In September 2011, the Chinese Academy of Meteorological Sciences (CAMS) of China Meteorological Administration (CMA) submitted a proposal conducting field experiments on “Deep Convection and Intense Rainfall in Southern China during the Monsoon Outbreak over South China Sea” to the Tropical Meteorology Research Working Group (WGTMR) Monsoon Panel of the World Meteorological Organization/World Weather Research Programme (WMO/WWRP). This proposal was later accepted as the WMO/WWRP Research and Development Project (RDP), and it was treated as the key subject of the WMO/WWRP’s “International Workshop on Heavy Monsoon Rainfall”, which was held at CMA on 12-14 October 2011. During the morning session of October 13, Prof. Renhe Zhang from CAMS introduced plans to conduct the above-mentioned field experiments. Scientists from CMA/CAMS and CMA/Numerical Prediction Center reported their related work on the numerical model development, and the modeling and analysis of monsoon rainfall. After one-day discussion, the workshop participants affirmed the scientific objectives of the experimental plans and CMA implement plan. Meanwhile, they made some recommendations to improve the plans. Based on these recommendations and those by Expert Group of the WWRP WGTMR Monsoon Panel, the above-mentioned plans were renamed as the "Southern China Monsoon Rainfall Experiment (SCMREX), with an improved version of the proposal. This revised proposal was submitted in January 2012 to and later accepted by the WMO Monsoon Panel, which was recommended to the WWRP in February 2012.
The SCMREX RDP proposal was approved during the 5th meeting of the WWRP Joint Scientific Committee which was held on 11-13 April 2012 at the WMO’s Headquarters in Geneva. The WMO Executive Council’s Annual Meeting, held in Geneva on 25 June – 3 July 2012 officially approved the SCMREX RDP as the WMO/WWRP RDP.
During the WMO/WWRP’s 2nd International Workshop on Heavy Monsoon Rainfall, which was held on 10-12 December 2012 in Kuala Lumpur, Malaysia, as an important part of the workshop, Prof. Yali Luo from CAMS introduced the origin, scientific objectives and implementation plans of SCMREX RDP, in the afternoon of December 11. Extensive discussions on the scientific goals, observing systems, data sharing were carried out by experts from China, the United States, Japan, South Korea and Australia, and other countries. Many useful recommendations and required actions were put forward.
On 26 December 2012, CMA/Division of Science and Technology and Climate Change organized a meeting to develop the SCMREX implementation plan, with divided tasks for several authors. This plan was completed a couple of months later.
In recent years, the WMO/WWRP has established several research development projects and Forecast Demonstration Project (FDP). They include the mesoscale Alpine Programme (MAP, Bougeault et al 2001; Ranzi et al. 2007) and its predecessor; the MAP Demonstration of Probabilistic Hydrological and Atmospheric Simulation of Flood Events (i.e., the MAP D-Phase, Zappa, et al 2008); the IMPROVE experiment over the Cascade Mountains of western North America aiming at improving cloud microphysical schemes (Stoelinga et al 2003); Sydney 2000 FDP (Anderson-Berry et al. 2004) aiming at promoting and showcasing summer convection nowcasting systems; Beijing 2008 FDP (Wilson et al 2010) demonstrating the progress in nowcasts and the technological advances in transforming research to applications since 2000; Beijing 2008 RDP (Duan et al. 2012) focusing on the mesoscale ensemble forecasts; Beijing 2008 FDP and RDP both emphasizing quantitative precipitation forecasts (QPFs), convective initiation, and summer severe weather; the Science of Nowcasting Olympic Weather for Vancouver 2010 Olympic (SNOW V10; Isaac et al. 2009) aiming at the winter weather nowcasts under complex terrain.
The SCMREX RDP aims at expediting our efforts to improve the prediction of heavy rainfall events in South China during the first rainy season through field campaigns and the subsequent data processing and sharing, numerical modeling and analysis. The project will improve our understanding of the structures and evolution of the South China heavy-rain-producing storms during the monsoon outbreak period, and improve our ability to predict these high-impact events through minimizing the errors in initial conditions and uncertainties in physics schemes in NWP models, and conduct/evaluate mesoscale ensemble prediction experiments. The unique background of the East Asian monsoon (and its associated abundant moisture) plus the complex surface conditions over South China (e.g., mesoscale mountains, plains, coastal, urban complexes; see Figs. 1 and 3d) gives rise to the uniqueness and complexity of heavy rainfall development during the first rainy season in South China. This also provides a great opportunity for SCMREX RDP to extend the WMO/WWRP RDP/FDP previous programs.
Southern China is one of the rainiest regions in China, with an annual amount of over 2000 mm. The rainfall amount, accumulated from the onset in April to the end in middle or late June, the so-called first rainy season, as the monsoon rainbelt moves northward to the Yangtze-Huai River basin (Ding, 1994), accounts for about 50% of the annual amount. This period often experiences the most frequent occurrences of heavy rainfall, leading to severe flooding and inundations. So these storms endanger the safety of lives, and cause marked property damages, often producing devastating economic losses. The first rainy season in southern China reaches its peak in terms of occurrence frequency and rainfall intensity during the period (Zhou et al., 2003; Fig.2). Most precipitation during the first rainy season in South China is of convective nature with mesoscale organizational characteristics, and more than 50% contributions to the total rainfall are from those heavy rainfall events with the rates of more than 50 mm day-1 (Fig. 3b). Major heavy rainfall centers are typically distributed over the coastal areas of Guangdong and Guangxi provinces, northern Guangxi as well as the western and northern Fujian, with the maximum rainfall located in Guandong Province (Fig. 3a). These rainfall centers correspond well to the most frequent occurrences of deep convection.
In order to better understand the heavy rainfall development during the first rainy season in South China, we have conducted for the first time in 1977-1980 “the Southern China Monsoon Rainfall Experiment”. Results indicate that these heavy rainfall events took place in the warm sector, and they are accompanied by the low-level jets, mesoscale convective systems (MCSs), and moist planetary boundary layer (PBL). The second such field experiment was conducted in 1998 (Zhou et al. 2003). Results from this experiment reveal some meso-b-scale structures of MCSs, and certain mechanisms by which they form. In particular, it is shown that high-resolution NWP models, initialized with large-scale observations, can simulate some observed meso-b-scale features associated with the heavy rainfall events. During the months of May-July 2008 and 2009, CAMS/the State Key Laboratory of Severe Weather conducted the South China Heavy Rainfall Experiment (SCHeREX) with four mesoscale observing networks distributed in South China, the low valley, and the middle valley of Yantze-River, and the Yangtze-Huai River basin, respectively (Zhang et al., 2011). One millimeter-wave radar and one C-band dual polarmetric Doppler radar were utilized; some dropsondes were used over the northern South China Sea. Mesos-analysis of the SCHeREX data and three-dimensional wind retrieval have revealed some mesoscale characteristics of heavy rainfall events.
The previous observational analyses show that the heavy rainfall events during the first rainy season in South China are mostly associated with mesoscale convective complexes (MCCs). However, the linkages from convective cells to convective clusters, to MCCs and further to regional heavy rainfall, as well as their multi-scale interactions are still unknown. Thus, it is extremely difficult to predict the timing and location of the heavy rainfall occurrence and its associated intensity changes. In general, we are lacking the predictability of heavy rainfall during the first rainy season (Fig. 4), and our ability to predict the warm-sector rainfall is much lower than that associated with the front rainfall.
To improve the forecast skill of heavy rainfall, it is desirable to explore the mechanisms whereby convective initiation and subsequent mesoscale organization occur, as well as the processes leading to rainfall intensity changes. On the other hand, it is important to improve the short- and medium-range quantitative precipitation forecasts (QPFs) with NWP models. To achieve this requires the accurate representation of clouds and precipitation as well as mesoscale information in the model initial conditions, the reasonable description of the model physical processes, and the development of ensemble prediction systems. For the model physics schemes, the PBL and cloud microphysics deserve special attention, because (a) these heavy rainfall events are highly associated with the PBL processes (b) the triggering of deep convection is closely related to the flows, moisture and thermal conditions in the PBL; (c) convective development and rainfall intensity are controlled by cloud-precipitation physical processes, especially as the grid size decreases to 1-3 km; and (d) few studies have been done to verify the model-simulated fine-scale structures of the PBL and convective systems, mainly due to the lack of suitable observations. Thus, we are uncertain about the ability of the current PBL and cloud microphysics schemes to simulate the development of heavy rainfall during the first rainy season in South China.
Through the previous field experiments, Chinese scientists have gained valuable experience in collecting and processing various non-conventional observational datasets (Ni et al., 2011), and in using the data in their research and operational forecasts (Zhang et al., 2011). However, the previous field experiments were unable to capture the inner-core characteristics of MCSs and the underlying PBL processes leading to heavy rainfall events. In addition, the high-resolution observations so obtained have not been applied to the verification and improvement of physical parameterization schemes in NWP models. Today, we have better observing facilities measuring the inner-core structures and PBL processes during the first rainy season in South China. For example, since 2007, we have installed 16 windprofiler radars, including 2 in Hong Kong, over Guangdong Province, which consists of 14 PBL windprofilers and 2 tropospheric windprofilers. These instruments will greatly improve the observational capability of low-level flows triggering deep convection, and help us capture the low-level shear or convergence line, and fluctuations in the LLJs. Moreover, as compared to SCHeREX, we now have more advanced portable remote sensing equipment, to be described in section 3.4.3, which can provide more comprehensive detection of the inner-core structures of MCSs. We have also made significant progress in data quality control and retrieval algorithm (Hu et al. 2010, 2012; Liu et al. 2010), which would allow us to obtain more accurate information of the microphysical and dynamic structures in the inner regions of MCSs. In addition, since 2009, CMA/National Meteorological Information Center, has began to perform the real-time quality control of all automated surface observatories, and established a real-time quality control system for basic meteorological variables (i.e., air temperature, air pressure, humidity, wind speed and direction, and precipitation), with error information feedback mechanisms (Ren et al. 2012; Ju et al. 2010a,b; Zhao et al. 2011). These high-resolution (i.e., hourly and a grid spacing of about 10 km) continuous surface observations have provided us with new opportunities to carry out surface meso-analysis, and investigate the mechanisms whereby heavy precipitation develops.
With the above-mentioned favorable background conditions, CMA conducts the international research and development project of “Southern China Monsoon Rainfall Experiment” with the following goals: (i) To obtain comprehensive observational datasets that could be shared among researchers of interest and then used to describe MCSs and their environmental conditions during the first rainy season in South China; (ii) To improve simulation and QPF of the torrential rain through advancing data assimilation technique and improving physics parameterizations in model and by conducting/evaluating mesoscale ensemble prediction (MEP) experiments. (iii) To increase our knowledge on the development of heavy rainfall in the warm sector and the associated dynamical and microphysical processes; and (iv) To establish a high-quality meteorological data bank to facilitate future scientific exchange.
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