Work

A Conceptual Model of Diurnal Recharge–Discharge of Energy for the Record-Breaking Heavy Rains over East Asia in Summer 2020

Year: 2025

Type: article

Abstract: The heavy rainfall in East Asia during summer 2020 is characterized by the strengthening of southwesterly monsoon flow over southeast China that provides a crucial regional source of warm moist energy. In this study, we examine the physical processes that govern the detailed variations in warm moist energy in the monsoon flow. During heavy rain days, the low-tropospheric equivalent potential temperature undergoes a pronounced diurnal cycle on land, with daytime increase (recharge) correlating highly with a decrease (discharge) in the following nighttime hours. The relatively strong events of diurnal recharge/discharge are found to strengthen the heavy rains with a dominant morning peak downstream, while weak events correspond to suppressed rainfall. Budget analysis shows that the daytime recharge of warm moist energy is mainly driven by the radiative heating on the surface and subsequent turbulence mixing in PBL. The diabatic heating intensity is strongly regulated by the cloud radiative forcing of weather states in southeast China. The relatively fair weather with prevalent shallow convection (SC) is favorable for enhancing daytime recharge, while the cloudiness condition with active deep convection (DC) tends to suppress the daytime recharge. The nighttime discharge is mainly caused by the advection of nocturnal low-level jets, rather than radiative cooling. The nighttime increase in wind speed is strongly regulated by the thermal difference as predicted by the Blackadar inertial oscillation theory, which represents a strong coupling between thermodynamic and dynamic processes. The diurnal recharge/discharge processes are highlighted as an effective regional forcing, fueling the growth of mesoscale convective systems in the downstream area ahead of extreme heavy rainfall over East Asia. Significance Statement Persistent heavy rainfall is found to be strongly associated with active monsoon flow on time scales ranging from days to weeks. In this study, we find that the monsoon flow gets the extra input of energy on a diurnal time scale as it passes the heated landmass of southeast China. By analyzing the physical processes governing warm moist energy, we find that cloud radiative forcing of daytime weather states can not only regulate the input of energy but also lead to the response in low-level winds. The enhanced low-level winds in the following night hours in turn help to redistribute warm moist energy downstream, which further triggers MCSs and heavy rainfall. These diurnal cycles of warm moist energy and associated low-level winds suggest the strong coupling between thermodynamic and dynamic processes. Therefore, monsoon flow passing over the heated landmass acts as a key regional forcing that determines heavy rainfall and related flooding in the summer monsoon regions.

Source: Monthly Weather Review

DOI: 10.1175/mwr-d-24-0196.1

Authors: Rongjiang Wang, Guixing Chen

Institutions: Sun Yat-sen University, Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai)

Language: English

Topic: Meteorological Phenomena and Simulations

Subfield: Atmospheric Science

Field: Earth and Planetary Sciences

Domain: Physical Sciences

SDG: Climate action

Open Access status: bronze

Funders: National Natural Science Foundation of China, European Centre for Medium-Range Weather Forecasts