• ADVANCES IN ATMOSPHERIC SCIENCES, 2018, 35(8): 918-926
    doi: 10.1007/s00376-018-7210-y
    Seasonal Forecasts of the Summer 2016 Yangtze River Basin Rainfall
    Philip E. BETT1,, Adam A. SCAIFE1,2, Chaofan LI3, Chris HEWITT1, Nicola GOLDING1, Peiqun ZHANG4, Nick DUNSTONE1, Doug M. SMITH1, Hazel E. THORNTON1, Riyu LU5, Hong-Li REN4

    Abstract:

    The Yangtze River has been subject to heavy flooding throughout history, and in recent times severe floods such as those in 1998 have resulted in heavy loss of life and livelihoods. Dams along the river help to manage flood waters, and are important sources of electricity for the region. Being able to forecast high-impact events at long lead times therefore has enormous potential benefit. Recent improvements in seasonal forecasting mean that dynamical climate models can start to be used directly for operational services. The teleconnection from El Niño to Yangtze River basin rainfall meant that the strong El Niño in winter 2015/16 provided a valuable opportunity to test the application of a dynamical forecast system. This paper therefore presents a case study of a real-time seasonal forecast for the Yangtze River basin, building on previous work demonstrating the retrospective skill of such a forecast. A simple forecasting methodology is presented, in which the forecast probabilities are derived from the historical relationship between hindcast and observations. Its performance for 2016 is discussed. The heavy rainfall in the May-June-July period was correctly forecast well in advance. August saw anomalously low rainfall, and the forecasts for the June-July-August period correctly showed closer to average levels. The forecasts contributed to the confidence of decision-makers across the Yangtze River basin. Trials of climate services such as this help to promote appropriate use of seasonal forecasts, and highlight areas for future improvements.

    Key words: seasonal forecasting; flood forecasting; Yangtze basin rainfall; ENSO; hydroelectricity;
    摘要: 长江历史上一直遭受着洪涝灾害的影响. 近年来的严重灾害, 如1998年的大洪水, 造成了重大的人民生命财产损失. 洪水带来的径流在沿江的大坝的控制下, 同时是该地区重要的电力来源. 因此, 能够提前对这种灾害性事件进行有效预测, 有巨大的潜在价值. 最近季节预测能力的提高表明动力气候模式可以直接进行业务化的气候服务. 长江流域降水与厄尔尼诺联系密切, 因而2015/16年冬季强厄尔尼诺事件的发生为我们提供了一个检验动力预测系统应用到长江流域夏季降水预测的宝贵机会. 因此, 在前期回报工作呈现出一定预测技巧基础上, 本文对长江流域的实时季节预测进行了实例研究. 本文使用了一种简单的预测方法, 根据历史回报和观测的关系推算出预测事件发生的概率, 进而讨论了2016的预测结果. 结果表明, 2016年5月至7月的强降水预测准确. 8月份降水异常偏低, 而模式预测的6月至8月降水准确接近气候平均的结果. 这些成功的预测结果为长江流域防洪减灾并进行决策提供了信心. 此类气候服务的展开可以促进季节预测结果的应用推广, 并有助于未来气候预测服务领域的提升.
    关键词: 季节预测 ; 洪水预测 ; 长江流域 ; ENSO ; 水力发电
    1. Introduction

    The Yangtze River basin cuts across central China, providing water, hydroelectricity and agricultural land for millions of people. The Yangtze has been subject to flooding throughout history (e.g., Plate, 2002; Yu et al., 2009), linked to variations in the East Asian monsoon that are sometimes driven by factors such as the El Niño-Southern Oscillation (ENSO; e.g. Zhang et al., 2016a, b). Large hydroelectric dams along the river and its tributaries, such as the Three Gorges Dam (Jiao et al., 2013), have flood defense as their primary responsibility. However, by lowering the water level behind the dam to protect against flooding, less electricity will be produced. There are therefore clear benefits of forecasting impactful rainfall events at long lead times, allowing mitigation planning for flooding and electricity production.

    The relationship between ENSO and the East Asian monsoon is complex and not fully understood. However, it has long been clear that a strong El Niño peaking in winter is likely to be followed by above-average rainfall in China the following summer (e.g., Stuecker et al., 2015; He and Liu, 2016; Xie et al., 2016; Zhang et al., 2016a, b), although this response is not symmetric under La Niña conditions (Hardiman et al., 2017). The extreme El Niño event of 1997/98 was followed by devastating floods in the Yangtze River basin (Zong and Chen, 2000; Ye and Glantz, 2005; Yuan et al., 2017): thousands of people died, millions were made homeless, and the economic losses ran into billions of CNY. In the subsequent years, much work has gone into better water management and flood prevention, and into improving both the accuracy and communication of climate forecasts, to prevent such a disaster happening again.

    Seasonal rainfall forecasts across China have long been produced based on statistical relationships with large-scale climate phenomena, rather than forecasting precipitation directly from dynamical models. For example, (Zhu et al., 2008) and (Li and Lin, 2015) both examined the skill of 500 hPa geopotential height (z500) data, from multi-model ensembles of dynamical seasonal forecast systems, for forecasting summer monsoon and Yangtze river valley rainfall, respectively. (Tung et al., 2013), however, found that using sea level pressure performed better than using z500 when forecasting station-scale summer rainfall in southern China. (Kwon et al., 2009), (Peng et al., 2014), (Wu and Yu, 2016), (Xing et al., 2016) and (Zhang et al., 2016b) all investigated different statistical approaches to forecasting summer precipitation in China, based on various observational indices derived from sea surface temperatures (SSTs), air temperature and pressure. In many cases, these showed an improvement over dynamical models. (Wang et al., 2013) found that both dynamical models and a statistical model based on SSTs and pressure were able to predict the variability in the West Pacific subtropical high, which was itself shown to be a good predictor of East Asian summer monsoon rainfall. Statistical downscaling techniques have also been shown to improve predictions of summer precipitation in China over global dynamical forecast models (e.g., Ke et al., 2011; Liu and Fan, 2012).

    Recent advances in the dynamical seasonal forecast system developed at the UK Met Office, GloSea5 (MacLachlan et al., 2015), have resulted in the development of operational and prototype climate services for the UK in many sectors (e.g., Svensson et al., 2015; Palin et al., 2016; Clark et al., 2017). Recent work has shown that GloSea5 also has useful levels of skill for various processes in China (Bett et al., 2017; Lu et al., 2017), including for summer precipitation over the Yangtze River basin (Li et al., 2016), without having to use statistical models based on larger-scale drivers.

    In parallel to these findings, (Golding et al., 2017) demonstrated that there was a clear demand from users for improved seasonal forecasts for the Yangtze, both from the flood risk and hydropower production communities. The very strong El Niño that developed during the winter of 2015/16 (Zhai et al., 2016) provided a perfect opportunity to develop a trial operational seasonal forecast using GloSea5 for the subsequent summer of 2016.

    We therefore produced forecasts for the upcoming three-month period each week, from February (forecasting March-April-May) to the end of July 2016 (forecasting August-September-October); our focus, however, was on forecasting the June-July-August (JJA) period, as that was where (Li et al., 2016) had demonstrated skill. In the last week of each month, a forecast for the coming season was issued by the Met Office to the China Meteorological Administration (CMA).

    In this paper, we describe the observed rainfall in the Yangtze region in summer 2016, and assess how the real-time forecasts for May-June-July (MJJ) and JJA performed, with a range of lead times from zero to three months. We describe in section 2 the datasets used, and in section 3 our forecast production methodology. In section 4 we compare the forecasts to the observed behavior, and in section 5 discuss possible future developments.

    2. Datasets

    The current operational version of GloSea5 (MacLachlan et al., 2015) is based on the Global Coupled 2 (GC2) configuration of the HadGEM3 global climate model, described in detail in (Williams et al., 2015) and references therein. Within HadGEM3-GC2, the atmospheric component [the Met Office Unified Model (Walters et al., 2017)] is coupled to the JULES land surface model (Best et al., 2011), the NEMO ocean model (Madec, 2008; Megann et al., 2014) and the CICE sea ice model (Hunke and Lipscomb, 2010; Rae et al., 2015). The atmosphere is modelled on a grid of 0.83° in longitude and 0.55° in latitude, with 85 levels vertically, including a well-resolved stratosphere; the ocean model is modelled on a 0.25° horizontal grid, with 75 levels vertically.

    Using this configuration, GloSea5 runs operationally, producing both forecasts and corresponding hindcasts (intended to bias-correct the forecasts). Each day, two initialized forecasts are produced, running out to seven months. To produce a complete forecast ensemble for a given start date, the last three weeks of individual forecasts are collected together to form a 42-member lagged forecast ensemble.

    At the same time, an ensemble of hindcasts is produced each week. As described by (MacLachlan et al., 2015), three members are run from each of four fixed initialization dates in each month (the 1st, 9th, 17th and 25th), for each of the 14 years covering 1996-2009. The full hindcast ensemble is made by collecting together the four hindcast dates nearest to the forecast start date, yielding a 12-member, 14-year hindcast. Note that the hindcast was extended at the end of April 2016 to cover 23 years (1993-2015).

    This operational hindcast is not intended to be used for skill assessments: with only 12 members, skill estimates would be biased low (Scaife et al., 2014). However, a separate, dedicated hindcast was produced for skill assessment, with 24 members and 20 years. Using that hindcast, we find a correlation skill of 0.56 for summer Yangtze rainfall, statistically indistinguishable from the previous value of 0.55 found by (Li et al., 2016).

    We use precipitation data from the Global Precipitation Climatology Project (GPCP) as our observational dataset. This is derived from both satellite data and surface rain gauges, covering the period from 1979 to the present at a spatial resolution of 2.5° (Adler et al., 2003). The verification we present here uses version 2.3 of the data (Adler et al., 2016). Only version 2.2 was available when we started our operational trial, although we have confirmed that the choice of version 2.2 or 2.3 makes negligible difference to our forecasts or results.

    Fig. 1. Forecasts for MJJ (produced 25 April 2016) and JJA (produced 23 May 2016), as labeled, using GPCP observations. Observation/hindcast points are color-coded according to their observed winter ENSO index: red points are El Niño years, blue points are La Niña years, and gray points are neutral. The horizontal width of the green forecast bars is the standard error on the ensemble mean, i.e., the forecast ensemble spread divided by the number of ensemble members. The 75% and 95% prediction intervals are shown as gray shading. The variability in the observations is indicated by the pink horizontal dotted lines, at 1 and 2 standard deviations. The correlation r between hindcast and observations is marked on each panel (coincidentally the same when rounded).

    3. Forecast production

    Typically, when producing a seasonal forecast, the distribution of forecast ensemble members is used to represent the forecast probability distribution directly. However, experience has shown that the GloSea5 ensemble members may contain anomalously small signals, such that the predictable signal only emerges through averaging a large ensemble (Eade et al., 2014; Scaife et al., 2014). While this effect is less pronounced in subtropical regions like the Yangtze Basin, it is still present (Li et al., 2016).

    We therefore implemented a simple precipitation forecasting methodology, based entirely on the historical relationship between the hindcast ensemble means and the observed precipitation, averaged over the Yangtze River basin region (25°-35°N, 91°-122°E), following (Li et al., 2016), for the season in question. The prediction intervals, derived from the linear regression of the hindcasts to the observations (e.g., Wilks, 2011), provide a calibrated forecast probability distribution.

    This is illustrated in Fig. 1, where we show the precipitation forecasts issued in late April for MJJ, and in late May for JJA. The distribution of hindcasts and observations is shown as a scatter plot, with the ensemble mean forecast also included as a green circle. The uncertainty in the linear regression (gray) determines the forecast probabilities (green bars). The GloSea5 data are shown in standardized units——that is, the anomaly of each year from the mean, as a fraction of the standard deviation of hindcast ensemble means. The observations on the vertical axis are presented as seasonal means of monthly precipitation totals. The relationship with ENSO is indicated though color-coding of the hindcast points: years are labelled as El Niño (red) or La Niña (blue) according to whether their Oceanic Niño Index (http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml), based on observed SST anomalies in the Niño3.4 region, is above 0.5 K or below -0.5 K, respectively.

    Forecasts like those shown in Fig. 1 were produced each Monday starting in February 2016, using the forecast model runs initialized each day of the preceding three weeks to generate the 42-member ensemble, and the four nearest weeks of hindcast runs for the 12-member hindcast ensemble. The forecast produced near the end of each month was issued to the CMA: the MJJ release was produced on 25 April and the JJA release on 23 May.

    It is important to note that, due to the linear regression method we employ, our forecast probabilities are explicitly linked to both the hindcasts and the observations. The correlations between hindcasts and observations are biased low due to the smaller size of the hindcast ensemble compared to the forecast ensemble——a larger hindcast ensemble would not necessarily alter the gradient of the linear regression, but would reduce its uncertainty. Our forecast probabilities are therefore conservative (likely to be too small).

    The forecast information provided was designed to show very clearly and explicitly the uncertainties in the forecast system, to prevent overconfidence on the part of potential decision-makers. In addition to the scatterplot showing the forecast and the historical relationship (Fig. 1), we also provided the probability of above-average precipitation as a "headline message". This was accompanied by a contingency table showing the hit rate and false alarm rate for above-average forecasts over the hindcast period. For the MJJ and JJA forecasts, these are shown in Tables 1 and 2.

    Table 1. Contingency table for forecasts of above-average precipitation for the Yangtze region in MJJ, produced on 25 April 2016. The event counts are based on the GPCP observations and ensemble mean hindcasts shown in Fig. 1. The hit rate is the ratio of the number of hits to the number of times above-average conditions were observed. The false alarm rate is the ratio of the number of false alarms to the total number of observed below-average years.
    Observed
    Above-average precipitation Yes No
    Misses Correct rejections
    Hit rate: 60%
    False alarm rate: 30%

    Table 1. Contingency table for forecasts of above-average precipitation for the Yangtze region in MJJ, produced on 25 April 2016. The event counts are based on the GPCP observations and ensemble mean hindcasts shown in Fig. 1. The hit rate is the ratio of the number of hits to the number of times above-average conditions were observed. The false alarm rate is the ratio of the number of false alarms to the total number of observed below-average years.

    Table 2. Contingency table for forecasts of above-average precipitation in JJA, produced on 23 May 2016, similar to Table 1.
    Observed
    Above-average precipitation Yes No
    Misses Correct rejections
    Hit rate: 75%
    False alarm rate: 25%

    Table 2. Contingency table for forecasts of above-average precipitation in JJA, produced on 23 May 2016, similar to Table 1.

    To assess the sensitivity of our results to individual years, we have performed leave-one-out cross-validation for the MJJ and JJA forecasts. We find that the correlation between hindcasts and observations in the case of each left-out year does not vary much: 75% of the cases have correlations between 0.41 and 0.47. However, leaving out 1998 does reduce the performance, as expected: the correlation over the remaining 22 years in that case reduces to r=0.37 (MJJ) and r=0.24 (JJA), and the observed value falls outside the 95% prediction range of the forecasts; our procedure does require similar signals to be present in the hindcast period in order to calibrate the forecasts. Note that this cross-validation procedure is not directly analogous to our actual forecasts: with only 12 members per year, the hindcast ensemble means are much more uncertain than our actual 42-member forecasts for 2016, and our cross-validation does not account for this.

    Fig. 2. Observed precipitation from GPCP (version 2.3) for May, June, July and August (as labeled), in standardized units with respect to the 1993-2015 period. The Yangtze box used for the forecasts is marked as a red rectangle, with a pink polygon showing the physical Yangtze River catchment. Major rivers are marked in blue.

    4. Results

    The observed precipitation in May, June, July and August 2016 is shown in Fig. 2. We use standardized units here to show the precipitation anomaly relative to the historical variability over the hindcast period (1993-2015). It is clear that the most anomalously high rainfall was in May and June, and largely in the eastern half of the basin. July was close to normal overall when considering the box we were forecasting for, although there were disastrous floods further north. August had anomalously low rainfall across most of the region. (Yuan et al., 2017) examined the observed summer 2016 rainfall in China and the Yangtze River basin in detail, including its relationship to larger-scale drivers: the anomalously low rainfall in August 2016 is in marked contrast to the situation in 1998, and is related to the behavior of Indian Ocean temperatures and the Madden-Julian Oscillation (MJO) during the summer.

    Fig. 3. Mean precipitation for 2016-MJJ in the GPCP observations and GloSea5 forecast signal, (as labeled), in standardized units. The GloSea5 data have been regridded to match the lower-resolution observations.

    Fig. 4. Mean precipitation for 2016-JJA in the GPCP observations and GloSea5 forecast signal, (as labeled), in standardized units. The GloSea5 data have been regridded to match the lower-resolution observations.

    Figures 3 and 4 show the three-month mean precipitation anomalies for MJJ and JJA respectively, for both GPCP and the forecast averages from the GloSea5 model output. While we do not expect the spatial patterns to match in detail [considering the skill maps of (Li et al., 2016)], the overall signal is similar to the observations, with stronger anomalous precipitation in the eastern region in MJJ, and closer-to-average precipitation in JJA.

    We examine our forecasts for the Yangtze basin box more quantitatively in Figs. 5 and 6, where we show the variation with lead time of the hindcast-observation correlation, the 2016 forecast signal, and the probability of above-average precipitation, for MJJ and JJA respectively. Neither the hindcast-observation correlation nor the forecast signal vary significantly with lead time; indeed, they are remarkably consistent back to three months before the forecast season, and when the 23-year hindcast is introduced at the end of April.

    The forecasts did a good job of giving an indication of precipitation in the coming season. For MJJ, the forecast gave a high probability of above-average precipitation (80%), and it was observed to be above average. In JJA, the mean precipitation was observed to be slightly below average, due to the strong drier-than-average signal in August, although it was within a standard deviation of the interannual variability. While our forecast marginally favored wetter than average conditions (65% probability of above-average rainfall), it was correctly near to the long-term mean, and the observed value was well within the forecast uncertainties.

    Fig. 5. Time series showing the behavior of the MJJ forecasts and hindcasts with lead time: (a) Correlation between observations and the operational hindcasts available each week. The final point was produced using 23 years, whereas only 14 were available before that. The shading indicates 95% confidence intervals using the Fisher Z-transformation. (b) The forecast signal shown as 95% and 75% prediction intervals (boxes) and the ensemble mean (blue line). The observed precipitation is marked as an orange horizontal line from May. The observed historical mean and standard deviation over the hindcast period are marked as a dashed line and orange shading respectively. (c) The forecast probability of above-average precipitation. The final forecast issued for MJJ, produced 25 April, is highlighted with a gray vertical bar.

    Fig. 6. Time series showing the behavior of the JJA forecasts and hindcasts with lead time, following the same format as Fig. 5. In (a) (hindcast-observations correlation), the line becomes thicker when 23 years of hindcasts are available. We mark with a blue cross and error bar the correlation skill derived from the assessment hindcast (see text for details). The final forecast issued for JJA, on 23 May, is highlighted across all panels.

    5. Discussion and conclusions

    The heavy rainfall in the Yangtze River region in early summer 2016 was at a similar level to that of 1998, and caused heavy flooding (WMO, 2017; Yuan et al., 2017). While deaths due to the flooding were roughly an order of magnitude fewer than those caused by the 1998 floods (i.e., hundreds rather than thousands of lives), the economic losses nevertheless ran into tens of billions of CNY. Furthermore, it was reported that insurance claims, mostly from agricultural losses, amounted to less than 2% of the total economic loss, suggesting significant levels of underinsurance (Podlaha et al., 2016). The prior experience of the 1998 El Niño-enhanced flooding, and the high levels of awareness of the strong El Niño in winter 2015/16, meant that dams along the Yangtze were prepared in anticipation of high levels of rainfall. Our forecasts from GloSea5, produced using the simple methodology described here, contributed to the confidence of users adapting to the impending rainfall.

    Our verification has shown that our forecasts gave a good indication of the observed levels of precipitation for both MJJ and JJA averages over the large Yangtze Basin region. A greater degree of both spatial and temporal resolution——splitting the basin into upper and lower sections, and producing additional forecasts at a monthly timescale——would of course be preferable to users. However, smaller regions and shorter time periods may well be less skillful, so further work is needed to assess how best to achieve skillful forecasts in these cases.

    One significant improvement would be to increase the ensemble size of the hindcast. During 2017 the GloSea5 system was changed from three hindcast members per start date to seven. This could result in noticeable improvements in forecasts like those described here, as the hindcast-observations relationship will be less uncertain, especially when a predictable signal is present, such as from El Niño. Improvements in the underlying climate model, such as to parametrized convective precipitation, and the simulation of the monsoon and features like the MJO, could also improve the forecast skill.

    We will be issuing forecasts again in 2017. However, unlike 2016, in 2017 there are no strong drivers, such as El Niño. Nevertheless, understanding the behavior of the forecast system under such conditions will be informative, for both the users and the producers of the forecasts. Ultimately, trial climate services such as this help to drive forecast development, improve understanding of forecast uncertainties, and promote careful use by stakeholders in affected areas.

    Acknowledgements. This work and its contributors (PB, AS, ND, DS, CH, NG) were supported by the UK-China Research & Innovation Partnership Fund through the Met Office Climate Science for Service Partnership China as part of the Newton Fund. CL and RL were supported by the National Natural Science Foundation of China (Grant No. 41320104007). HR was supported by the Project for Development of Key Techniques in Meteorological Operation Forecasting (Grant No. YBGJXM201705). The trial forecast service was first suggested by AS in 2015. The GPCP precipitation data were provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, via their website at http://www.esrl.noaa.gov/psd/. The Yangtze River basin shapefile used in the maps was obtained from http://worldmap.harvard.edu/data/geonode:ch_wtrshed_30mar11 and is based on the watersheds shown in the China Environmental Atlas (2000), copyright Chinese Academy of Sciences, Environmental Data Center.

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    Key words
    seasonal forecasting
    flood forecasting
    Yangtze basin rainfall
    ENSO
    hydroelectricity

    Authors
    Philip E. BETT
    Adam A. SCAIFE
    Chaofan LI
    Chris HEWITT
    Nicola GOLDING
    Peiqun ZHANG
    Nick DUNSTONE
    Doug M. SMITH
    Hazel E. THORNTON
    Riyu LU
    Hong-Li REN