The Synoptic Patterns Associated with Extreme Precipitation over the Middle Reaches of the Yangtze River during the Mei-Yu Season and Its Application in Model Assessment

The large-scale synoptic patterns associated with extreme precipitation over the middle reaches of Yangtze River (MRYR) in early summer (June and July) are classified into three canonical patterns via hierarchical clustering. The clustering results reveal a clear connection between the MRYR extreme precipitation and anomalous moisture convergence over this region with the eastward expansion of South Asia High and intensified westerly jets providing additional forcing for local rising motion. In all three clusters, the anomalous moisture convergence results from anomalous low-level southwesterlies encountering anomalous northerlies from mid-high latitudes. The southwesterly anomaly is associated with the expansion of the Western Pacific Subtropical High (WPSH). However, the anomalous northerlies weakening the northward advance of the Mei-yu front are mainly driven by different extratropical circulation anomalies in the three clusters. These anomalies range from zonally-elongated barotropic disturbances to developing baroclinic disturbances that are potentially tied to upstream storm tracks. All three clusters are characterized by a meridional dipole in geopotential height anomaly over the tropical-subtropical East Asia. The northern and also more pronounced node of the dipole is located over the MRYR with a cyclonic (anti-cyclonic) height anomaly in the lower (upper) troposphere, suggesting the critical role played by anomalous latent heating of extreme MRYR rainfall in driving the formation of this dipole. It is verified here that the presence of these patterns indeed increases the likelihood of the occurrence of extreme rainfall over the MRYR. This relationship between the synoptic-scale circulation pattern and extreme rainfall is reproduced by only a subset of models from the phase 5 of the Coupled Model Intercomparison Project (CMIP5), where the underestimated frequency of the corresponding synoptic-scale circulation patterns partly explains the underestimated frequency of extreme rainfall thus the summer total over the MRYR. Our analysis also reveals that a few models could "accidentally" simulate a realistic rainfall total and probability distribution of daily rain rate over the MRYR region during summer by generating their own model-dependent synoptic-scale circulation patterns that are not typically seen in observations. These findings suggest that a projection of future changes in extreme rainfall over the MRYR will be better constrained dynamically if we use a subset of models that can reproduce the "rainfall-circulation" relationship, given the diverse response of different circulation patterns to radiative forcing changes in the atmosphere. The results presented here also demonstrate the importance of tracking biases of synoptic-scale circulations in understanding model deficiencies in precipitation simulation, in addition to investigating problems in model physics such as cumulus schemes and microphysics parameterization. Nevertheless, these results are based on the analyses of extreme rainfall in one region and one season, further research covering other regions/seasons is needed.