East Asian Summer Monsoon Season Research Articles

Diurnal Variation of Summer Monsoon Season Precipitation Over Southern Hainan Island, China: The Role of Boundary Layer Inertial Oscillations Over Indochina Peninsula

AbstractIn June after the onset of the East Asian summer monsoon season, rainfall on the south side of Hainan Island (HNI) of China exhibits a trimodal pattern with an unusual peak near noon. A typical case of 20 June 2017 is chosen to analyze the mechanisms through a convection‐permitting simulation together with rainfall observations. Clockwise rotation in low‐level winds is found at and off the coast of south HNI, leading to maximum onshore southwesterly winds in the late morning that is closely linked to the diurnal variation of rainfall. The offshore diurnal variations of low‐level winds are tracked upstream and attributed to boundary layer inertial oscillations over the inland plain of Indochina Peninsula. In addition, a short wide gap comprised mostly of hills lower than 700 m is located across the higher Annamite Range along the east coast of the Peninsula, right upstream of HNI. Nocturnal boundary layer low‐level jet (LLJ) forming over the plain passes through the gap with significant acceleration due to gap channeling and downslope flow effects, reaching peak intensity off the Peninsula coast at around 09 local standard time (LST). The enhanced LLJ propagates downstream towards HNI and impinges on the mountains at around 12 LST when rainfall reaches peak there. As the inertial‐oscillation‐indued perturbation winds approaching the Island weaken and eventually reverse direction, lifting weakens and low‐level stability increases as descending motion develops offshore. Rainfall on the southern Island therefore dissipates in late afternoon.

Size Distributions of Water-Soluble Inorganic Ions in Atmospheric Aerosols During the Meiyu Period in the Yangtze River Delta, China

In order to investigate the chemical composition distributions and pollution characteristics of Total water-soluble inorganic ions (TWSII) in the rain period (Meiyu) in the East Asian summer monsoon season, including the impact of Meiyu on air pollution in the Yangtze River Delta, East China, the gaseous pollutant concentrations, the 9 sizes segregated particles, and water-soluble inorganic ions of aerosols were measured on the north shore of Taihu Lake from June 4 to July 5, 2016. Results show that the mass concentrations of atmospheric particulate matters (PM2.5 and PM10) and main gaseous pollutants (SO2, NO2, CO, and O3) decrease during the Meiyu period, with the largest decline in PM10 and the smallest in CO. TWSII in atmospheric particles are mainly concentrated in fine particles during the Meiyu period. The values of ρ (TWSII) for PM1.1, PM1.1–2.1, and PM2.1–10 before the Meiyu onset are generally greater than those during the Meiyu period. During the first pollution process, the ρ(TWSII) for PM1.1 and PM1.1–2.1 first increase to the peak values, and then decrease during the moderate rainfall period, when the ρ(TWSII) in PM2.1–10 increase to its maximum before the Meiyu onset. The mass concentrations for anions, cations, and total ions at different particle-size sections all exhibit bimodal distributions before and after the Meiyu onset. The mass concentration peaks at a particle size of 1.1–2.1 μm for fine particles, while at 5.8–9.0 μm (before the Meiyu onset) and 9.0–10.0 μm (during the Meiyu period) for coarse particles, respectively. The peak particle size for mass concentration of coarse particles moves toward larger sizes during the Meiyu period. The mass concentrations of SO42− at different particle-size sections show a bimodal distribution before the Meiyu onset and a multi-modal distribution during the Meiyu period. The mass concentrations of NO3− at different particle-size sections show a bimodal distribution before the Meiyu onset and a unimodal distribution during the Meiyu period. The mass concentrations of NH4+ at different particle-size sections present a bimodal distribution before and after the Meiyu onset, with the particle-size for peak concentrations distributing in 1.1–2.1 and 5.8–9.0 μm before the Meiyu onset, and 9.0–10.0 μm during the Meiyu period. The mean value of nitrogen oxidation ratio (NOR) is higher before the Meiyu onset than after, indicating that the secondary conversion of NO2 before the Meiyu onset is enhanced. The sulfur oxidation ratio (SOR) values are greater than NOR values, but the concentrations of NO2 in the same period during the Meiyu period are higher than those of SO2, which indicates that the secondary conversion of SO2 during the Meiyu period on the north bank of Taihu Lake is stronger than that of NO2. During the whole observation, the contribution of stationary sources mainly contributed to the atmospheric particulate matters during the Meiyu period. The contributions of vehicle exhaust and coal combustion to fine particles are more obviously affected by the changes in meteorological conditions during the Meiyu period, and the vehicle emissions contribute more to PM1.1–2.1 than to PM1.1.

Efficient transport of atmospheric microplastics onto the continent via the East Asian summer monsoon

The presence of microplastics (MPs) in the atmosphere is a global concern because of its environmental and health impacts; however, the monsoonal transport of atmospheric MPs has not yet been investigated. To fully understand the effect of the monsoon on atmospheric MP transport, we conducted a study along the southeast coast of China during the East Asian summer monsoon (EASM). We found that the EASM transports atmospheric MPs back onto the continent at a flux of up to 212.977–213.433 kg/EASM/year. The backward trajectory and wind field results indicate that the EASM provides an effective MP transport pathway from Vietnam, the Philippines, and Malaysia to southeastern China. This suggests that only some of the airborne MPs over the ocean enter the marine ecosystem. The average abundance of atmospheric MPs over the sampling area was 0.39 items/100 m3 (0.39 ± 0.43 items/100 m3) during the EASM season, with high variability among the sampling sites. This study improves our understanding of the impact of the EASM on atmospheric MP transport, which can help quantify the contributions of atmospheric MPs to marine or terrestrial ecosystems.

Preliminary Study of Land–Sea Microphysics Associated with the East Asian Summer Monsoon Rainband and Its Application to GPM DPR

AbstractRaindrop size distribution (DSD) characteristics during the East Asian summer monsoon (EASM) were studied, using measurements from three OTT Particle Size Velocity (Parsivel) disdrometers in Nanjing, Chuzhou, and the northwestern Pacific (NWP), respectively. Western and eastern parts of the monsoon rainband were separated for a comparative study of the DSD variability. Along with disdrometer data, GPM Dual-Frequency Precipitation Radar (DPR), Fengyun-2E (FY-2E), MODIS, GPCP, ERA-Interim, and in situ radiosonde datasets are combined to illustrate the possible microphysical mechanisms for the significant DSD variability in two parts, in terms of convective intensity, cloud structure, and aerosol effects. The DSD characteristics of six rain-rate classes and two rainfall categories (convective and stratiform) were studied. The western part has larger mass-weighted mean diameter Dm while smaller normalized intercept log10(Nw) than the eastern part, and the convective clusters of the western part (land) could be identified more maritime-like than continental-like due to moisture transport from the tropical ocean, while that of the eastern part (sea) is between maritime-like and continental-like. Cross validation of GPM rainfall products are implemented based on surface disdrometer observations. DPR products manifest better performance over sea than land areas of the EASM rainband. Empirical Dm–Ze and Nw–Dm relations were also derived preliminarily to improve the GPM rain-retrieval algorithms in the EASM season.

Atmospheric rivers impacting mainland China and Australia: climatology and interannual variations

In this study we have built two atmospheric river (AR) databases for mainland China and Australia using Japanese 55-year Reanalysis data with manual detections. By manually checking the magnitude, shape and orientation of vertically integrated vapour transport fields calculated from the reanalysis data and analysing its embedded synoptic patterns and other meteorological information, we detected 625 AR events over mainland China during 1986–2016 and 576 AR events over the Australian continent during 1977–2016. This manuscript documents the mean climatology, spatial distributions, seasonality and interannual variations of ARs occurring in these two regions. We also assessed possible underlying drivers influencing AR activities. Our results showed that: (i) most ARs over mainland China occured in its lower latitudes, including southern, eastern and central China, but ARs also reached its far north and northeast regions. In Australia, most ARs occurred in the states of Western Australia, South Australia and part of New South Wales and Victoria. These regions of high AR frequencies also frequently experienced Northwest Cloud Bands during the cool season; (ii) ARs in China reached their peak during the East Asian summer monsoon season (May–September). This was also the period when AR frequency in the Australian region tended to be higher, but its seasonal variation was weaker than in China; (iii) ARs exhibited large interannual variations in both regions and a declining trend in central and eastern China; (iv) there was a notable influence of tropical sea surface temperatures (SSTs) on the AR activities in the region, with the ARs in Australia being particularly affected by Indian Ocean SSTs and El-Niño Southern Oscillation (ENSO) in the tropical Pacific. ARs in China appear to be affected by ENSO in its decaying phase, with more ARs likely occurring in boreal summer following a peak El Nino during its preceding winter; (v) the Western Pacific Subtropical High plays a dominant role in forming major moisture transport channels for ARs in China, and South China Sea appears to be a key moisture source. In the Australian region, warm and moist air from the eastern part of the tropical Indian Ocean plays a significant role for ARs in the western part of the continent. In addition, moisture transport from the Coral Sea region was an important moisture source for ARs in its east. Results from this study have demonstrated the value of using AR diagnosis to better understand processes governing climate variations in the A–A region.

Paleoceanographic changes in the Northern East China Sea during the last 400 kyr as inferred from radiolarian assemblages (IODP Site U1429)

The East China Sea (ECS) is a shallow marginal sea that is sensitive to glacio-eustatic sea-level changes and is influenced by warm oligotrophic water of the Kuroshio Current (KC), the nutrient-rich Taiwan Warm Current, and freshwater discharges from rivers in southern China during the East Asian summer monsoon season. In this area, local paleoceanographic changes for times prior to 40 ka remain poorly studied because of high sediment accumulation rates on the seafloor. During Integrated Ocean Drilling Program Expedition 346, long sediment cores representing the last 400 kyr were retrieved from the northern part of the ECS (Site U1429). In these cores, radiolarians are abundant and well-preserved, thus using the ecological properties of radiolarians, we analyzed how glacio-eustatic sea-level variations have influenced the paleoceanography of the ECS over the last 400 kyr, with a focus on changes in water properties at intermediate depths. Additionally, the summer sea surface temperature (SST) and intermediate water temperature at about 500 m were quantified by means of data on selected radiolarian species. The KC influenced the shallow water at Site U1429 during each interglacial period over the last 400 kyr (marine isotope stages [MISs] 1, 5, 7, 9, and 11), causing a high summer SST (about 27 °C), although inflow of the KC into the ECS was probably delayed until after the sea-level maximum of interglacial MIS 1 and MIS 5. During this lag time, ECS shelf water was the dominant influence on the system. During glacial periods (MISs 2–4, 6, and 10), our data suggest that coastal conditions prevailed, probably because of a sea-level drop of more than 90 m. At these times, the summer SST was colder, ca. 20 °C. Changes in the relative abundance of Cycladophora davisiana indicate that the most significant changes in the bottom water occurred during MIS 6, when the bottom water likely became poorer in oxygen. An increase in the shallow-water primary productivity during MIS 7 and MIS 6 was probably the key factor causing the oxygen-poor conditions.

Contributions of GCM/RCM Uncertainty in Ensemble Dynamical Downscaling for Precipitation in East Asian Summer Monsoon Season

Contributions of GCM/RCM Uncertainty in Ensemble Dynamical Downscaling for Precipitation in East Asian Summer Monsoon Season

Evaluation of the East Asian Summer Monsoon Season Simulated in CMIP5 Models and the Future Change

Evaluation of the East Asian Summer Monsoon Season Simulated in CMIP5 Models and the Future Change

Simulation of the Summer Monsoon Rainfall over East Asia Using the NCEP GFS Cumulus Parameterization at Different Horizontal Resolutions

Abstract The most recent version of the simplified Arakawa–Schubert (SAS) cumulus scheme in the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) (GFS SAS) is implemented in the Weather Research and Forecasting (WRF) Model with a modification of the triggering condition and the convective mass flux in order to make it dependent on the model’s horizontal grid spacing. The East Asian summer monsoon season of 2006 is selected in order to evaluate the performance of the modified GFS SAS scheme. In comparison to the original GFS SAS scheme, the modified GFS SAS scheme shows overall better agreement with the observations in terms of the simulated monsoon rainfall. The simulated precipitation from the original GFS SAS scheme is insensitive to the model’s horizontal grid spacing, which is counterintuitive because the portion of the resolved clouds in a grid box should increase as the model grid spacing decreases. This behavior of the original GFS SAS scheme is alleviated by the modified GFS SAS scheme. In addition, three different cumulus schemes (Grell and Freitas, Kain and Fritsch, and Betts–Miller–Janjić) are chosen to investigate the role of a horizontal resolution on the simulated monsoon rainfall. Although the forecast skill of the surface rainfall does not always improve as the spatial resolution increases, the improvement of the probability density function of the rain rate with the smaller grid spacing is robust regardless of the cumulus parameterization scheme.

Relationship between the initiation of a shallow landslide and rainfall intensity—duration thresholds in Japan

The empirical rainfall intensity and duration ( I– D) threshold for the initiation of shallow landslide is newly defined for Japan where heavy rainfalls frequently occur during the East Asian summer monsoon season. The rainfall causes sediment-related disasters annually. This paper presents an examination of 1174 rainfall-induced shallow landslides that occurred during 2006–2008. Their I– D conditions were analyzed objectively from rainfall data (Radar-Raingauge Analyzed Precipitation) to derive the I– D threshold using the quantile-regression method: I = 2.18 D − 0.26 , where I is measured in millimeters per hour and D in hours, as measured from the beginning of rainfall to the landslide occurrence. Rainfall events are separated by the absence of rainfall for 24 h. We then established a rescaled I– D threshold by dividing the rainfall intensity by the mean annual precipitation ( MAP), as I MAP = 0.0007 D − 0.21 , where I MAP is the rescaled average per-hour rainfall intensity. These thresholds were defined by the second percentile regression line for D of 3–537 h. The new thresholds are considerably lower than those previously reported for the world, humid subtropical regions, the Asian monsoon region, and Japan. The result suggests that Japan is highly prone to rainfall-induced shallow landslides because of its high-relief topography, geologic conditions, human interference, and rainfall characteristics during the East Asian summer monsoon season.

Global Warming Projection by an Atmospheric Global Model with 20-km Grid

We projected global warming on the Earth Simulator using a very high horizontal resolution atmospheric global general circulation model with 20-km grids, targeting tropical cyclones (TCs) and the rain band (Baiu) during the East Asian summer monsoon season because these bring typical extreme events and global climate models have not yielded reliable simulations or projections due to insufficient resolutions. Our model reproduces TCs and a Baiu rain band reasonably well under present-day climate conditions. In a warmer climate at the end of this century, the model projects, under A1B scenario of the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES), that the annual mean formation frequency of TCs decreases by about 30% globally but increased in the North Atlantic and TCs with largemaximumsurface winds increase. The Baiu rain band activity tends to intensify and last longer until August, suggesting more damages due to heavy rainfalls in a warmer climate. This is a review paper mainly originated from published articles on tropical cyclone by Oouchi et al. (2006) [26] and on the East Asian summer monsoon by Kusunoki et al. (2006) [17].

Regional Differences of Daily Rainfall Characteristics in East Asian Summer Monsoon Season

Regional differences of daily rainfall characteristics are examined during the East Asian summer monsoon season (May to August) over a 30-year (1961-90) period. It is found that the contribution of heavy rainfall with a daily total >50 mm to total seasonal precipitation exceeds 50% around Taiwan, in the northwestern part of Kyushu Island, coastal Southwest Japan, and the southern coast of China. It exceeds 40% in northern Korea, the middle of the Yangtze and lower Huai-he River Basins. On the other hand, daily rainfall characteristics in Southwest, Northwest and North China are quite different from these regions. In Southwest China, although seasonal total precipitation is considerably large ( > 1000 mm), the contribution of daily heavy rainfall to total precipitation is only less than 20% to the west of 105°E. In Northwest China, both total precipitation and contribution of daily heavy rainfall to total precipitation are small. In North China, precipitation by weak daily rainfall with a daily total < 20 mm is relatively high. The seasonal changes of heavy rainfall events are also examined. In general, the area of both maximum precipitation and heavy rainfall occurrence progresses northward from early June to late July in the major part of both East China and Japan in accordance with northward displacement of Baiu/Meiyu season. But some different features are found to the west of ∼ 110°E in China.