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 of 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 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 is favorable for enhancing daytime recharge, while the cloudiness condition with active deep convection tend 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 of 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.
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