Balloon drift estimation and improved position estimates for radiosondes.

When comparing model output with historical radiosonde observations, it is usually assumed that the radiosonde has risen exactly above its starting point and has not been displaced by the wind. This has changed only relatively recently with the availability of Global Navigation Satellite System (GNSS) receivers aboard the radiosondes in the late-1990s, but even then the balloon trajectory data were often not transmitted, although this information was the basis for estimating the wind in the first place. Depending on the conditions and time of year, radiosondes can sometimes drift a few hundred kilometres, particularly in the mid-latitudes during the winter months. The position errors can lead to non-negligible representation errors when the corresponding observations are assimilated. This paper presents a methodology to compute changes in the balloon position during its vertical ascent, using only limited information, such as the vertical profile of wind contained in the historical observation reports. The sensitivity of the method to various parameters is investigated, such as the vertical resolution of the input data, the assumption about vertical ascent speed of the balloon, and the departure of the surface of the Earth from a sphere. The paper considers modern GNSS sonde data reports for validation, for which the full trajectory of the balloon is available, alongside the estimated wind. Evaluation is also conducted by comparison with ERA5 and by conducting low-resolution data assimilation experiments. Overall, the results indicate that the trajectory of the radiosonde can be accurately reconstructed from original data of varying vertical resolution and that the more accurate balloon position reduces representation errors, and, in some cases, also systematic errors. [ABSTRACT FROM AUTHOR]