Evolution of Precipitation Efficiency of Mesoscale Extreme Rainfall Event on 25–28 August 2015 over the South China Sea

In this study, the evolution of precipitation efficiency (PE) and water budget of mesoscale convective systems (MCSs), which produced extreme rainfall with the peak amount of more than 500 mm over the South China Sea and southern Taiwan on 25–28 August 2015, are investigated using the satellite and radar observations and cloud-permitting model simulations. The MCSs were embedded in the southwesterly monsoon flow from Indo China with abundant moisture. Firstly, the evolution of PE and water budget is examined in a semi-Lagrangian framework by following the movement of targeted MCS. Secondly, the changes of PE and water budget are confirmed over the broad-scale areas where the target MCS are located in an Eulerian framework. Thirdly, the sensitivity of PE and water budget to moisture fluxes within the southwesterly monsoon flow (in both moisture amount and horizontal wind speed) is investigated.

Water budget in the semi-Lagrangian framework shows that as the low-level large-scale moisture was increased (decreased) by 10%, the total condensation and deposition were increased (decreased) by 10% (30–40 %). Horizontal convergence of moisture flux was significantly enhanced within the MCSs to generate precipitation, and evaporation was more pronounced over the region of weak convection. Similar results are found in an Eulerian framework. For the monsoon MCSs with radar reflectivity of greater than 35 dBZ, the calculated large-scale PE was 20–25% and the microphysical PE was 35–40%, both of which were less than those for the principal rainband of Typhoon Morakot (2009) producing the peak rainfall amount near 3000 m in 4 days. Microphysical ratios are also calculated in the semi-Lagrangian framework. Condensation ratio remained a peak value of 60% during the MCS’s mature stage and then decreased to approximately 30% at the decaying stage. Deposition ratio was increased from 10% to 30% and evaporation ratio was increased from 20% to 30%, when the MCS evolved from the mature to the decaying stage. Finally, the surface precipitation was highly sensitive to the large-scale moisture change. In particular, 10% decrease of low-level (below 700 hPa) relative humidity could result in 10–20% decrease of moisture flux and 10–40% reduction of surface precipitation. These results highlight the importance of amount and spatial distribution of low-level environmental moisture to the extreme rainfall event.