypes and their evolution is in particular hampered by the lack of available vertical profile observations of temperature, humidity and wind speed at an appropriate vertical resolution and at high enough sampling rates, as these variables may vary strongly in time and space.

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In the WSBL, turbulence can be properly scaled following the local scaling hypothesis proposed by Nieuwstadt (1984), an extension of the original MOST.For the VSBL, classical scaling relations break down and a comprehensive theory is virtually absent.Previous studies successfully applied gradient-based scaling as a function of the gradient Richardson number, Ri (Sorbjan and Grachev 2010;Sorbjan 2010).This method is formally equivalent to MOST, but does not suffer from poorly defined scaling parameters (i.e., fluxes that are particularly difficult to measure in the VSBL) and it is also not affected by self-correlation (Sorbjan and Grachev 2010).
Further insights into the SBL are crucial for further progress in climate modeling and NWP (Holtslag et al. 2013).Atmospheric circulation models tend to require more drag at the surface than can be justified from local field observations on drag due to vertical shear (Beare 2007;Svensson and Holtslag 2009).This may be due to differences between processes captured by local observations and those acting on the scale of a grid cell, in particular over complex terrain with additional drag resulting from horizontal shear (Goger et al. 2018) or gravity waves (Steeneveld et al. 2008).Without the enhanced drag, the predicted weather systems are typically too persistent.Hence, climate and NWP models have utilized a so-called enhanced mixing approach (Louis 1979) for decades.This approach comes, however, at the cost of the representation of the SBL that is often too warm near the surface, too deep, and the modeled LLJ are often "diluted."This has large consequences for applications such as air quality modeling (Fernando and Weil 2010), road state forecasting (Karsisto et al. 2017), wind energy production (Heppelmann et al. 2017) and visibility forecasts for aviation (Román-Cascón et al. 2019).In climate models, enhanced mixing may result in a positive surface temperature bias (Holtslag et al. 2013), increasing the upwelling longwave radiation (temperature feedback) and decreasing the reflected shortwave radiation through enhanced snow and ice melt (albedo feedback).To overcome the shortcomings of the enhanced mixing approach without impacting the model performance on larger scales, future SBL parameterizations would have to take into account all sources of mechanical drag, for which detailed observations are essential.
A number of earlier field campaigns have been dedicated to SBL studies, either over midlatitude grass fields, such as the Cooperative Atmosphere-Surface Exchange Study in 1999 (CASES-99) in Kansas (Poulos et al. 2002) and Stable Atmospheric Boundary-Layer Experiment in Spain (SABLES 98) in Spain (Cuxart et al. 2000); in hilly terrain with a focus on mountain weather, such as Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) in Utah (Fernando et al. 2015); or in polar regions such as the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment in the Arctic Ocean (Uttal et al. 2002).These studies provided a wealth of observational data and their analysis offered highly valuable insights into SBL behavior.All these campaigns were, however, limited by their in situ measurements being from rather low meteorological masts and with supporting atmospheric profiling, e.g., by radiosondes, having rather poor temporal resolution.The availability of new instruments, observation techniques and measurement platforms for probing the SBL, UAS in particular, now offers unique and unrivaled opportunities for a new generation of polar SBL observations (Kral et al. 2018).
The application of unmanned, at that time remotely controlled, aircraft for atmospheric research, started at the end of the 1960s.Konrad et al. (1970) used a commercially available hobby model airplane with a wingspan of around 2.5 m to measure profiles of temperature and humidity up to 3 km above ground.About two decades later, more systematic attempts for atmospheric investigations were conducted, mainly based on relatively large military drones modified for scientific applications (Langford and Emanuel 1993;Stephens et al. 2000).A breakthrough on the path toward smaller and more cost-efficient systems was the Aerosonde, with a wingspan of 2.9 m, an overall take-off weight of 15 kg, and about 5 kg of scientific payload capacity (Holland et al. 2001).

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A rapid development of small airframes, autopilots and meteorological sensors from around 2000 is the direct result of the substantial progress in microelectronics and component miniaturization.One of the pioneering attempts was the still remotely controlled system Kali that performed more than 150 flights in Nepal and Bolivia to investigate thermally driven flows in the Himalayas and the Andes (Egger et al. 2002(Egger et al. , 2005)).During the following decade, a number of different research groups developed small meteorological UAS systems with the aim of providing reasonably priced airborne sensing capabilities for boundary layer research.Some of the most prominent examples are Small Unmanned Meteorological Observer (SUMO; Reuder et al. 2009), Meteorological Mini Aerial Vehicle (M 2 AV; Spiess et al. 2007), Multi-Purpose Airborne Sensor Carrier (MASC; Wildmann et al. 2014), Smartsonde (Chilson et al. 2009;Bonin et al. 2013), andPilatus (de Boer et al. 2015).A comprehensive overview of small UAS for atmospheric research can be found in Elston et al. (2015).
Many ABL campaigns have relied on UAS based data sampling (e.g., Houston et al. 2012;Reuder et al. 2012b;Bonin et al. 2013;Lothon et al. 2014;Reuder et al. 2016;de Boer et al. 2019).Several of the aforementioned systems have also been operated successfully in polar environments and provided unique profiles of basic meteorological parameters that have been used for process studies (Curry et al. 2004;Cassano et al. 2010;Cassano 2013;Knuth and Cassano 2014;Jonassen et al. 2015;de Boer et al. 2018), mesoscale model validation (Mayer et al. 2012b,c) and the evaluation of the benefit of UAS data assimilation (Jonassen et al. 2012;Sun et al. 2020).However, as fixed-wing systems, they have shortcomings and limitations with respect to accurate measurements in the stable surface layer close to the ground.Rotary-wing multicopter systems, with their ability to hover and to slowly ascend and descend vertically, have here clear advantages (Neumann and Bartholmai 2015;Palomaki et al. 2017;Wrenger and Cuxart 2017;Bell et al. 2020;Segales et al. 2020).
On the basis of previous field campaigns, it is evident that the SBL is often highly heterogeneous over a variety of horizontal scales (e.g., Martínez et al. 2010;Cuxart et al. 2016).Hence, we have to question the classical assumption that sampling over time at one point is equivalent to sampling instantly in space.Accordingly, there is a need for the use of mobile sensor platforms, allowing for observations over a broad range of spatial scales.In ISOBAR we respond to this need by operating a variety of UAS with different capabilities, supported by point and profile observations.

The ISOBAR17 and ISOBAR18 field campaigns
As an integral part of the ISOBAR project, we carried out two field campaigns over the sea ice of the northern Baltic Sea close to the Finnish island Hailuoto in February 2017 and 2018 (see Table 1 for a list of all participants).Hailuoto is located in the Bothnian Bay, the northernmost part of the Baltic Sea, about 20 km west of the city of Oulu (Fig. 1).It covers roughly 200 km 2 , with its highest point reaching only about 20 m MSL.Our field site was located at 65.037°N and 24.555°E, just offshore of Hailuoto Marjaniemi, the westernmost point of the island, which is also the location of a WMO weather station, operated by the Finnish Meteorological Institute (FMI).Besides the solid sea ice conditions that can be expected for the Bothnian Bay in February (Uotila et al. 2015), the daylight periods are still relatively short, favoring the VSBL development.In addition, this field site provided a solid infrastructure, easy access and the Finnish air traffic regulations allowed for an unbureaucratic flight permission process that enabled very flexible and science-driven UAS operations during the two campaigns.
The observational setup largely relied on micrometeorological masts installed on the sea ice, a few hundred meters southwest of the FMI weather station (Fig. 1).In 2017 we installed a 4-m mast on the sea ice, equipped with one eddy-covariance (EC) system, three levels of slow-response instrumentation, net radiation and its components (upward and downward for both solar shortwave and thermal longwave radiation), and two ground heat flux sensors.

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This setup was extended in 2018 by erecting a 10-m mast (referred to as GFI2), equipped with the same set of sensors and two additional EC systems.An additional 2-m mast (GFI1), consisting of an EC system and a net radiometer, was placed about 65 m to the north-northwest of the 10-m mast.The nearby WMO station provides observations of temperature, humidity, pressure, cloud base height, cloud fraction, visibility, and precipitation every 10 min at a height of 2 m AGL and observations of wind speed, wind direction, and sonic temperature at the height of 46 m MSL.Details on station location, sensor placement and specifications for the two campaigns and the different automatic weather stations are summarized in Table 2.
For continuous observations of the vertical wind profile and the turbulent structure of the lower atmosphere, we deployed a number of different ABL remote sensing systems: a vertically pointing 1D LATAN-3M sodar in 2017 and 2018 (Kouznetsov 2009;Kral et al. 2018), a Leosphere WindCube 100S (WC100s) scanning wind lidar in 2017 (Kumer et al. 2014;Kral et al. 2018), a 3D Scintec MFAS phased array sodar in 2018, and a 3D Leosphere WindCube v1 (WCv1) Doppler wind lidar in 2018 (Kumer et al. 2014(Kumer et al. , 2016)).Table 3 provides an overview of the specifications of these systems and the observed variables.
Complementing the observations from the stationary systems, we made intensive use of a number of meteorological UAS, in order to sample profiles of the most important thermodynamic and dynamic properties of the ABL and the lower free atmosphere.A summary of the different UAS and their specifications with corresponding references is given in Table 4 and Fig. 2. The three UAS shown in Fig. 2 but not listed in Table 4 were still at an experimental stage and their data were not shown in this article.
For atmospheric profiles of temperature, humidity and wind up to 1,800 m (just below flight level 65, our altitude operation limit defined by the aviation authorities) we used the fixed-wing system SUMO, with repeated profiles every 3-4 h during intensive observational periods (IOPs).Multicopter profiles based on the Bebop2Met (abbreviated B2M), Q13, and

Name
Affiliation Year CopterSonde (CS) were carried out roughly every 15-30 min during IOPs to gain profiles of the lowermost 200-300 m at high vertical resolution.To capture prevailing strong gradients within the SBL, we operated the multicopters at fairly low climb rates between 0.5 and 1 m s −1 .
The second fixed-wing UAS, MASC-2/3, measured turbulence properties along horizontal straight legs at fixed altitudes between 10 and 425 m, vertically separated by 10-25 m.An overview of the different IOPs, including a basic description of the observed conditions and the number of performed UAS flights is given in Table 5.
Postprocessing including thorough quality checks resulted in two extensive datasets on the SBL over sea ice.The overall data availability (see Fig. 3 for an overview for the different systems) was significantly improved for ISOBAR18 compared to the previous year.UAS data availability during the first days of the campaigns is very limited since the preparation of the UAS

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causing high variations in wind speed and direction.From 24 February on, the Bothnian Bay was again under the influence of high pressure, creating favorable conditions for SBL development.Relatively, the temperature was mostly mild, with only few days below −10°C.
Consistent with the mild weather, the sea ice extent of the Baltic sea in February 2017 was considerably smaller than usual (compared to a reference period of 2006-18, not shown).

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during the relatively cold period associated with the high pressure system in the beginning of the campaign.From mid-February, the large-scale flow packed the ice toward the northeast of the Bothnian Bay, resulting in a local minimum in the sea ice concentration on 18 February (Fig. 4d).Afterward, the sea ice concentration gradually increased until the end of the month (Fig. 4e).
In contrast to the varying synoptic conditions the year before, the weather during ISOBAR18 was dominated by high pressure (Fig. 4f).In February 2018 temperatures were low, winds were relatively weak and mostly from the north and there were many days with clear skies.An exception to these meteorological conditions occurred during the passage of a low pressure

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system from the North Sea toward northern Sweden and Finland around 8-16 February, resulting in strong southerly winds and temperatures up to 0°C.Before and after this period, daily mean temperatures were typically below −10°C and the wind speed was mostly low to moderate.The high pressure blocking situation during ISOBAR18 is consistent with a colder sea ice season compared to ISOBAR17, with gradually increasing sea ice concentration and thickness during the cold periods of 1-8 February (Figs.4g,h) and 15-23 February (Figs.4i,j).The Bothnian Bay was more or less ice covered throughout the ISOBAR18 campaign.
Overall, the sea ice conditions and weather situation were more favorable for the formation of VSBL during ISOBAR18.An overview of the large-scale and corresponding boundary layer conditions during the 14 IOPs is provided in Table 5.

Synthesis of UAS and ground-based in situ and remote sensing observations
The two ISOBAR field campaigns comprised a variety of observation systems, thus the synthesis of observations on the basic meteorological parameters, such as wind speed, direction,

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temperature and humidity, required carefully designed postprocessing procedures.In particular the UAS data underwent procedures for sensor calibration, reprocessing of altitude data based on observed pressure and air temperature instead of assuming a standard atmosphere lapse rate, response time correction (UAS thermodynamic parameters) and QA/QC procedures, especially for the wind estimation algorithms.Excellent examples for the quality of this synthesis are the profiles from 1510 to 1530 UTC 20 February 2018 when all four profiling UAS (SUMO, B2M, CS2, and Q13) were operated quasisimultaneously together with the ground-based observations from GFI2, FMI, MFAS and WCv1.The resulting profiles in Fig. 5, reveal a very good agreement between the different systems.All UASs and the 10-m mast sample a well-mixed layer up to ~100 m topped by a sharp inversion.The observed wind speed profiles also agree very well with light winds below 2 m s −1 in the lowermost 60 m and increasing wind speeds up to 4-5 m s −1 , peaking at about 200 m.CopterSonde, lidar (WCv1) and sodar (MFAS) show slightly higher wind speeds at this level with the CopterSonde indicating this being related to a LLJ.The SUMO did not reproduce the same peak wind speed at this level, as its wind estimation algorithm (Mayer et al. 2012a) takes data over one full circular flight track into account, which results in a smoother wind profile.Furthermore, the presented wind speed profiles from MFAS and WCv1 represent 30-min averages, whereas UAS profiles are based on quasi-instantaneous observations.Science highlights SBL evolution.During IOP-14, 1615-2030 UTC 23 February 2018, UAS based atmospheric profiling with high temporal resolution gives detailed insight into the temporal evolution of the SBL at a spatial resolution on the order of 1 m.This allows for the direct capture of a considerable portion of the turbulent fluctuations, in particular at higher levels, as the size of turbulent eddies is expected to increase with height.Hailuoto was located at the southeastern flank close to the center of the high pressure system and under the influence of weak northeasterly flow (Table 3).Clear-sky conditions favored the development of an SBL, transitioning between the weakly stable and very stable regime.Temperature profiles from the three UAS operated during this IOP, i.e., SUMO, B2M and Q13, indicate an overall cooling of the ABL associated with strengthening

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of the surface-based inversion and increase in inversion depth (Fig. 6a).The corresponding near-surface temperature observations (Fig. 6b) confirm the trend of surface cooling and intensification of the inversion, which is initiated by longwave radiative cooling after sunset.Various UAS profiles indicate remarkable, finescale structures of thermal instabilities in layers between the surface and approximately 70 m.In particular, the profiles at 1718, 1741, and 1819 UTC consistently resemble these features.At the same time, we observe a series of rapid temperature changes, most pronounced at the 0.6-and 2.0-m levels.During the cold episodes, the near-surface wind directions change from about 60° to 10° and exhibit a signature of wind veer resembling an Ekman spiral (Fig. 6b).The observed shift in wind direction occurs, however, on time scales much shorter than expected from pure Ekman adjustment, indicating the importance of local advective processes.With the geostrophic wind shifting gradually from roughly 60° to 100°, this results in a surface angle of at least 50°.Note that NWP models in the first GEWEX Atmospheric Boundary Layer Study (GABLS1) show roughly a surface angle of 30° (Svensson and Holtslag 2009), while theory of Nieuwstadt (1985) predicts 60°.The period from about 1815 until 2000 UTC is characterized by a strong surface inversion and meandering of the flow can be observed at all levels up to 46 m.The second weather station on the sea ice (GFI-1, not shown) recorded a very similar temperature and wind signal; however, the changes occur a couple of minutes earlier and the cold periods last longer.Based on these observations, we conclude that these events are related to the passage of microfronts (i.e., the advection of airmasses with different properties).The measured wind direction suggests the warmer airmass being modified by the presence of land, whereas the colder air originates from a rather clean sea ice fetch.The observed finescale instabilities in the vertical profiles lead us to the hypothesis that these microfronts are rather irregular in their shape, potentially triggered by directional shear.
Disentangling the complexity of the SBL.During IOP-10, 18-19 February 2018, groundbased in situ and remote sensing systems alongside UAS captured a variety of SBL phenomena during two major periods with very stable stratification, the first of which was from 1330 to 1615 UTC while the second was from about 1930 to 0040 UTC.The large-scale situation was characterized by a high pressure system forming in the Barents Sea and associated weak pressure gradients at its southeastern flank, but varying cloud cover (Table 5).The start of

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these periods corresponds well with strongly negative net radiation (indicated as colored shading at the top of Fig. 7a), due to clear sky conditions.The temperatures observed at GFI2 (10-m mast on the sea ice) and FMI (permanent 46-m tower) reveal strong vertical gradients during the VSBL cases and are subject to rapid variations, especially at the 4.5-and 6.9-m levels.The LATAN-3M sodar echogram indicates a surface-based turbulent layer extending to a maximum altitude of roughly 100 m, but frequently as shallow as 20 m (or even lower) and with occasional elevated turbulent layers above (Fig. 7c).The wind profile above the ABL is fairly constant with a weak flow from east-northeast (wind barbs in Fig. 7d).Within the ABL, the wind profile is, however, influenced by a variety of processes (e.g., LLJ or submesoscale motions) resulting in strong variability in both wind direction and magnitude (Fig. 7b).In general, IOP-10 was characterized by near-calm conditions, with 31% (63%) of the 10-min averaged 2-m wind speed below 0.5 m s −1 (1.0 m s −1 ), which makes the SBL susceptible to sporadic mixing events generated by wave-like and other submesoscale motions (Mahrt 2011).
In the following paragraphs we will highlight some of the observations during the subintervals I-III.The complexity of these cases (i.e., nonlinear interactions between a variety of different scales, including turbulent and nonturbulent motions) is likely to cause severe problems not only in state-of-the-art NWP but also in other atmospheric research models (e.g., Fernando and Weil 2010;Sun et al. 2015).

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IOP-10/I, IntensIfIcatIOn and cOllaPse Of the llJ.The first VSBL interval is initiated by a rapid temperature drop close to the surface of 2 K within 20-30 s (Fig. 8a), accompanied by a reduction in wind speed (Fig. 8b) and a wind direction shift of 180° from north to south (Fig. 8c).During the following minutes (until ~1400 UTC) the near-surface winds almost completely calm down, thus increasing the dynamic stability, while the flow at elevated layers around 100 m slightly accelerates and forms a weak LLJ.All three EC systems of GFI2 show weak intermittent turbulence during this period (see w� in Fig. 8d and -w� -T�in Fig. 8e).Nevertheless, the lowest layers remain at a rather constant temperature; the reason for this is not quite clear.Our mast observations, however, show small-scale oscillations in wind speed and direction at the three lowest levels, which seem rather independent of each other.Occasionally, the

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local wind and directional shear might be large enough to trigger small-scale mixing events.
At about 1535 UTC, the 10-m wind speed accelerates to about 2 m s −1 triggering a strong intermittent event, which also influences the two EC levels below, although to a weaker extent.Investigating the evolution of the vertical wind profile (Fig. 8g) based on WCv1 lidar and 10-m mast data, suggests that the acceleration of the 10-m wind is related to an increase in wind shear due to the intensification and lowering of the LLJ; eventually this causes a shear instability.The sodar echogram (Fig. 7c) supports this interpretation, as it indicates an elevated weak turbulent layer merging with lower levels around 1440 UTC, followed by an increase in turbulence below 80 m and the lowering of the elevated inversion layer (Fig. 8f).After this event, the wind speed profiles take a more logarithmic shape again.The vertical temperature profiles in Fig. 8f also feature a shift from a very shallow and strong surface-based and an additional elevated inversion to a more logarithmic profile after this event.A reduction in radiative cooling due to increased cloud cover initiates the end of this VSBL period.
IOP-10/II, near-surface wave InstabIlIty.During IOP-10 the instrumentation on the 10-m mast recorded a series of amplifying temperature oscillations, most pronounced at 4.5, 6.9, and 10.3 m (Fig. 9a).At 2234 UTC this oscillation results in a remarkable cooling of the 10.3-m temperature, dropping by 4 K within approximately 1 min.Associated with this main cooling event is a temporary shift to neutral static stability and enhanced near surface turbulence

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(Fig. 9b).The near-surface stability before this event was characterized by a sharp temperature gradient, ∆T 10-0.6m≈ 4.5 K and weak winds at about 1 m s −1 , meandering between southsouthwest and north-northeast (Fig. 9a).The three sonic anemometers of GFI2 sampled very weak to intermittent turbulence (Fig. 9b), whereas the remote sensing systems (e.g., 45-and 85-m lidar levels in Fig. 9b) indicate some wave activity aloft (see also Fig. 7b).The signature of this wave can also be detected in the 10-m vertical velocity data.
From 2232 UTC the wind at the lowermost levels shifts to a northerly direction, whereas at 10 m it stays south-southeast for two more minutes.This results in enhanced local shear as shown in Fig. 9c, while the bulk shear is still fairly weak.At the same time, the amplitude of the wave starts to grow rapidly, causing an upward transport of cold, near-surface air at the wave crest at 2233 UTC.This is also associated with a shift to near-neutral stratification as reflected in the substantial drop in the gradient Richardson number (Fig. 9d).During the next wave trough, the static stability becomes stable again but the directional shear remains.The following wave crest results in the aforementioned strong elevated cooling event, contributing to further destabilization of the surface layer (Fig. 9d) and the breaking of the wave at 2234 UTC.This wave instability causes enhanced turbulence and a uniform northerly wind direction at all observation levels of the 10-m mast.Also the gradient and bulk Richardson numbers drop to values between 0 and 0.25.The following period is characterized by weak but increasing stability with continuous turbulence.Some weaker wave activity remains clearly visible in our observations.
Although the origin of the process leading to the shift in wind direction near the surface and the resulting enhanced directional wind shear remains unclear, this case nicely illustrates the importance of local wind shear for triggering the instability of near-surface wave.
IOP-10/III, turbulence IntensIfIcatIOn thrOugh wave breakIng.The LATAN-3M sodar recorded a very clear and strong harmonic signal starting at 0010 UTC (Fig. 10f) between 100 m and 200 m, which resulted in a major instability at 0037 UTC.Near the surface, the turbulence was enhanced substantially, as observed by the EC systems at 2.0, 4.5, and 10.3 m (Figs.10d,e).The harmonic oscillations with a period of about 4 min can also be observed in the horizontal and vertical velocity components (Figs.10b,d) of the WCv1 and the 10-m mast.The oscillations in horizontal and vertical velocity are 180° out of phase, consistent with internal gravity waves (Sun et al. 2015).
The preceding period is at first characterized by a strong, surface-based inversion (Fig. 10a) topping out at about 100 m with light, meandering winds roughly from southeast (Figs.10b,c), occasionally showing the signature of wind veer resembling an Ekman spiral (e.g., as seen around 2310 UTC).The turbulence detected by the three sonic anemometers is very weak and of intermittent character.Between 2340 and 2350 UTC the wind direction shifts to a rather northerly direction at all levels below 200 m (see also Fig. 7d) and the wind speeds drop.The upper level winds, at heights between 46 and 85 m, accelerate gently until the wave breaks at 0037 UTC.
For an in-depth analysis, the UAS temperature and lidar wind speed profiles sampled at 2342, 0009, and 0030 UTC (mean UAS flight time) offer further insight in the dynamics of this event (Fig. 11a).These profiles indicate some cooling above 50 m, whereas wind speeds increase below 75 m and decrease above this level, resulting in the formation of a LLJ as seen in the last profile.This corresponds to strong downward transport of momentum as seen in the time height diagram for wind speed (Fig. 11c).Estimates of Ri for the three profiles (Fig. 11b) indicate the formation of a dynamically weakly stable layer (Ri < 0.25) right below 150 m, between the time of the first and second profile, which then propagates downward.This locally weakly stable layer is largely related to the sharp wind speed gradients above the LLJ core.Just about 7 min after the last UAS profile, the wave breaks and strongly enhances

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the turbulence near the surface.The wavelet spectral energy estimate of the vertical wind component observed at 10 m (Fig. 11d) indicates very weak turbulence before 0037 UTC.The wave-breaking event is characterized by a very strong signal with a period of about 3 min, which triggers a turbulence cascade to smaller scales.After 0100 UTC, the strong 3-min signal begins to vanish and the small-scale turbulence weakens again.
The wave breaking event ends at 0050 UTC and after this the wave appears to have higher frequency (Fig. 10f).This is most likely due to the Doppler shift caused by the increasing wind speeds at the levels above 125 m (Fig. 11c).

Summary and outlook
ISOBAR is an experimental research project targeting the SBL over Arctic sea ice by means of a novel observational approach based on a combination of ground-based in situ and remote sensing instrumentation with multiple unmanned aircraft systems.Two major field campaigns, ISOBAR17 and ISOBAR18, were carried out at the Finnish island Hailuoto in the

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ice-covered Bothnian Bay, each lasting for about one month in February 2017 and 2018.These campaigns were characterized by contrasting conditions, with little sea ice and relatively mild temperatures in 2017, whereas conditions were closer to the climatological mean in 2018, favoring more frequent VSBL formation.With our observation strategy of extensive UAS-based measurements supplemented by surface-based mast and remote sensing observations, we have sampled comprehensive SBL datasets, including 14 IOPs; these datasets offer unprecedented spatiotemporal resolution, while also displaying good agreement between the different systems.
Frequent UAS profiles allow for detailed insight into the evolution of the SBL, which may be subject to rapid temperature changes affecting the entire ABL, and small-scale thermal instabilities within the otherwise stably stratified ABL.These data also allow for detailed studies on various VSBL processes and their interaction with near-surface turbulence, of which we highlight three examples, all observed during the same IOP: first, a shear instability caused by the lowering and intensification of the LLJ; second, an unusual rapid-cooling event at elevated levels around 10 m, which appears to be caused by the interaction of a near-surface wave with local shear and the modulation of the surface-layer static stability associated with this nonlinear wave; third, a wave instability related to the intensification of shear at the top of a forming LLJ, triggering enhanced turbulence near the surface.The nature and interactions of such VSBL processes, as well as the potential deviations from similarity theory associated with them, will be subject to more systematic studies also making use of other SBL datasets such as SHEBA (Grachev et al. 2008) or CASES-99 (Poulos et al. 2002).
Furthermore, the ISOBAR datasets provide an excellent opportunity to study the transition from WSBL to VSBL, which is important for a better understanding of the conditions leading to strong surface-based temperature inversions and associated extremely cold temperatures.In particular, we aim to investigate the relative importance of local and large-scale conditions.In a follow-up project, we aim to identify and classify the various mechanisms behind the

SBL model simulations
To illustrate current challenges in SBL modeling, three different types of numerical models were used to simulate the SBL evolution during IOP-14 (23-24 February 2018): the MetCoOp Ensemble Prediction System (MEPS), the Weather Research and Forecasting Model in its single-column mode (WRF-SCM), and the Parallelized Large-Eddy Simulation Model (PALM).MEPS (Müller et al. 2017) is an operational NWP system covering the Nordic countries, forced at its boundaries by the global ECMWF-IFS (Bauer et al. 2013).There are 65 vertical model levels, with the first level at 12.5 m AGL and decreasing vertical resolution aloft.Surface-atmosphere and surface-soil processes are described by the Surface Externaliseé (SURFEX) model (Masson et al. 2013).WRF-SCM utilizes the full WRF physics (Skamarock et al. 2008), with Mellor-Yamada-Nakanishi-Niino turbulence parameterization (Nakanishi and Niino 2006), within an atmospheric column with 200 vertical levels.The vertical spacing is about 2 m in the lower atmosphere.Hourly geostrophic winds and advection of momentum, temperature and humidity are derived from a mesoscale WRF simulation (Sterk et al. 2015).PALM (Maronga et al. 2015(Maronga et al. , 2020) ) runs at a grid spacing of 2 m and a model domain of 500 3 m 3 using a standard configuration but with a modified Deardorff subgrid-scale closure as described by Dai et al. (2021) and coupled to the Rapid Radiative Transfer Model (Clough et al. 2005).PALM is initialized by the same vertical profiles as WRF-SCM and forced by skin temperatures observed during IOP-14.
Figure SB1 shows that even though all three models are capable of forming a very stable stratification and cold air at the surface, the model results differ considerably.The formation of cold air above the surface and the associated strong vertical (temperature) gradients are best captured by PALM, while both MEPS and WRF-SCM display a deeper SBL with weaker gradients.At heights between 50 and 300 m, both WRF-SCM and PALM produce weaker temperature gradients, which can be ascribed to deficiencies in the model initialization.MEPS here captures the stratification much better.Overall, the three different models show substantial deviations from the observations in the lower atmosphere.
Likely sources for these deviations are the turbulence parameterizations that overestimate turbulent mixing and the associated downward heat flux from the atmosphere to the cold surface, and the different boundary conditions and initial conditions applied.As PALM resolves most of the turbulent transport, it can more adequately represent the SBL evolution close to the surface.It is noteworthy that PALM and WRF-SCM, despite being initialized with the same profiles, produce quite different SBLs.Research models like WRF-SCM and PALM are highly sensitive to the initial profiles and boundary conditions, which are either derived from measurements or largerscale model data and thus come with an inherent uncertainty.All three models depend on accurate surface properties, for which a combination of measurements and ad hoc estimations was used here.The differences present in these simulations epitomize the necessity for deeper understanding of the SBL and its representation in atmospheric models; an understanding that is expedited by unique, finescale observational datasets, such as ISOBAR.Sensitivities to model physics and surface properties during IOP-14 are subject of an ongoing study, following the process-based analysis by Sterk et al. (2016).A1 provides a list of abbreviations that appear in the paper along with their expansions.

Fig. 1 .
Fig. 1.Overview maps showing the ISOBAR field site: The two inlay maps at the top display the area of Northern Europe (left) and around Hailuoto and Oulu (right).The large map is based on a Sentinel-2 L2A satellite image from 18 Feb 2019 (https://apps.sentinel-hub.com/eo-browser).

MASCFig. 2 .
Fig. 2. The different UAS systems used during the two campaigns.The numbers of flights are indicated in parentheses for ISOBAR17 and ISOBAR18, respectively.

Fig. 3 .
Fig. 3. Data availability for the measurement systems during the ISOBAR17 and ISOBAR18 campaigns.For the profiling systems the data availability is given as a function of height.

Fig. 4 .
Fig. 4. Overview of the meteorological and sea ice conditions during the two campaigns in February 2017 and 2018.(a),(f) The time series of temperature, cloud cover, wind speed and direction, and pressure observed by the WMO weather station (FMI) during February 2017 and 2018, respectively.(b)-(e),(g)-(j) The ice charts in represent the extrema of the ice coverage during the corresponding period based on data provided by the Finnish Meteorological Institute.Black dotted lines indicate the time of observation and the corresponding location on the maps.

Fig. 5 .
Fig. 5. Combined temperature T and wind speed (ws) profiles based on mast, UAS, and remote sensing (wind only) data, observed between 1510 and 1530 UTC 23 Feb 2018.Solid lines and shaded areas indicate the mean and standard deviation (bin averaged for all UAS and time averaged for ground based systems), respectively.The observation period for the ground-based systems is from 1510 to 1530 UTC, and the slightly shorter periods for the UAS flights are given in the legends.

Fig. 6 .
Fig. 6.Series of (a) UAS boundary layer profiles and (b) corresponding time series of surface based measurements of T (contours) and ws (wind barbs), observed during IOP-14, 1615-2130 UTC 23 Feb 2018.The UAS flight times for the data presented in (a) (ascent up to 250 m) are indicated by colors, and additional markers in (b), applying the corresponding color scheme.

Fig. 9 .
Fig. 9. Time series of various atmospheric parameters during IOP-10, 2210-2245 UTC 18 Feb 2018: (a) temperature T (contours) and wind speed and direction (wind barbs), observed by GFI2; (b) vertical velocity perturbation w� (GFI2, WCv1); (c) wind shear S (GFI2); (d) Richardson number Ri (GFI2).The w� data are shifted by increments of 0.25 m s −1 to reveal structures.Wind speed at all levels and w� data at the upper two levels are smoothed applying a 1-min sliding mean average; T, S, and Ri data are 10-s sliding mean averages.

Fig. 11 .
Fig. 11.Observations of (a) UAS profiles of potential temperature and corresponding lidar wind speed profiles; (b) resulting profiles of Ri; (c) time-height diagram of lidar wind speed; and (d) wavelet energy of 10-m sonic vertical velocity component during IOP-10, 2330 UTC 18 Feb-0130 UTC 19 Feb 2018.The black dotted line in (b) indicates Ri = 0.25.The UAS flight times from (a) and (b) are indicated as vertical lines in the same color in (c) and (d).

Fig. SB1 .F
Fig. SB1.Time-height plots of potential temperature from the MEPS forecast (MEPS-NWP), the WRF single-column simulation (WRF-SCM), and the PALM LES simulation (PALM-LES).The data cover the lowermost 500 m and the first 24 h at the measurement site.Observations from SUMO, GFI, and FMI are superimposed as circles.

Table 3 . Remote sensing systems specifications with measured parameters as in Table 2 and radial wind speed u rad ; standard deviation of wind velocity components σu, συ, σw; attenuated backscatter signal strength, bsc; carrier to noise ratio, CNR.
was started after the installation of most ground-based systems was finished.The UOWL team operating the Q13 UAS could not participate for the full campaign period and decided to focus on the last week of ISOBAR17 and the last two weeks of ISOBAR18.In addition, the Bebop2Met (in 2017) and the CopterSonde (in 2018) were operated for the first time during a scientific campaign and required extensive preparation, resulting in limited data availability from these UAS during approximately the first week of the corresponding campaign.Furthermore, icing Unauthenticated | Downloaded 12/14/23 05:11 PM UTC F E B R UA RY 2 0 2 1 E226 on the inside of the WindCube 100S lens (in 2017) and the late arrival of the WindCube v1 (in 2018) caused the major data gaps in the remotely sensed wind profiles.