Impact of rapid urbanization on temporal and spatial pattern change of heavy rainfall in China during the past 60 years

Recent studies have noted a worldwide increase in the occurrence of extreme-precipitation events. Here we use daily rainfall data from 1951 to 2010 of 659 meteorological stations in China and on the basis of duration days defined long duration heavy rainfall. Results indicate that: on the spatial distribution, short duration heavy rainfall shows gradually decreasing phenomenon from the southeast coastal to northwest inland in China from 1951 to 2010. And long duration heavy rainfall is concentrated in the southeast coastal areas, such as Guangdong, Guangxi and Hainan. On the temporal change, the interannual and interdecadal short and long duration heavy rainfall both show increasing trend. In precipitation contribution ratio, the proportion of total heavy rainfall amounts to total rainfall amounts and total heavy rainfall days to total rainfall days are 6.1%∼27.7% and 0.6%∼27.7% respectively from 1951 to 2010 in China. Short duration storm to occupy the dominant position that the proportion of short duration heavy rainfall amounts to total heavy rainfall amounts and short duration heavy rainfall days to total heavy rainfall days are 75.9%∼89.4% and 75.6%∼89.4% respectively in the same period. Long duration heavy rainfall occupy a secondary position that the proportion of long duration heavy rainfall amounts to total heavy rainfall amounts and short duration heavy rainfall days to total heavy rainfall days are only 10.6%∼24.1% and 10.8%∼24.4% respectively. On the trend of precipitation contribution, China’s the contribution rate of total heavy rainfall to the total rainfall show increasing trend with contribution of rainfall amounts and rainfall days trend are 2.1%/10a and 0.2%/10a respectively. The contribution rate of short duration heavy rainfall to the total heavy rainfall show increasing trend with contribution of rainfall amounts and rainfall days trend are 0.5%/10a and 0.4%/10a respectively. On the contrary, the contribution rate of long duration heavy rainfall to the total heavy rainfall show decreasing trend with contribution of rainfall amounts and rainfall days trend are -0.5%/10a and -0.4%/10a respectively. The results suggest that the precipitation in China are changing to extremely accompanied by short duration storm increased significantly.

This study analyzed the trends of extreme daily rainfall indices over the Indochina Peninsula from 1960 to 2007. The trends were obtained from high-resolution gridded daily rainfall data compiled by APHRODITE with coordinates of 4°N–25°N and 90E°–112°E. The indices were selected from the list of climate change indices recommended by ETCCDI, which is a joint group of WMO CCl, CLIVAR and JCOMM. The indices are based on the number of heavy rainfall days (≥10 mm), number of very heavy rainfall days (≥20 mm), number of extremely heavy rainfall days (≥25 mm), consecutive dry days (<1 mm), consecutive wet days (≥1 mm), daily maximum rainfall, five-day maximum rainfall, annual wet-day rainfall total, Simple Daily Intensity Index, very wet days, and extremely wet days. The indices were simulated by calculating different extreme characteristics according to wet and dry conditions, frequency, and intensity. Linear trends were calculated by using a least squares fit and significant or non-significant trends were identified using the Mann–Kendall test. The results of this study revealed contrasting trends in extreme rainfall in eastern and western Indochina Peninsula. The changes in extreme rainfall events in the east primarily indicate positive trends in the number of heavy rainfall days, very heavy rainfall days, extremely heavy rainfall days, consecutive wet days and annual wet-day rainfall total, with significant trends at times. These events correlated with the northeastern monsoon that influences the Indochina Peninsula from October to February annually. The results in the west primarily indicate negative trends in consecutive wet days, where significant trends were correlated with decreasing number of annual wet-day rainfall total, heavy rainfall days, very heavy rainfall days, and extremely heavy rainfall days. Daily maximum rainfall, five-day maximum rainfall, very wet days, and extremely wet days show random positive (negative) significant (non-significant) trends, while the simple daily intensity index shows positive trends that dominate the southern part of the Indochina Peninsula, with some grids show significant trends.

The daily weather at a particular place is largely influenced by the synoptic circulation and thermodynamic profile of the atmosphere. Heavy rainfall occurs from a particular subset of synoptic and thermodynamic states. Baseline climatologies provide objective information on heavy rainfall-producing circulation patterns and thermodynamic variables. This is how climatologically large or extreme values associated with heavy rainfall are identified. The aim of this research is to provide a heavy rainfall sounding climatology in austral summer over Gauteng, South Africa, using self-organising maps (SOMs). The results show that the SOM captures the intra-seasonal variability of heavy rainfall soundings by clearly distinguishing between the atmospheric conditions on early summer (October–December) and late summer (January–March) heavy rainfall days. Conditions associated with heavy early summer rainfall are large vertical wind shear and conditional instability, while the atmosphere is drier and cooler than when heavy rainfall occurs in late summer. Late summer heavy rainfall conditions are higher convective instability and small vertical wind shear values. The SOM climatology shows that some heavy rainfall days occur in both early and late summer when large-scale synoptic weather systems cause strong near-surface moisture flux and large values of wind shear. On these days, both the conditional and convective instability of the atmosphere are low and heavy rainfall results from the strong synoptic forcing. In contrast, heavy rainfall also occurs on days when synoptic circulation is not very favourable and the air is relatively dry, but the atmosphere is unstable with warm surface conditions and heavy rainfall develops from local favourable conditions. The SOM climatology provides guidelines to critical values of sounding-derived parameters for all these scenarios.

This paper discusses projections of heavy rainfall events in China during the 21st century based on daily precipitation data from the Fourth Assessment Report's (AR4) Coupled General Circulation Models (CGCM). Results show that all three experimental scenarios (scenarios A2, A1B, and B1) project consistent changes in frequency and intensity of heavy rainfall at the end of 21st century. In the regions of Northeast China and North China, there are no significant changes in frequency but there are remarkable increases in intensity of heavy rainfall, indicating that enhanced intensity is the main contributor to increased ratios of heavy rainfall to total annual precipitation in these regions. In regions of the lower reaches of Yangtze River and South China, increases in the amount of heavy rainfall are closely associated with increased frequency and increased intensity. Projected frequencies of heavy rainfall at the end of 21st century increase by 30.9 ∼ 56.6% in the Yangtze River and 35.9 ∼ 50.2% in South China compared to the period of 1980–1999, and projected intensities increase by 1.0 ∼ 5.7% and 2.8 ∼ 6.3%, respectively. Additionally, the ratios of heavy rainfall to total annual precipitation increase by 2.3 ∼ 5.4% in the Yangtze River and 1.8 ∼ 3.8% in South China. The significant increases of heavy rainfall ratios indicate that as the climate warms, heavy rainfall events are expected to increase at rates that are much faster than increases in total precipitation amounts, indicating that China will experience increased amounts of flooding. These results are substantially consistent among the three IPCC (Intergovernmental Panel on Climate Change) scenarios.The increased probability of heavy rainfall events in China is closely connected with increased transportation of water vapour from the Arabian Sea and the South China Sea. Additionally, atmosphere stratification has become increasingly unstable, which has provided a favorable background for the initiation of heavy rainfall at the end of the 21st century. Copyright © 2010 Royal Meteorological Society

ContextLand use/land cover change and other human activities contribute to the changing climate on regional and global scales, including the increasing occurrence of extreme-precipitation events, but the relative importance of these anthropogenic factors, as compared to climatic factors, remains unclear.ObjectivesThe main goal of this study was to determine the relative contributions of human-induced and climatic factors to the altered spatiotemporal patterns of heavy rainfall in China during the past several decades.MethodsWe used daily precipitation data from 659 meteorological stations in China from 1951 to 2010, climatic factors, and anthropogenic data to identify possible causes of the observed spatiotemporal patterns of heavy rainfall in China in the past several decades, and quantify the relative contributions between climatic and human-induced factors.ResultsOur analysis suggests that a total of 84.7–87.5% of the variance in heavy rainfall factors could be explained by large-scale climate phenomena and the local/regional anthropogenic activities. In particular, urbanization and air pollution together explained 58.5–65.5% of the variance. The spatial distribution of heavy rainfall amount and days over time shows a significant and increasing correlation with the spatial distributions of population density and annual low-visibility days.ConclusionsOur results suggest that the substantial increase in heavy rainfall across much of China during the past six decades is likely triggered by local and regional anthropogenic factors. Our results call for a better understanding of local and regional anthropogenic impacts on climate, and the exacerbated extreme climate events as a potential consequence of urbanization and air pollution.

The increase in the variability of rainfall as the climate of the world warms up is a concern for many regions. Studies in the past have associated this change in variability to rise in heavy rainfall events. Thus a regional analysis of heavy rainfall characteristics to evaluate its response to a changing climate becomes important. In this study, we are putting this focus on the North East Region (NER) of India, which boasts of being one of the wettest regions of the world. This study examines the heavy rainfall characteristics from 1901 till 2020 over NER using the IMD gridded daily rainfall product. The examination showed that although the annual and monsoonal rainfall over the NER has been decreasing, the intensity of heavy rainfall has increased by ~5mm over the decades. Singular Spectrum Analysis is used to identify the long-term trend, which showed a change in heavy rainfall characteristics around 1970 and hence further investigation is carried out over two time blocks (Pre-1970: 1901-1970 and Post-1970: 1971-2020). The investigation of the frequency of heavy rainfall events showed that its increasing trend, prior to 1970, transitions to a decreasing trend, post 1970. It also showed that the area under a negative trend of frequency has increased significantly after 1970. Furthermore, a non-parametric probability distribution approach has also been implemented to interpret the frequency and intensity relationship of heavy rainfall together. This showed that post 1970, the probability of occurrence of a very heavy or extreme rainfall events has increased. The increase in probability did show a spatial variability. The increase in probability is more for the pre-monsoon season compared to the monsoon. This finding corresponds to the fact that the contribution of pre-monsoon rainfall to annual rainfall has increased while that of the monsoon rainfall has decreased over the decades. To investigate the local causes of the observed changes, the 2m temperature (T2), 2m Dew-point temperature (TD2) are investigated using a cross-sample entropy analysis. Interestingly, both T2 and TD2 showed a significant increasing trend over NER. Coincidentally, locations with increasing heavy rainfall intensity and frequency are also the locations with increasing TD2. Also, the pre-monsoon show a stronger increase in TD2, i.e., more moisture is available for convection, compared to T2, which could explain the higher probability of heavy rainfall events. Thus, increasing intensity and decreasing frequency can be explained by the inter-relationship between T2, TD2 and the convective processes.

In the austral summer, parts of southeastern Australia are prone to heavy rainfall that causes major riverine flooding and fatalities. Easterly flow associated with an anticyclone in the Tasman Sea, large moisture transports from the Coral Sea, and upper tropospheric cyclonic disturbances all contribute to these heavy rainfall episodes. However, questions regarding their synoptic dynamics remain, including which of these ingredients are the most critical. These questions are addressed by comparing composite pressure and moisture fields of heavy rainfall days over selected regions with non‐heavy rainfall days that have a similar synoptic pattern. A synoptic climatology is constructed for this purpose by ‐means cluster analysis of 500 hPa geopotential height anomalies from the European Centre for Medium‐range Weather Forecasts Reanalysis v5, for all December to March days over a period of 40 years. Heavy rainfall days in the wettest clusters have negative 500 hPa geopotential height anomalies immediately west of the affected region that are stronger on average than those of non‐heavy rainfall days. Their accompanying distributions of surface pressure, precipitable water, and vertical motion are consistent with cyclonic baroclinic development and are preceded by anticyclonic Rossby wave breaking. Heavy rainfall days also show an increased frequency of blocking near E; however, this peaks 1–2 days after the onset of heavy rain. Regional rainfall in these clusters shows strong sensitivity to lower pressure immediately westward but little sensitivity to high pressure in the Tasman Sea until after the commencement of rain. A companion study using the same cluster analysis illustrated the link between anticyclonic Rossby wave breaking and heatwaves in southeastern Australia. These latest results highlight the upper cyclonic anomalies that often form on the equatorward flank of anticyclonic Rossby wave breaking as the key ingredient separating days with a favourable synoptic‐scale pattern of surface high pressure into those that rain heavily and those that do not.

This study analyzed trends in extreme precipitation based on daily rainfall data provided by Bénin Méteo Agency for the Upper Ouémé valley in Benin over the period 1951–2014. Eleven indices divided into two groups were considered. The first group consists of frequency indices: number of heavy rainfall days, very heavy rainfall days and extremely heavy rainfall days; and maximum number of Consecutive dry days and wet days. The second group concerns intensity: daily maximum rainfall (RX1day), maximum five-day rainfall (RX5day), annual total wet-day rainfall (PRCPTOT), simple daily intensity index (SDII), very wet day (R95P) and extremely wet day rainfall (R99P). The non-parametric Mann-Kendall test was used to assess trends in those indices. The results show that only 30% of the stations experienced decreasing trends for the number of heavy rainfall days (R10mm) and daily maximum rainfall (RX1day). For the annual total wet-day rainfall (PRCPTOT), the simple daily intensity index (SDII) and the very wet day rainfall (R95P), 20% of stations faced significant negative trends. In addition, the decreasing trends are observed for 10% stations considering the number of very heavy rainfall days (R20mm), the maximum five-day rainfall (RX5day) and the extremely wet day rainfall (R99P). About the increasing trend, 10% stations are identified for the number of consecutive dry days (CDD), very heavy rainfall days (R20mm), the daily maximum rainfall (RX1day), the simple daily intensity index, and the extremely wet day rainfall (R99P). These results show the absence of clear trend of climate indices evolution in almost all stations. Consequently, uncertainties in the evolution of rainfall indices must be taken into account in the definition of adaptation strategies for flood or drought risks. Similarly, these results show a slight drop in the dry sequences of the 1970s and 1980s revealed in the region by previous studies.

To assess whether changes in the frequency of heavy rainfall events are occurring over time, annual maximum records from 21 rainfall gauges in Ontario are examined using frequency analysis methods. Relative RMSE and related boxplots are used to characterize assessment for selecting distributions; the Gumbel distribution is verified as one of the most suitable distributions to provide accurate quantile estimates. Records were divided into two time periods, and tested using the Mann-Kendall test and lag-1 autocorrelations to ensure that data in each period are identically distributed. The confidence intervals of design rainfalls for each return period (2, 5, 10, and 25-year) are derived by using resampling method, and compared at 90 % confidence levels. The changes in heavy rainfall intensities are tested at gauges across the Province of Ontario. Several significant decreases in heavy rainfall intensities are identified in central and southern Ontario. Increases in heavy rainfall intensities are identified in gauges at Sioux Lookout and Belleville. The sensitivity analysis of changes identified with respect to the year of splitting indicates changes are occurring during the 1980s and 1990s.

The seasonal variations of heavy rainfall days over Taiwan are analyzed using 6-yr (1997–2002) hourly rainfall data from about 360 rainfall stations, including high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System stations and 25 conventional stations. The seasonal variations and spatial variations of nontyphoon and typhoon heavy rainfall occurrences (i.e., the number of rainfall stations with rainfall rate >15 mm h−1 and daily accumulation >50 mm) are also analyzed. From mid-May to early October, with abundant moisture, potential instability, and the presence of mountainous terrain, nontyphoon heavy rainfall days are frequent (>60%), but only a few stations recorded extremely heavy rainfall (>130 mm day−1) during the passage of synoptic disturbances or the drifting of mesoscale convective systems inland. During the mei-yu season, especially in early June, these events are more widespread than in other seasons. The orographic effects are important in determining the spatial distribution of heavy rainfall occurrences with a pronounced afternoon maximum, especially during the summer months under the southwesterly monsoon flow. After the summer–autumn transition, heavy rainfall days are most frequent over northeastern Taiwan under the northeasterly monsoon flow. Extremely heavy rainfall events (>130 mm day−1) are infrequent during the winter months because of stable atmospheric stratification with a low moisture content. Typhoon heavy rainfall events start in early May and become more frequent in late summer and early autumn. During the analysis period, heavy rainfall occurrences are widespread and dominated by extremely heavy rainfall events (>130 mm day−1) on the windward slopes of the storm circulations. The spatial distribution of typhoon heavy rainfall occurrences depends on the typhoon track with very little diurnal variation.

China, as are other countries, is greatly affected by summer monsoons and their associated heavy rainfalls. A notable example for severity and duration of rainfalls is Meiyu (Plum Rains) occurring over the Yangtze River Valley. Almost every year, much damage is caused in China by flash floods or persistent floods caused by excessively heavy rains. In some years, the regional loss of property and lives reaches disaster proportions, exemplified by the extensive and devastating flood of the Huaihe River (in the central part of China) in August 1975. Much emphasis has been placed in recent years in China on research dealing with heavy rain with the aim of improving the prediction of heavy rainfall. The summer monsoon rainfalls in China mainly include the pre-summer heavy rainfall in South China, the Meiyu (Plum Rains) over the Yangtze River and Huaihe River Valleys and the mid-summer heavy rainfall in northern China. For years, Chinese meteorologists have paid more attention to these problems. In the 1950’s, the research into this aspect was focused on large-scale circulation conditions and rain-bearing synoptic systems (Xie et al., 1956; Tao et al., 1957). During the past 20 years, however, emphasis has been placed on the diagnostic analysis and theoretical study of meso- and small-scale systems.

Trends in climate extreme indices and implications for coffee farming in The Kafa Biosphere Reserve, southwest Ethiopia

Yaku-shima is a mountainous island, with area of approximately 500 km2 and peak of 1, 935 m, located in the western North Pacific around 30° N, 131° E (Fig. 1). This island is known as one of the highest precipitation areas in Japan where annual mean precipitation in the mountainous region exceeds 7, 000 mm. In the present study, distribution of monthly and annual mean precipitation is presented, and that of heavy rainfall days when daily precipitation exceeding 100 mm is observed at least one station in the island is investigated by using station data in so far as possible. For the mean distribution, data of 16 stations are available, while for heavy rainfall distribution data of 23 stations are utilized (Table 1).First, distribution of monthly and annual mean precipitation is presented. Monthly mean precipitation at some stations is over 1, 000 mm in June or September (Fig. 2). Annual mean precipitation exceeds 5, 000 mm in the inland area, with the highest value of 7, 373 mm in the southeastern part (Fig. 3).Next, precipitation distribution of heavy rainfall days from 1996 to 1998 is analyzed as follows : for each heavy rainfall day when rainfall amount over 100 mm/day is observed at anystations in the island, “relative precipitation (RP)” is calculated as a ratio of daily precipitation at each station to the maximum daily precipitation observed in the island on the same day. The cause of rainfall on each heavy rainfall day is identified as typhoon (Ty), extratropical cyclone (Ec), stationary front (Sf), or cold front (Cf), by inspecting daily surface weather charts analyzed by the Japan Meteorological Agency. Then, “averaged relative precipitation of heavy rainfall days (ARP)” is computed by averaging RP at each station classified by each synoptic system (Fig. 4).For ARP distribution due to Ty, ARP is very high in the inland area but in contrast it is very low in the coastal area. For ARP distributions due to Ec, Sf, and Cf, the highest ARP (over 0.6) area is located in the southeastern part of the inland area. However, the moderate ARP (0.3-0.5) area is expanding more westward in case of Cf, but it is located in only eastern part in case of Ec.Applying a cluster analysis to monthly mean precipitation and ARP, Yaku-shima Island is divided into 5 regions (Fig. 7). In region A, located in the coastal area except for the eastern coast, precipitation is relatively low in each month and ARP is also low in all cases. In region B of the southeastern coastal area, precipitation is high in March and April, and ARP is high in case of Ec, Sf, and Cf. In region C of the southeastern inland area, precipitation is high in each month and ARP is high in all cases. In region D of the northern inland area and the northeastern coastal area, precipitation is high in August and September, and ARP is high in case of Ty. In region E of the central inland area, ARP is high in case of Ty, Sf, and Cf.

Heavy precipitation in Beijing is modulated by both synoptic forcings and local thermodynamic characteristics of troposphere, which has yet to be well known. This study investigated the large‐scale synoptic patterns and local sounding features associated with the summertime heavy precipitation in Beijing, based on long‐term surface meteorological observations, radiosonde measurements, in combination of reanalysis data from 2008 to 2017. The results show that the heavy rainfall occurs more frequently in late July, which is associated with the movement of subtropical anticyclone. The sounding parameters during the heavy rainfall days are examined as well. It is found that the heavy rainfall is often related to favourable convective conditions characterized by abundant water vapour and high unstable energy. The soundings for around 45% of heavy rainfall days in Beijing exhibit the pattern of “thin tube” (TT), and those for ~25 and ~20% of heavy rainfall days show the patterns of “loaded gun” (LG) and “inverted V” (IV), respectively. On average, the rainfall amount of TT is 55.8 mm/day, which is ~3 mm/day (~15 mm/day) higher than that of LG (IV). The more frequent and heavier rainfall observed for TT pattern is due to the high values of precipitable water and wind shear. On the large scale, three dominant synoptic patterns associated with heavy rainfall in Beijing have been identified using the T‐mode principle component analysis. These synoptic patterns are all characterized by prevailing southerly winds within the lower troposphere, resulting in water vapours being easily transported from southern regions to Beijing, which in turn favours the occurrence of heavy precipitation. These dominant synoptic patterns and thermodynamic characteristics associated with the heavy rainfall in Beijing revealed in this study have important implications for better understanding of heavy rainfall in the North China Plain.

The smoothed time series of northeast monsoon rainfall over south peninsular (SP) India has revealed two extreme active epochs (1877–1887 and 2005–2015) and two extreme weak epochs (1899–1909 and 1980–1990) during 1871–2016. Only the recent two epochs 1980–1990 and 2005–2015 were chosen due to data constraints. For these two epochs, the distribution of zonal wind pattern at the 850 and 200 hPa levels, latent heat flux, vertical shear in the zonal wind between 850 and 200 hPa, sea surface temperature and outgoing long wave radiation (OLR) were studied. The vertical profile of zonal wind over the mean position of the subtropical westerly jet and the maximum wind reversal were also analysed. Further, the frequencies of dry days, little rainfall days (0–20 mm), moderate rainfall days (20–60 mm), heavy rainfall days (60–100 mm) and very heavy rainfall days (>100 mm) were evaluated. SP India has experienced active/weak monsoon conditions every one to two decades alternately. An active monsoon epoch shows a higher north–south temperature gradient over the Bay of Bengal than a weak monsoon epoch. An upper level subtropical westerly jet over north India, strong tropical easterlies in the lower troposphere and low vertical shear in the zonal wind over the Bay of Bengal were observed during the active epoch, which facilitates the transportation of good amounts of moisture to SP India. The low OLR values over SP India in the active monsoon epoch indicate more convective activity over that region. Enhanced rainfall activity over SP India in the active monsoon epoch is due to the enhanced frequency of very heavy, heavy, moderate and little rainfall events and the reduced frequency of dry days.