Study of the Application of WRF-FLEXPART in Airborne Balloon Movement Simulation

In the present era, national borders frequently encounter the risk and threat of unidentified airspace objects such as airborne balloons; however, there is a lack of effective methods for predicting their trajectories. To address this gap, we examine the utilization of a Lagrangian diffusion model named FLEXPART for forecasting the movement of airborne balloons. The simulation results show that when taking into account the radius of the airborne balloon, there is a potential increase of up to 1.07 km and 1.45 km in the mean absolute error (MAE) and root mean square error (RMSE), respectively, in the horizontal direction. Similarly, considering the density of the airborne balloon can also result in an increase of 3.46 km (MAE) and 4.71 km (RMSE) horizontally. Therefore, simplifying them into mass points enables FLEXPART to predict their trajectories with higher accuracy. On this basis, we simulate and comparatively analyze the forward and backward trajectories of an airborne balloon observed in China in 2019 using the FLEXPART model. The results demonstrate that within 11 h, FLEXPART predicts the horizontal trajectory of the balloon, with both RMSE and MAE falling within 50 km. Moreover, when it comes to the vertical direction bias within the 11 h prediction, RMSE and MAE are both below 500 m. Furthermore, trajectory bias gradually diminishes as the forecast time increases. The combination of the sensitivity tests and the results of the actual balloon trajectory simulation confirm that the FLEXPART model is capable of predicting the airborne balloon trajectory.