The variational echo tracking method and its application in convective storm nowcasting

The objective extrapolation forecast is the main method for 0-1 h convective storm nowcasting. Radar echo extrapolation is performed by using radar mosaics at 6 min intervals obtained from the radar images provided by eight radars in Beijing-Tianjin-Hebei region. A comparative study of two suites of extrapolated forecasts of eighteen typical convective precipitation events occurred in Beijing-Tianjin-Hebei region from 2016 to 2018 was conducted. Compared with the method of tracking radar echoes by correlation method, the variational echo tracking method utilizes variational technique to compute the motion vector field, and uses two strict constraints to get a better motion vector field. The results indicate that the variational echo tracking method performs better in prediction of the radar echo pattern, echo location, and echo intensity at 30 and 60 min forecast lead times. (1) A comparative study of two suites of forecasts of four precipitation events in Beijing-Tianjin-Hebei region has been conducted. The results indicate that the radar echo location, the echo pattern and echo intensity produced by the variational echo tracking method are closer to observations within 1 h. (2) Quantitative evaluation of the two suites of forecasts of eighteen typical convective precipitation events has been conducted. Compared with the correlation method of tracking radar echoes, the variational echo tracking method yields higher probability of detection and the critical success index of the 30 or 60 min extrapolated forecast of echoes larger than 35 and 45 dBz, while the false alarm rate is lower. Also, a quantitative evaluation classified by the weather type indicates that the variational echo tracking method performs better than the correlation method of tracking radar echoes for most weather types.