Sensing the Wind with Hot-air Balloons and their Application in NWP Models

Numerical Weather Prediction (NWP) models with a horizontal grid of 2 km or finer need detailed information for estimating the initial state of the atmosphere. Ground-based remote-sensing instruments like Sodars, Doppler lidars and Profilers provide already meteorological information of the Atmospheric Boundary Layer (ABL). Although observational networks have been extended over the years, there are still gaps in data gathering particular on the finer scales. Therefore we have commenced research to investigate data from third parties. Here we focus on wind-information in the ABL from recreational Hot-air Balloon (HaB) flights. In the basic equipment of a HaB pilot there is a professional navigation device, which is compulsory for safety reasons. Similarly to routinely launched weather balloons, the Global Navigation Satellite System (GNSS)-data from consecutive positions and the elapsed time are the basis of the calculation of the horizontal wind vector. On a yearly basis about 6000 flights are taken place in the Netherlands, mainly during the morning- and evening transition. As soon as the surface is covered with snow and when convection is strongly reduced, flights may also take place during the day. The HaB data are validated with observations from the meteorological site of Cabauw and we compare the HaB winds with mast data and other available observations like a RASS wind profiler and a wind lidar. We also compare the HaB data with the results of an NWP model and we will report about a first attempt to assimilate the HaB data in a NWP model. To explore the possibilities of this new type of wind observations in more complex terrain, we will present also the results of an intriguing HaB flight in Austria showing a typical mountain-valley circulation.