Measuring low-altitude winds with a hot-air balloon and their validation with Cabauw tower observations

A field experiment with a hot-air balloon was conducted in the vicinity of the meteorological observatory of Cabauw in The Netherlands. Recreational hot-air balloon flights contain useful wind information in the atmospheric boundary layer (ABL). On a yearly basis between 8000 and 9000 flights are taking place in The Netherlands, mainly during the morning and evening transition. An application (app) for smartphones has been developed to collect location data. We report about a feasibility study of a hot-air balloon experiment where we investigated the accuracy of the smartphone’s Global Navigation Satellite System (GNSS) receiver using an accurate geodetic GNSS receiver as a reference. Further, we study the dynamic response of the hot-air balloon on variations in the wind by measuring the relative wind with a sonic anemometer, which is mounted below the gondola. The GNSS comparison reveals that smartphones equipped with a GNSS chip have in the horizontal plane an absolute position error standard deviation of 5 m, but their relative position error standard deviation is smaller. Therefore, the horizontal speeds, which are based on relative positions and a time step of 1 s, have standard deviations of σu = 0.8 m s-1 and σv = 0.6 m s-1. The standard deviation in altitude is 12 m. We have validated the hot-air balloon derived wind data with observations from the Cabauw tower and the results are encouraging. We have studied the dynamics of a hot-air balloon. An empirical value of the response length has been found which accounts for the balloon’s inertia after a changing wind, and which compared favorable with the theoretical derived value. We have found a small but systematic movement of the hot-air balloon relative to the surrounding air. The model for the balloon dynamics has been refined to account for this so-called inertial drift.