Thermodynamic and dynamic effects of anomalous dragon boat water over South China in 2022

Abstract

In 2022, persistent extreme precipitation events occurred during the Dragon Boat Water (DBW) period (May to June) over South China, with rainfall-breaking historical extremes at many stations. In this study, the relative importance of the thermodynamic and dynamic effects on this anomalous event was explored through budget equations of moisture and moist static energy (MSE). The moisture budget analysis suggests that the thermodynamic effect contributes to the precipitation anomaly by less than 10% through the advection of abnormal moisture by mean horizontal and vertical motion. However, the dynamic effect, associated with the mean moisture advection by anomalous vertical motion, makes the largest contribution. This seems to imply that the increase in water vapor due to global warming does not play a critical role in this extreme event. Nevertheless, further analysis of the MSE budget equation reveals that the anomalous vertical motion is governed by both the dynamical effect of atmospheric circulation changes and the thermodynamic effect of atmospheric thermal state changes. The thermodynamic effect, which accounts for 47.21% of the total effect (sum of the thermodynamic and dynamic effects), is caused by anomalous moist enthalpy (mainly dry enthalpy) advection due to the mean horizontal motion. In contrast, the dynamical effect is related to the warm and humid air advection caused by the anomalous southwest wind caused by the anomalous anticyclone over the northwest Pacific. Simultaneously, the anomalous northeast wind brings dry cold air, resulting in the convergence of cold and warm air over the Pearl River basin and persistent heavy rainfall events. The results of this study provide valuable insights into the direct and indirect physical mechanisms leading to anomalous DBW period over South China in 2022.