One of the main goals of the HALO-(AC)3 Arctic airborne campaign conducted in spring 2022 was a quasi-Lagrangian sampling of air masses. This means that the same air masses were to be sampled twice. Such a measurement strategy allows for concrete observations of air mass transformations, which can for example be used to benchmark weather models. After finishing the campaign, trajectory calculations were conducted for all flights of the HALO aircraft to check whether the flight strategy was a success. For this, the trajectory calculation tool Lagranto was used in conjunction with wind fields from the ERA5 reanalysis. Latter has an output resolution of around 30 km and one hour. The hourly data was bi-linearly interpolated to one minute resolution. Air masses were initialized temporally every one minute along the flight track of HALO, and vertically every 5 hPa between 250 hPa and 10 hPa above the ground level. Horizontally, air masses were started within a 30 km radius circle centered around the location of HALO. In each circle, 30 air masses were initialized evenly spaced approximately every 10 km, allowing for a better statistical analysis. Trajectories were then calculated in one-minute steps up to 10 hours forward in time. At typical HALO flight times of up to 10 hours, around 2.7 million trajectories were calculated per research flight. A quasi-Lagrangian match is registered if the same air mass is seen again below HALO at a later time of the same day and within the same 30 km radius. In order to allow air masses to evolve, a minimum temporal threshold of one hour was applied between matches.

For every research flight (RF) of the HALO aircraft during HALO-(AC)3, quasi-Lagrangian matches on the same day are calculated using Lagranto and ERA5 input. For every RF, two files were uploaded: a quicklook png file, and the match data itself. The match data looks like this:Reference date 20220312_0800 / Time range 600 mintime lon lat p alt------------------------------------0.22 19.66 67.81 715 22918.40 21.20 67.94 776 17750.22 19.61 67.72 720 21588.40 21.15 67.96 786 1675....In the header, the reference date is given as YYYYMMDD_HHMM (UTC time). Trajectories had been calculated 10 hours forward (time range: 600 min). Then, a long list of matches is given. Let's look at the first pair of matches. An air mass is seen first at 08:22 UTC (08 + 0.22) below HALO. The lon and lat coordinates are then given, as well as the pressure level of 715 hPa. This corresponds to an altitude of approx. 2291 m above ground level, using the ERA5 geopotential height. This same air mass is seen more than 8 hours later, at 16:40 UTC (08 + 8.40). As it was moving along the prevailing winds, it is now seen at slightly different lon/lat coordinates and a different altitude (776 hPa, which corresponds to 1775 m above ground).In the quicklook, the radar reflecitivity is plotted in the lowest 10 km above ground. At the bottom of the figure, the ERA5 sea-ice concentration is shown, to highlight the surface type HALO was passing over. Finally, all quasi-Lagrangian matches are overlaid. For each orange dot (first match), there is a corresponding red dot (second match), meaning that the same air mass was observed again at a later stage and somewhere below HALO.