Wind measurements from hot-air balloon flights

Trajectories from commercial hot-air balloon flights can provide interesting wind information of the lower atmosphere.

Introduction
Hot-air balloons float in the air and travel with the wind. Global Navigation Satellite System (GNSS) navigation data acquired during the flight, provide a displacement during a time interval which is a measure of the airspeed. This wind information is obtained in the Atmospheric Boundary Layer (ABL) where in general only few observations are present.

Response time
During the flight the balloon feels the drag as soon as the relative speed is not zero anymore, for instance when the balloon enters a layer with vertical wind shear.  In the beginning the response is fast and levels off as the speed difference becomes less. From the theory we know that the balloon reacts with a response length  [1]. Due to the inertia of the hot-air balloon, its displacement does not capture the small scale variations in the wind pattern. The wind observations represent  an average in space and time.

Thermal wind
In Figure 2 we compare the balloon wind data during the last 20 minutes of the flight of 18 June 2013 with Numerical Weather Prediction (NWP) model  data and with the observations at the Cabauw observatory. The NWP model data consist of the +01 and +02 forecasts starting from the analysis at 18:00 UTC. The wind-profiler is located at Cabauw and the data is available as a 30 min average. In the observations the height-dependence of the wind is stronger than in the NWP model.