The balloon drift calculation involves estimating an updated latitude, longitude and observation time on each reported level as the radiosonde or PIBAL balloon ascends through the atmosphere. The horizontal drift is calculated on each valid wind and height level based on the mean wind vector in the layer beneath and the estimated time over which the balloon traversed this layer. The bottom level of this layer is the previous valid wind and height level in the profile. The drift time is calculated from the layer thickness and an assumed constant balloon ascent rate of 5 meters per second.
The horizontal drift calculation is performed on all levels with valid wind and height, where both have passed all quality control checks to this point. In order to generate a smooth profile of drift latitude/longitude, radiosonde heights are integrated from reported height and temperature levels to levels where no height information was reported (e.g., significant temperature and winds-by-pressure levels) and likewise radiosonde and PIBAL winds are interpolated to levels where no wind data were reported (e.g., significant temperature levels). For PIBAL reports, the U.S. Standard Atmosphere height is used on missing height levels. If any of the reported data used in either the height integration or the wind interpolation have failed quality control checks, then the generated data are also considered to be of bad quality and the generated data levels will not be used in the horizontal drift calculation. In addition, if the pressure difference between bracketing wind levels exceeds 150 mb, then no wind interpolation is done onto levels in between and these levels not be used in the horizontal drift calculation.
Several checks are applied to the horizontal drift calculation on each level. If any check fails, no calculation is done and the drift latitude/longitude from the level beneath is transferred to the current level. These checks include: a valid height and wind, a positive calculated drift time interval in the layer below, a calculated drift time interval of less than one-hour in the layer below, and a calculated latitude and longitude change both less than one degree from the level beneath. In addition, if, on any level, the pressure difference in the layer below is greater than 150 mb, then the drift latitude/longitude from the level beneath is transferred to all levels above. Finally, the horizontal balloon drift cannot be calculated at the South Pole station, 89009. Here, the station's latitude and longitude are transferred to all levels.