During the presummer rainy season (April–June), southern China often experiences frequent occurrences of extreme rainfall, leading to severe flooding and inundations. To expedite the efforts in improving the quantitative precipitation forecast (QPF) of the presummer rainy season rainfall, the China Meteorological Administration (CMA) initiated a nationally coordinated research project, namely, the Southern China Monsoon Rainfall Experiment (SCMREX) that was endorsed by the World Meteorological Organization (WMO) as a research and development project (RDP) of the World Weather Research Programme (WWRP). The SCMREX RDP (2013–18) consists of four major components: field campaign, database management, studies on physical mechanisms of heavy rainfall events, and convection-permitting numerical experiments including impact of data assimilation, evaluation/improvement of model physics, and ensemble prediction. The pilot field campaigns were carried out from early May to mid-June of 2013–15. This paper: i) describes the scientific objectives, pilot field campaigns, and data sharing of SCMREX; ii) provides an overview of heavy rainfall events during the SCMREX-2014 intensive observing period; and iii) presents examples of preliminary research results and explains future research opportunities.
In this study, the presummer diurnal cycle of rainfall (DCR) over southern China is examined using the merged 0.1°-resolution gridded hourly rain gauge and satellite rainfall dataset and the National Centers for Environmental Prediction Final Global Analysis during April to June of 2008–2015. Results show pronounced diurnal variations in rainfall amount, frequency, and intensity over southern China, with substantially different amplitudes from southwestern to southeastern China, and from the pre- to postmonsoon-onset period. Southwestern China often encounters significant nocturnal-to-morning rainfall under the influence of enhanced nocturnal low-level southwesterly winds. Southeastern China is dominated by afternoon rainfall, as a result of surface heating, likely aided by local topographical lifting. Both the pre- and postmonsoon-onset periods exhibit two diurnal rainfall peaks: one in the early morning and the other in the late afternoon. But the latter shows the two peaks with nearly equal amplitude whereas the former displays a much larger early morning peak than that in the late afternoon. Three propagating modes accounting for the presummer DCR are found: (i) an eastward- or southeastward-propagating mode occurs mostly over southwestern China that is associated with enhanced transport of warm and moist air from tropical origin and the induced low-level convergence, (ii) a quasi-stationary mode over southeastern China appears locally in the warm sector with weak-gradient flows, and (iii) an inland-propagating mode occurs during the daytime in association with sea breezes along the southern coastal regions, especially evident throughout the postmonsoon-onset period.
The cloud-to-ground (CG) lightning climatology and its relationship to rainfall over central and eastern China is examined, using data from 32 million CG lightning flashes and Tropical Rainfall Measuring Mission measurements during a 6-yr period covering 2008–13. Results show substantial spatial and temporal variations of flash density across China. Flash counts are the highest (lowest) in summer (winter) with the lowest (highest) proportion of positive flashes. CG lightning over northern China is more active only in summer, whereas in winter CG lightning is more active only in the Yangtze River basin. The highest CG lightning densities, exceeding 9 flashes per kilometer squared per year and more than 70 CG lightning days per year, are found in the northern Pearl River delta region, followed by the Sichuan basin, the Yangtze River delta, and the southeastern coast of China in that order. Lower-flash-density days occur over mountainous regions as a result of the development of short-lived afternoon storms, while higher-flash-density days, typically associated with nocturnal thunderstorms, appear over the north China plain and Sichuan basin. The highest number of CG lightning flashes is found in August whereas monthly convective rainfall peaks in May or July. Flash rates during the warm season are typically maximized in the afternoon hours in coincidence with a convective rainfall peak except for the Sichuan basin and its surrounding mountainous areas where a single late-night convective rainfall peak dominates. Much less lightning activity corresponds to a late-night to morning rainfall peak over the plains in eastern China because of the increased proportion of stratiform rainfall during that period.
A unique program is developed for improving heavy rainfall forecasts over southern China during the presummer rainy season through field campaigns and research on physical mechanisms and convection-permitting modeling.
Pre-summer rainy season (April to mid-June) over South China (SC) is characterized by high intensity and frequent occurrence of heavy rainfall in the East Asian monsoon region. This review describes recent progress in the research related to such a phenomenon. The mechanisms responsible for presummer rainfall consist of multi-scale processes. Sea surface temperatures over tropical Pacific and Indian Ocean are shown to have a great influence on the interannual variations of pre-summer rainfall over SC. Synoptic disturbances associated with regional extreme rainfall over SC are mainly related to cyclone-and trough-type anomalies. Formation and intensification of such anomalies can be contributed by surface sensible heating and mechanical forcing from the Tibetan Plateau. On a sub-daily scale, double rain belts often co-exist over SC. The northern rain belt is closely linked to dynamic lifting by a subtropical low pressure and its associated front/shear line, while the westward extension of the western North Pacific high and the intensification of the southwesterly monsoonal flows play important roles in providing highequivalent potential temperature air to the west-and east-inland regions, respectively. The southern rain belt with a smaller horizontal span is in the warm sector over either inland or coastal SC. The warm-sector rainfall over inland SC results from surface heating, local topographic lifting, and urban heat island effect interacting with the sea breeze. The warm-sector rainfall over coastal SC is closely associated with double low-level jets, land-sea-breeze fronts, and coastal mountains. A close relationship is found between convectivelygenerated quasi-stationary mesoscale outflow boundaries and continuous convective initiation in extreme rainfall events. Active warm-rain microphysical processes can play an important role in some extreme rainfall events, although the relative contributions by warm-rain, riming and ice-phase microphysical processes remain unclear. Moreover, to improve the rainfall prediction, efforts have been made in convection-permitting modeling studies.
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