I would like to simulate the trajectory of a weather balloon in an altitude of 12 to 20 km. For this I would like to know how the velocity-field (i.e. wind-speed and direction as a function of position and time) looks like and how constant it is in the time and space domain.

Is there some publicly available data about air-circulation in the stratosphere? Or do you have an idea how to generate test-data for my simulation?