1 Antarctic atmospheric boundary layer observations with the Small Unmanned Meteorological Observer (SUMO)

Between January 2012 and June 2017 a small unmanned aerial system (UAS), known as the Small Unmanned Meteorological Observer (SUMO), was used to observe the state of the 20 atmospheric boundary layer in the Antarctic. During 6 Antarctic field campaigns 116 SUMO flights were completed. These flights took place during all seasons over both permanent ice and ice free locations on the Antarctic continent and over sea ice in the western Ross Sea. Sampling was completed during spiral ascent and descent flight paths that observed the temperature, humidity, pressure and wind up to 1000 m above ground level and sampled the entire depth of the 25 atmospheric boundary layer as well as portions of the free atmosphere above the boundary layer. A wide variety of boundary layer states were observed including very shallow, strongly stable conditions during the Antarctic winter and deep, convective conditions over ice free locations in the summer. The Antarctic atmospheric boundary layer data collected by the SUMO sUAS, described in this paper, can be retrieved from the United States Antarctic Program Data Center 30 (https://www.usap-dc.org). The data for all flights conducted on the continent are available at https://www.usap-dc.org/view/dataset/601054 (Cassano 2017; https://doi.org/10.15784/601054) and data from the Ross Sea flights, are available at https://www.usap-dc.org/view/dataset/601191 (Cassano 2019; https://doi.org/10.15784/601191). (Bonin et al. 2013), as well as estimates of large-scale advective changes, based on state changes above the boundary layer. The relatively high temporal resolution atmospheric profiles observed by the SUMO were after take-off. As noted in section 2.1 there is a noticeable sensor lag evident in the temperature and humidity measurements. The sensor lag is short, of the order of a few seconds, for the temperature 260 measurements but is much longer for the humidity observations. These lags are obvious when boundary layer and a portion of the free atmosphere above, with the SUMO flying a spiral ascent and descent flight path. A wide variety of boundary layer states were observed including very shallow, strongly stable


Introduction
The turbulent lower portion of the atmosphere, known as the atmospheric boundary layer, is the part of the atmosphere that interacts directly with the underlying surface. In lower and middle latitudes atmospheric properties in the boundary layer change diurnally in response to the diurnal 40 cycle of net radiation at the surface. During the day downwelling shortwave radiation often results in a positive surface radiation budget, surface heating, and the development of a convective boundary layer with temperature decreasing with height at a rate of approximately 10 K km -1 . At night, longwave radiative cooling from the surface results in surface cooling and the development of a statically stable boundary, often characterized by a surface based inversion, where 45 temperature increases with height. The presence of clouds or changes in large-scale winds will alter this typical diurnal boundary layer evolution (Stull 1988).
In the polar regions a weaker, or absent, diurnal cycle in radiative forcing results in a less pronounced diurnal cycle in boundary layer evolution compared to that observed in lower latitudes, although some locations, such as the McMurdo Dry Valleys, do experience a pronounced diurnal 50 cycle during the austral summer (Katurji et al. 2013). The presence of extensive ice covered surfaces reduces the amount of solar radiation absorbed at the surface during the day and weakens, or eliminates, the presence of convective boundary layer conditions. During the long polar night extended periods of radiative cooling at the surface lead to the development of stably stratified boundary layers with strong temperature inversions, although strong winds or clouds can 55 cause the surface inversion to dissipate and well mixed conditions to develop (King and Turner 1997;Cassano et al. 2016a;Nigro et al. 2017).
The vast majority of in-situ atmospheric observations in the Antarctic are surface observations within the lowest 10 m of the atmosphere with very few observations made above the surface (Summerhayes 2008). This lack of information on the vertical structure of the atmosphere, 60 even in the lowest 10s to 100s of meters, limits our ability to study the Antarctic boundary layer.   This paper describes the SUMO sUAS and flight strategy employed during our Antarctic 95 field campaigns (section 2), the data processing and quality control applied to the data (section 3), provides examples of boundary layer features that were observed with the SUMO sUAS (section 4) and the SUMO data availability (section 5). A brief summary is presented in section 6. Reuder et al. (2012), Cassano (2014), and Jonassen et al. (2015) provide technical descriptions of the SUMO sUAS. The SUMO is a small fixed wing pusher prop drone with 0.80 m wingspan and 580 g take-off weight that is constructed from high-density foam (Fig. 2). The airframe is based on the commercially available Multiplex Funjet model remote control airplane. 105

SUMO sUAS
The SUMO uses a single lithium polymer (LiPo) battery to power the electric motor, which allows for a flight time of ~30 minutes. Further details about the SUMO sUAS are given in Table 1   Vertical range 4 km a.g.l.

Flight duration ~30 min
The SUMO sUAS is launched by hand and lands on its underside. Usually a two-person team operates the SUMO. One person, the remote control pilot, maintains visual sight of the aircraft and controls the SUMO via a remote control while the second flight team member, the ground station pilot, manages the ground control software. A standard model airplane remote control can 120 be used to manually control the SUMO sUAS but it is typically flown in a semi-autonomous or autonomous mode via the onboard Paparazzi autopilot (ENAC 2008) and ground control software (Table 2). A 2.4 GHz radio modem is used for two-way communication between the SUMO and the ground control software. The SUMO observations are relayed to the ground station computer and the pre-programmed flight plan can be modified with the ground station software via the radio 125 modem link. In case of a ground station communication failure the remote control pilot can take control of the aircraft at any time. The SUMO records temperature, humidity, pressure and aircraft location at 2 to 8 Hz frequency (Table 2). Temperature is measured with a reported accuracy of ± 0.2 K (± 0.3 K) for the 135 Pt 1000 (Sensirion) sensor. The temperature sensor specifications indicate a time lag of 3 to 30 s but comparison between the two temperature sensors indicate that both have a similar lag of 2 to 5 s. The sensor lag appears as an offset between ascending and descending temperature profiles during flights with continuous spiral ascent and descent flight patterns as shown in Cassano (2014) and discussed in Section 4. As a result of this sensor lag most of the SUMO flights described in 140 this paper used stepped ascent or descent profiles (described in more detail below). Each step in these ascent / descent patterns occurred over roughly 65 s so the temperature sensor lag becomes unimportant as it is much shorter than the orbit time at each height. The reported sensor time constant for relative humidity measurements, made with the Sensirion SHT 75 sensor, is approximately 8 s although at temperatures well below 0ºC we found that the humidity data was 145 largely unusable due to very long time lags. The Mayer et al. (2012) "no flow sensor" method was used to estimate wind speed and direction by evaluating differences in ground speed throughout a circular flight path and is described in greater detail in section 3.  Table A1 lists the date and time, location and maximum altitude for each flight.

Flight
Additional Antarctic SUMO flights had been planned to take place after the PIPERS cruise in 2017, but challenges in attempting to schedule a mid-winter campaign as well as the recent 165 Antarctic field work restrictions due to COVID-19, resulted in the 2017 flights being the last SUMO flights conducted in Antarctica. As such, this paper provides a description of all Antarctic SUMO UAS flights that have been, or will be, conducted by our research group. Future sUAS flights in Antarctica, led by our group, will use the DataHawk2 sUAS, developed at the University of Colorado The boundary layer depth in the Antarctic can vary from 10s of m or less during strongly 195 stable, light wind conditions in winter to more than 1000 m in summer (King and Turner 1997). The maximum SUMO spiral profile heights ranged from 89 m to 1371 m agl (Table A1)

Data processing and quality control
Data from the SUMO sUAS were logged using an onboard datalogger controlled through the Paparazzi autopilot software. This data was telemetered, via 2.4 GHz radio modem, to a laptop 205 computer running the Paparazzi ground control software and also logged to an onboard SD https://doi.org/10.5194/essd-2020-284 Preprint. Discussion started: 4 November 2020 c Author(s) 2020. CC BY 4.0 License. memory card. The data in the telemetered and SD data streams were identical, other than a reduced temporal resolution and occasional gaps in the record in the telemetered data. The SD data stream was used as the source data for the archived data except for cases where the SD data was not available due to memory card failure or other issues. 210 The data recorded by the SUMO, in both the telemetered and SD data stream, is written sequentially with each record marked with the elapsed time since the SUMO was powered on.
Each data record reported a single variable -aircraft status, navigation information or data from one of the scientific instruments. This data was stored in text log files on the ground station computer and the onboard SD card. 215 Following each flight the SUMO SD, or telemetered, log files were processed into a comma separated text file. A header was written to this file that listed the flight location, sUAS pilots and start date and time of the flight. Each subsequent line of the file listed the date and time, elapsed time since SUMO power on, location and meteorological data (Table A2)  The _SD indicates that the data is from an SD file. If the data came from a telemetered SUMO log file the _SD was omitted in the filename.
A second data processing step linearly interpolated all variables in time to replace the missing data values and provide data for all variables at each time step in the data file. No interpolation was performed before a given variable was first reported in the log file so these records 230 retain missing data values. This data was written to files with the same naming convention as above but with _interpolation added to the filename before the .txt filename extension to indicate the temporal interpolation was applied to the original data. The interpolated data was then used to calculate averages from each constant altitude orbit completed during the flight and vertical bin averages. As described above, most SUMO flights 235 were conducted such that the sUAS orbited at multiple fixed heights over the flight altitude range.
These fixed height orbits helped address sensor time lag issues and also allowed the Mayer et al.  Table A2.
Vertical bin averages were calculated for each 5 m altitude bin over the full altitude range 250 of each SUMO flight. For each vertical altitude bin the average, standard deviation and number of observations in the bin was calculated. These values were calculated from all data during the flight as well as from data from the ascent-only and descent-only portions of the flight. This data is stored in files named yy_mm_dd__HH_MM_SS_SD_vert_avg.txt (Table A2).
Data from each flight, in the interpolated data file as well as from the vertical bin average 255 and constant altitude orbit data files was visually inspected. The following issues were observed, but no data was removed from the archived data files. For each flight, there is a short period, of a few seconds, when the temperature sensors adjust to the ambient conditions immediately after take-off. As noted in section 2.1 there is a noticeable sensor lag evident in the temperature and humidity measurements. The sensor lag is short, of the order of a few seconds, for the temperature 260 measurements but is much longer for the humidity observations. These lags are obvious when https://doi.org/10.5194/essd-2020-284 comparing observations from the ascent and descent portions of the flight (Fig. 3). Since the original time resolution data is provided users of this data can apply whatever corrections are needed for their application to account for the sensor lag, but no corrections were applied to the archived data.   convective boundary layer was present. Figure 3a shows the temperature observed during the stepped ascent (blue) and spiral descent (red) portions of this flight. The ascent portion of the flight is consistently warmer than the descent and given the decreasing temperature with height on this day, is consistent with a short time lag in the temperature measured by the SHT sensor. By applying a 2.5 s offset to the temperature, relative to the height, the ascent and descent profiles more closely 285 match (Fig. 3b). This time lag is consistent with results presented by Cassano (2014). The humidity observations also exhibit a time lag (Fig. 3c). Here, humidity increases with height and the time lag results in the ascending observations being shifted to slightly lower relative humidity values, at a given height, compared to the observations during the descent. Applying a 30 s time shift to the humidity measurements results in much closer correspondence between the humidity profiles 290 observed during the SUMO ascent and descent (Fig. 3d), although does not fully remove the discrepancies between the ascent and descent profiles as seen for temperature (Fig. 3b).

Examples of Observed Features
With  (Fig. 4). The timing between 300 most of these flights was ~1.5 hours which provided a detailed depiction of the temporal evolution of the boundary layer and free atmosphere above.
From 1916 UTC 20 January to 0204 UTC 21 January a shallow, well mixed boundary was observed, with a dry adiabatic temperature profile up to a maximum height of 250 m. After 0204 UTC the boundary layer transitioned to a weakly stable temperature profile. The temperature in the 305 boundary layer initially cooled (1916 to 2144 UTC 20 January) ~1 K and then warmed ~5 K by 0731 UTC 21 January before cooling ~2 K over the next approximately 2 hours.  Contrasting the summer conditions seen in Fig. 4, Fig. 5 shows temperature profiles 330 observed during 11-12 September 2016 at the Pegasus runway at the end of the Antarctic winter.
Six SUMO flights were completed between 1426 UTC 11 September and 1658 UTC 12 September 2016. The surface temperature during these flights was near -40°C, which is the lower observation limit of the SHT temperature sensor. Unlike the temperature profiles shown in Fig. 4, the temperature profiles observed during the 24+ hour period from 11-12 September 2016 (Fig. 5)  335 showed a remarkable lack of temporal variability, exhibiting nearly steady state conditions. During this time a very strong, shallow surface inversion was present with the temperature warming ~7 K in the lowest 50 m of the sounding and then warming another several K up to 200 m.

Data Availability
The SUMO sUAS data described in this paper can be retrieved from the United States conditions during the Antarctic winter (Fig. 5) and deep, convective conditions over ice free locations in the summer (Fig. 6). 390 Data from these flights was processed from the native SUMO log files into comma separated text files at the original and an interpolated time resolution. Additional data processing, of the interpolated time resolution data, created vertical bin averaged data and averages over constant altitude SUMO orbits. Errors noted in the data include short periods of time, at the start of the flight, when the meteorological sensors equilibrate with the ambient atmospheric conditions and 395 time lags in the temperature (~2.5 s) and humidity (30 or more s) measurements.
The Antarctic atmospheric boundary layer data collected by the SUMO sUAS, described in this paper, are freely available from the United States Antarctic Program Data Center