Abstract:The round-trip horizontal drift radiosonde can do the three-stage observation of “ascending-drifting-descending stages” by releasing one sounding balloon at only one time. In particular, in the descending stage the radiosonde can increase the vertical observation of the atmosphere at 06 UTC and 18 UTC respectively, and it has the potential to significantly improve the prediction skills of the regional high-resolution rapid assimilation cycle prediction system at 06 UTC and 18 UTC. In order to achieve the assimilation of the round-trip horizontal drift sounding in the high-resolution regional model and analyze its impacts on the forecast, one vertical thinning method by “selecting the closest observation according to the model layer” for assimilation is preliminarily proposed, and the influence of this thinning method on model analysis is deeply analyzed. On this basis, a one-month batch assimilation impact experiment was carried out by using the networked observations in the middle and lower reaches of Yangtze River, and the impacts on model forecasts are investigated in detail in this paper. The results of the thinning sensitivity tests show that, compared with the assimilation of traditional operational soundings, the analysis and prediction root mean square errors (RMSE) of the assimilation of non-thinning round-trip horizontal drift radiosonde observations are significantly increased, while the precipitation prediction scores are significantly reduced. On the contrary, the model performances in analysis and prediction fields are improved by assimilating the data after vertical thinning， which indicates that the vertical thinning of round-trip horizontal drift radiosonde observations must be carried out before assimilation. The results of batch experiments show that at the cold start time (00 UTC and 12 UTC, time with conventional radiosonde), the assimilation of round-trip horizontal drift sounding (ascending stage) observations has smaller changes in analysis error and prediction error relative to assimilation of traditional operational radiosonde data. At the warm start time (03 UTC, 06 UTC, 09 UTC, 15 UTC, 18 UTC, 21 UTC, time without conventional radiosonde), however, compared with the control experiment, the accuracy of the analysis field is improved by about 0.4% by assimilating the data of descending section of the horizontal drift sounding. The ETS scores of the 0-12 h accumulated precipitation forecast change less, but the ETS scores of the 12-24 h accumulated precipitation forecast at 0.1, 1.0, 5.0, 10.0， 25.0 mm thresholds increase by about 0.5%. Moreover, the ETS score at 50.0 mm threshold increases by about 2.3%. All above results show that the round-trip horizontal drift sounding must be thinned before assimilation, and the assimilation of the round-trip horizontal drift sounding can improve the precipitation forecast skill of the regional high-resolution rapid assimilation cycle forecast system at the warm start time.