Abstract:

Background synoptic circulations over Northeast China from May to September in 2018 were classified using a SANDRA (SAN) method, and heavy precipitation (>10 mm/24 h) forecast using the European Center for Medium-range Weather Forecast (ECMWF) model was spatially verified and quantitatively analyzed.Dominant background synoptic circulation over Northeast China can be classified into the North Low Disturbance (NLD) pattern, the Subtropical High Northward Shift (SHNS) pattern, the Northeast Low Vortex Disturbance (NLVD) pattern, and the Low Disturbance Eastward Shift (LDES) pattern.Heavy precipitation events occur more frequently in the presence of the former three patterns, hence we specifically analyze these precipitation events predicted by the ECMWF model.The results indicate that rainfall areas of medium-heavy precipitation events and above under the NLD pattern are generally underestimated by 45%-60%, and that of medium precipitation events is overestimated by 40% for 36 h forecast and is underestimated by 19% for the 84 h forecast.In 36 h, 60 h, and 84 h forecast, the locations of heavy precipitation are deviated by 0.19°, 0.53°, and 1.39° westward, respectively, and the average (maximum) precipitation values are underestimated by 2.9 (7.3) mm, 3.1 (8.1) mm, and 3.4 (9.4) mm, respectively.Under the SHNS pattern, the deviations between the predicted and observed precipitation areas are not always consistent in signs, the predicted rainfall locations are deviated by 0.19°, 0.53°, and 1.39° southward in 36 h, 60 h, and 84 h forecast, respectively, and weaker average and maximum precipitation values occur among 65%-72% forecast cases.While under the NLVD pattern, the predicted precipitation locations are deviated by 0.18° eastward and 0.55° southward in 36 h forecast, by 0.20° eastward and 0.58° southward in 60 h forecast, and by 0.74° eastward in 84 h forecast.In addition, for the three forecast periods, the underestimation separately reaches 3.3 mm, 3.7 mm, and 3.9 mm for average precipitation values, and 10.2 mm, 10.6 mm, and 11.6 mm for maximum precipitation values.

Key words: SANDRA (SAN), Classification of background synoptic circulation, Summer precipitation in Northeast China, Quantitative forecast verification