Diagnosing the Dynamic and Thermodynamic Effects for the Exceptional 2020 Summer Rainy Season in the Yangtze River Valley


  • An exceptional rainy season occurred in the Yangtze River valley of eastern China in June–July 2020. The relative importance of the dynamic and thermodynamic effects on this unusual event is evaluated through the budget equations of moisture and moist static energy (MSE). The moisture budget analysis suggests that the thermodynamic effect contributes to the precipitation anomaly by 8.5% through the advection of abnormal water vapor by mean verti-cal motion, while the dynamic effect, related to water vapor advection by anomalous vertical motion, has the dominant contribution. The MSE budget analysis further reveals that the anomalous vertical motion is both constrained by the dynamic effect related to changes in atmospheric circulation and the thermodynamic effect related to changes of the atmospheric thermal state, with a ratio of thermodynamic versus total effects estimated at 45.3%. The dynamic effect is linked to the advection of warm and humid air by the abnormal southwesterly wind, which is related with an anomalous anticyclone over the Philippine Sea. The thermodynamic effect is partly induced by the positive advection of anomalous MSE (mainly latent energy) by the mean vertical motion. This analysis of the dynamic and thermodynamic effects is useful to understand the underlying physical mechanisms leading to the unusual rainy season in the Yangtze River valley in summer 2020. It is also helpful to put forward a few speculations on the potential role of global warming whose primary effect is, after all, to change the thermal state of the atmosphere.
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