Objectively classified patterns of atmospheric circulation for rainstorm events in flood season in Hunan

In this study, both subjective and objective classification techniques are employed to identify features of synoptic weather patterns for 117 severe precipitation events in Hunan Province between April and September from 2006 to 2014. The subjective classification is based on the operational experience accumulated by forecasters in Hunan in the past. With the scheme above, we have classified weather patterns of the 117 severe precipitation events into six types, i.e., low vortex with cold trough pattern, frontogenesis in the warm ground inverted trough pattern, edge of subtropical high pattern, typhoon pattern, Meiyu front with shear line pattern, and South China quasi-stationary front pattern. Based on these subjective patterns, we collect meridional wind at 500 hPa and 850 hPa between 20°N and 25°N, 105°E and 120°E at 08:00 BT for the 222 rainstorm days from the 117 severe precipitation events as parameters, and then apply the K-means clustering method and the iterative repetition to obtain six types of objective synoptic patterns of flood-season severe precipitation in Hunan. It is shown that more than 60% of synoptic situation and the severe rainfall distribution for the first, second and sixth type of severe precipitation days corresponded well with those of typhoon pattern, low vortex with cold trough pattern and frontogenesis in the warm ground inverted trough pattern severe precipitation. The third, fourth and fifth type of severe precipitation days occur during two or more patterns of severe precipitation events, and their synoptic situation and severe precipitation distribution corresponded to the different stages of the severe precipitation events. The 6 types of objective clustering results can well represent the synoptic situation and rainfall feature of rainstorms in flood season in Hunan, and also can be used as an important basis for objective classifications of rainstorm.