Can the Accumulated Precipitation Intensity and Structure Be Captured by CINRAD VCP21 Using Bi‐Directional Optical Flow (BIO) Algorithm?

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Abstract
This study evaluates the effectiveness of the Bi‐directional Optical Flow (BIO) method in capturing accumulated precipitation intensity and structure by China New Generation Weather Radar (CINRAD) operated in volume coverage pattern 21 (VCP21) scan mode, aiming to improve the accuracy of radar quantitative precipitation estimation (QPE). The BIO method was analyzed across 13 precipitation events, including three types of precipitation events: convective, typhoon, and stratiform precipitation events. High‐resolution X‐band phased array radar QPE data and rain gauge observations were used to validate the precipitation intensity and structure obtained using the BIO method. The results of elliptical parameter analysis demonstrated that the BIO method's precipitation showed high positional accuracy, precise alignment of precipitation orientation, and spatial distribution closely consistent with original X‐band phased array radar precipitation data. Statistical comparisons further highlighted that accumulated precipitation intensity of the BIO method (BIOQPE) significantly reduced errors compared to intensity from VCP21 mode, with an average decrease of 11.2% in RMSE, 14.0% in RMAE, and an average increase of 4.7% in CC across the 13 events. Particularly, BIOQPE provided superior accuracy and continuity in intensity and structure for convective and typhoon precipitation events. For stratiform precipitation, BIOQPE improved spatial continuity and demonstrated greater accuracy than original radar data. The results reveal that the BIO method is effective in describing precipitation intensity and structure within observations, and significantly improves the accuracy of accumulated precipitation, especially under limited temporal resolution conditions.

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