Characteristics of atmospheric rivers over the Asia Pacific monsoon region with the modified AR detection algorithm

Research on the Atmospheric River (AR) in regions beyond the midlatitudes has gained increasing attention in recent years. However, detection algorithms basically rely on image recognition, which often results in errors when scrutinizing the moisture filament embedded with tropical cyclones (TCs). Therefore, the ARs over the Asia–Pacific monsoon region, where the coexistence of AR and TC frequently occurs, from 1981–2020 are investigated. To reduce erroneous identification and specifically separate the AR and TC, an additional objective approach to filter out the TC circulation according to the objectively estimated radius of the outermost closed isobar (ROCI) is incorporated into the existing detection algorithm. In addition, two modern reanalysis datasets, MERRA-2 and ERA5, are utilized to evaluate the sensitivity of the modified algorithm to the geometric criteria and intensity thresholds of the moisture features and different reanalysis inputs. Based on the level of spatial overlap between the detected ARs and contiguous rainfall areas (CRAs) and the sensitivity of AR frequency to different thresholds and algorithms, the detection scheme performs better when the integrated vapor transport anomaly (IVTA) used for identification is computed from ERA5 and its intensity threshold is lowered to 200 kg⋅m⁻¹⋅s⁻¹. The CRAs are classified into those associated with AR, TC, both of them, or neither of them, depending on the algorithm outputs. An analysis of the rainfall intensity of the CRAs revealed that, compared with TC (80–100 mm⋅day⁻¹), AR generally requires terrains to generate more rainfall of up to 80 mm⋅day⁻¹. Otherwise, the average rainfall intensity of AR is approximately 50–60 mm⋅day⁻¹. The average strongest rainfall tends to occur within the TC accompanied by the AR (100–120 mm⋅day⁻¹). Regional averages indicate that AR acts as the primary source of annual rainfall in Northeast Asia. In contrast, TC tends to contribute more than AR in the western North Pacific and South China Sea.