Identifying Regional Prolonged Low Temperature Events in China

Running title: Identifying Regional Prolonged Low Temperature Events in China Identifying Regional Prolonged Low Temperature Events in China ZHANG Zongjie ( 张宗婕 ) and QIAN Weihong ( 钱维宏 ) Monsoon and Environment Research Group, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871 (received 16 March 2010; revised 28 June 2010) Abstract This study examined regional prolonged low temperature (PLT) events in China from the observational station data for the period 1960 2008 using the new criteria. The new definition of a site PLT event is that the daily minimum temperature does not exceed the 10th percentile threshold of the local daily minimum temperature climatology for at least 5 days at a station. The regional PLT event is defined as at least five adjacent stations exhibiting site PLT simultaneously for >5 days. Under the new definition, 552 PLT events were identified, and three indices: duration, extent, and intensity, as well as a comprehensive index (CI) were used to quantify the event Corresponding author: QIAN Weihong, email- qianwh@pku.edu.cn 1

severity. In addition, geographical patterns and temporal variations of regional PLT events were investigated using three event categories: strong, moderate, and weak. Spatially, strong events were mainly located in the north of Xinjiang and along the Yangtze River to the south of the Yangtze River; moderate events occurred in Xinjiang and south of the Yangtze River; and weak events occurred south of the Yellow River. The variation for the annual frequency of regional PLT events in China in the last 49 years showed a significant decreasing trend with a rate of 1.99 times per decade, and the significant transition decade was the 1980s. Key words: regional prolonged low temperature, extreme event, duration index, comprehensive index, spatiotemporal variation Doi: 10.1007/s00376-010-0048-6. 1. Introduction Global mean temperatures began a discernible upward trend in the 1970s (IPCC, 2007). Climate and weather extremes have also changed remarkably in recent years. Under the changing climate, trend showing a decrease in the number of extremely cold days was observed over many countries (Karl et al., 1999; Easterling et al., 2000). Domonkos et al. (2003) suggested that, in the latest decades of the 20th century, a decline in the frequency of extremely cold winter days was found in South Central Europe. This trend was also observed in Canada, Australia, Southeast Asia, and the South Pacific (Collins et al., 2000; Bonsal et al., 2001; Manton et al., 2001). 2

In China, the average annual mean surface-air temperature has increased by 1.1 C over the past 50 years (Ren et al., 2005). Nationwide cold-wave activity has weakened, and the frequency of cold days in northern China has decreased significantly (Zhai et al., 1999). Decreasing frequencies of both cool days and cool nights have become a trend in China during the past 4 5 decades (Zhai and Pan, 2003; Qian and Lin, 2004). Regional daily extreme low temperatures also dropped in Eastern China in summer (Gong et al., 2004). These observational station data in China, indicate a significant decreasing trend in the frequency extremely cold days. Regional prolonged low temperature (PLT) events characterized by persistence, coverage, and intensity have severe impacts on social and natural environments. The most impressive regional PLT event in recent years, the early 2008 PLT and freezing-rain event, caused interruption of communication, great loss of crops, hundreds of related deaths, and an economic loss of >100 billion Yuan (National Climate Center (NCC), 2008; Gu et al.,2008; Ding et al., 2008; Qian and Fu, 2009). Additionally, regional PLT events in summer could also have caused decrease in crop production, and they should be taken into account. In 1969, 1972, and 1976, severe summer PLT occurred in Northeast China, and gross agricultural production dropped by ~20 30% due to severe events (Ding, 1980; Sun et al., 1983). Prolonged low temperature and overcast-rain could have also caused adverse impacts on the breeding, output, and quality of crops (Cai and Guan, 2007). As significant causes of weather-related disasters, regional PLT events should be studied more vigorously. Many previous studies mainly focused on the low temperature events in particular 3

regions, such as Northeast China (Wang and Wu, 1997; Cui et al., 2007), but none has comprehensively investigated the characteristics of regional or even nationwide PLT events throughout China over a long period. In this study, the Homogenized Daily Minimum Temperature data (with no missing records) at 549 National Standard Stations from Mainland China during the period 1960 2008 (Li and Yan, 2009) were used to examine all regional PLT events in the whole nation according to comparable definitions. In the analyses, the stations with elevations >2500 m were excluded. The paper is organized as follows: The definitions and methods used are described in section 2. Regional PLT events are illustrated in section 3 by several indices. Climatology of regional PLT events are shown in section 4, and their spatial temporal variations are described in section 5. Finally, a discussion and conclusions are given in section 6. 2. Definition and method The first requirement of this study was to define a site PLT event or regional PLT event. The NCC of China has created basic definitions for both site PLT events and regional PLT events. First, a site PLT event is one in which the daily minimum temperature is lower than the local daily minimum temperature climatology (running 11-day mean) for >5 consecutive days and <10 C for at least 2 days. Finally, site PLT events, with their intensity ordered by the series of low temperature courses, should be lower than the 10th percentile. A regional PLT event was defined by the NCC as one in which five adjacent stations (sites) exhibited site PLT events during the same 4

period. Daily temperature distribution has large spatial differences in China. Figure 1 shows the stations with no records of daily minimum temperature below 10 C in June August and in May September during 1960 2008. During these 49 years, no daily minimum temperatures <10 C were observed in summer (JJA) at 219 stations in Southeast China, and no daily minimum temperatures were recorded <10 C in May September at 79 stations in southern coastal regions. No site PLT events (according to the NCC s definition) were identified in Southeast China during June August and in southern coastal regions during May September. The absolute criterion with a threshold 10 C in the NCC s definition is not suitable in some particular regions for some seasons. Therefore, a relative criterion to define site and regional PLT event was used. For example, the daily extreme low temperature which had been used in many previous studies (Jones et al., 1999; Peterson et al., 2001; Yan et al., 2002; Gong et al., 2004; Trank et al., 2006) was used in which the daily temperature did not exceed the 10th percentile threshold of the local daily temperature climatology during 1960 2008. In our study, based on this relative criterion, a site PLT event occurred when the daily minimum temperature did not exceed the 10th percentile for at least 5 days at a station, and a regional PLT event occurred when at least five adjacent stations exhibited site PLT simultaneously for >5 days. These new definitions are based on percentile, i.e., they are similar to the definition of cold spell duration recommended by the WMO Expert Team on Climate Change Detection Monitoring and Indices (ETCCDMI). We examined the 5

daily minimum temperature difference of the 10th percentile threshold related to its climatology. For the average of all stations during July September, the difference was <3 C, while the difference was >5 C in February May. Short-term fluctuations of minimum temperature or cold surges are easily observed in spring, while the long-term low temperature events are more persistent in summer. The distributions for annual frequencies of site PLT events defined by both definitions are depicted in Figure 2. Based on the NCC s definition, high frequencies (>1.1 yr -1 ) were mainly located in northern China; the highest frequency (1.31 times/year) appeared at Qinghe (46.67 N, 90.38 E) in Xinjiang; and low frequencies (<0.7 yr -1 ) were mainly observed in southern China. Due to the limitation of 10 C in NCC s definition, stations in southern China experienced fewer site PLT events. Under the present definition, high PLT frequencies (>1.1 yr -1 ) were mainly scattered in Xinjiang and southern China. The highest frequency (2.04 yr -1 ) was found at Alashankou (45.18 N, 82.57 E), a station in Xinjiang. The monthly distributions of regional PLT events using both definitions are showed in Figure 3. Regional PLT events for the NCC s definition mainly occurred during November April (with two peaks in March and November), seldom in June and September, and never in July and August. Regional PLT events for the present definition can be found in each calendar month, and more events are found in August December, with the maximum number of 93 events in October, while few events were recorded in March May. The latter distribution is reasonable. In autumn, especially in October and November, cold air masses are strong and stable, and this 6

leads to more regional PLT events. In spring, cold air sometimes lasted <2 days, so rapid temperature increases after sharp temperature decreases yielded fewer regional PLT events. To further discuss the regional PLT events defined by NCC, we took the summer PLT event in Northeast China in 1969, 1972, and 1976 as an example. Based on the NCC s definition, no PLT events in Northeast China were obtained in the three typical years, whereas, using the present definition, two PLT events occurred in each year (Table 1). The present definition for site and regional PLT events is more effective in identifying events than that of NCC in different seasons. Therefore, the analysis of regional PLT events in the following parts is based on the present definition. Every day, there were a total of n sites reaching the low temperature condition, and at the ith site of geographical position lon(i) and lat(i). The distance between two adjacent sites i and j was calculated using D i j j j i j 2 2 = [lat( ) lat( )] + [lon( ) lon( )],. (1) If D 5, two sites were considered to be adjacent. In order to distinguish different regional PLT events that occurred during the same period, we used the starting date, the ending date, and the central position to distinguish them. For each event, the central position is calculated by the weighted-mean latitude and longitude of all sites that were involved in the event. The central position of a regional PLT event is calculated as follows: Lat = 549 i= 1 549 n lat( ) i i i= 1 n i (2) 7

Lon = 549 n lon( ) i i i= 1 549 i= 1 n i (3) Where Lat is the event s central latitude and Lon is the event s central longitude, and n i is the accumulated total days of the ith station that must satisfy the regional PLT criterion. The n i is zero if the ith station does not satisfy the regional PLT condition. Two statistical methods were used in the data analyses of this study. The least-squares method was applied to fit the linear trend, and the statistical t-test was used to detect the significance of regression equation for simulating time series (Huang, 2004). The scanning t-test was used to detect possible rapid transition points in time series (Jiang et al., 2001). In this study, the length of the entire sample was 49 years, and the subsample size varied from 3 to 24 with interval 1. 3. Regional PLT events Relying on the present definition, total 552 regional PLT events over China during 1960 2008 were identified. Severe regional PLT events were characterized by duration, geographic extent, and intensity, as well as comprehensive strength (CI). In order to measure severity of the extreme events, several indices similar as those in the study of persistent rainfall events (Tang et al., 2006) were used to quantify these events. 3.1. Duration index The duration index is the number of days for which a regional PLT event occurs. All 552 events were sorted by duration index. Table 2 lists the top ten regional PLT 8

events ranked by duration for the period 1960 2008. These long-lasting events mostly occurred during 1960s 1970s with the average duration of 31.3 days. The top five events lasted for >1 month, and the PLT of longest duration (46 days) occurred from November 1967 to January 1968. 3.2. Extent index The daily number of geographic 1 1 grid cells affected was calculated over whole events, and the extent index is the maximum number of 1 1 grid cells affected. All events were ranked according to extent index. Table 3 shows the top ten regional PLT events with their extent indices for the period 1960 2008. Nine of these ten events occurred during 1970s 1980s, and one in 1993. The average extent was 229.8 grid cells. The regional PLT event with a maximum extent 290 grid cells was observed in November 1976. 3.3. Intensity index The daily minimum temperature anomalies of all sites involved were calculated. The intensity index was defined as the minimum temperature anomaly that reached the lowest negative value during the regional PLT event. High values of the intensity index represent mild intensity, and low values represent severe intensity. All events were ranked according to intensity index. Table 4 shows the top ten intensive regional PLT events. The intensity indices of all ten events were < 14.0 C; the average value was 15.65 C. The most severe event reached the minimum temperature anomaly of 18.21 C: this occurred in December 1966 in Xinjiang. The central positions of nine intense events were located north of 40 N and west of 88 E. 9

3.4. Comprehensive index Duration, extent, and intensity indices are three different aspects of describing the extremity of regional PLT events, so it was necessary to construct an index to comprehensively judge the severity. A comprehensive index (CI) for a regional PLT event was constructed using three dimensionless indices for the jth event using CI (j) = ID (j) + IE (j) II (j). (4) Here, ID (j), IE (j), and II (j) are the standardized duration index, extent index, and intensity index, respectively. CI is the algebraic sum of the three dimensionless quantities. The larger the CI, the more severe the regional PLT event. In the present study, extremity of regional PLT events was judged without considering other meteorological quantities, such as rainfall or wind. All regional PLT events were ranked according to CI. The top ten regional PLT events indicated by CI during 1960 2008 are listed in Table 5. The regional PLT event with the largest CI occurred during November 1967 to January 1968: this event lasted for 46 days, influenced 184 grid cells, and caused the minimum temperature anomaly of 10.59 C. All ten events had CI of >6.5, with mean duration index of 25.8 days, mean extent index of 196.9 grid cells, and mean intensity index of 11.79 C. These events mainly occurred in winter. Table 6 shows the top ten regional PLT events in the south of 35 N, ranked by CI index. The severest event, covering the Jiang-huai region to southern China, occurred in September October 1979, with duration of 32 days, extent of 67 grid cells, intensity of 6.53 C, and a CI of 5.52. The early 2008 PLT event in southern China 10

had a duration of 22 days (from 13 January to 3 February), influencing 61 grid cells and reaching the minimum temperature anomaly 6.44, with a CI of 3.26. This event was ranked the fourth most severe regional PLT event in southern China during the period of 1960 2008. A summer regional PLT event among the ten events occurred in August 1974 in summer in southern China, with a geographic extent of 94 grid cells. This regional PLT event was an extreme event. 4. Climatology of regional PLT events Some distribution features of regional PLT events with various indices can be identified based on these 552 events. Figure 4 shows frequency distributions by major attributes: duration, extent, intensity, and CI, respectively. In Fig. 4a, duration index ranged from 5 to 46 days. There were 229 regional PLT events lasting for 5 to 6 days, 204 events lasting for 7 to 10 days, and 16 events lasting for >20 days. The mean duration of all events was 8.7 days. In Fig. 4b extent indices varied from 3 to 290 grid cells. Some 159 regional PLT events covered <10 grid cells, and only 36 events affected >100 grid cells. The mean extent index was 32.7 grid cells, and the peak was from 5 to 15 grid cells. In Fig. 4c, the intensity index distribution shows the lowest temperature of 18.21 C, the highest temperature of 1.88 C, and the average temperature of 6.98 C. There were 322 events with an intensity index of < 6 C, and 71 events had an intensity index of < 10 C. In Fig. 4d, CI indices varied from 3.12 to 12.85, with a peak range between 2.6 and 1.4. For further study of the climatic characteristics of regional PLT events, we classified 11

the 552 events into three groups according to their CI index. The events with CI index >0.5σ (standard deviation), between 0.5σ and 0.5σ, or < 0.5σ were classified as strong, moderate, or weak events, respectively (Table 7). There were 126 strong events with a CI index of >1.13. For these strong events, the mean duration index was 14.7 days, mean extent index was 78.5 grid cells, and mean intensity index was 9.28 C. There were 230 moderate events with a CI index between 1.13 and 1.13. For these events the mean duration index was 7.8 days, mean extent index was 25.4 grid cells, and the mean intensity index was 7.57 C. A total of 196 weak events occurred with CI index < 1.13. For these weak events, mean duration index was 6.0 days, mean extent index was 11.7 grid cells, and mean intensity index was 4.80 C. This confirms that the CI index defined in Eq. (4) was suitable for describing regional PLT events quantitatively. After classifying the regional PLT events, monthly mean indices were separated by duration, extents, intensity, CI index, and total monthly times of three groups of strong, moderate and weak events. These results are shown in Figure 5. Duration index was large in December January and was small in April June (Fig. 5a). For the PLT events with duration >2 weeks, >10 events per month were found in September, November, December, and January, while fewer were observed in July and August. For the PLT events with duration of 1 2 weeks, >25 events per month occurred in July January. Extent indices were highest in September December and lowest in April and June (Fig. 5b). From September to January, >24 events per month occurred with extents of >25 grid cells, and in July August there were also events with extents 12

>78 grid cells. For intensity (Fig. 5c), mild (high negative value) intensity was observed in July September, while severe (low negative value) intensity was observed in February April. There were more events with intensity indices of < 7.57 C in October February. Seasonally (Fig. 5d), high CI indices of >0.5 occurred during the cold season (December March), while low CI indices of < 0.5 appeared during the warm season (May August). Moderate CI indices were dominant in April, September, October, and November. This seasonal distribution of CI indices indicates that severe events occurred much more often during the cold season than during the warm season. Figure 5e shows the total number distribution of three groups events in each calendar month. Strong events with a high frequency of >15 occurred in September February, while none were observed in May June. Moderate events frequently appeared (>30 times) in October January. High frequencies (>20 times) of weak events were found in July November, and low frequencies (<5 times) in February May. The central positions of all events and total times affected by strong, moderate, and weak regional PLT events in 1960 2008 are shown in Fig. 6. The central positions of 126 strong regional PLT events were located from the middle reaches of the Yangtze River to Xinjiang and Northeast China. Stations scattered north of Xinjiang and along the Yangtze River to the south of the Yangtze River were frequently affected by strong events (>30 times). The central positions for 230 moderate events were distributed in the mainland China and mainly centered in Xinjiang, Northeast China, and southern China. High-frequency centers (>18 times) of moderate events were found in 13

Xinjiang and south of the Yangtze River, while a low-frequency center (<12 times) was observed in North China. The central positions of 196 weak events were mostly located south of 40 N, and weak events were frequently (>7 times) observed south of the Yellow River. 5. Spatial temporal variation Linear trends and temporal variations of regional PLT events were analyzed for 1960 2008. The annual frequency of regional PLT events showed a negative trend, with a magnitude of 1.99 per decade at the significance level of 0.01 (Fig. 7a). A significant transition point on the inter-decadal timescale was detected in the 1980s under the scanning t-test with various timescales (Fig. 7e). The two periods of regional PLT events in 1960 1980 and 1990 2008 are highlighted with mean annual frequency 13.9 times per year and 8.2 times per year, respectively. The variations of three groups of events were also examined. For the time series of strong events, a significant trend showing a decrease of 0.68 times per year was detected (Fig. 7b), and significant transition points occurred around 1980 from the 8 20-year timescale. The annual frequencies of strong events were reduced by 2 times per year after transition (Fig. 7f). For moderate and weak events, the decreasing trends were 1.06 times per year and 0.25 times per year respectively, but no transition points were tested. Annual-mean variations and their trends of four indices were analyzed using scanning t-test calculations with different timescales from 3 years to 24 years (Fig. 8). 14

The variation of annual-mean CI, duration, and extent indices of regional PLT events were similar in 1960 2008, high in the 1960s 1970s, and low in the 1990s. The variation of intensity index was different: low in the 1970s and high in the 1990s. For annual CI, annual duration and annual intensity indices, the significant transition point was detected around 1980 at the 10 24-year timescale. The mean values of CI, duration, and intensity indices were 0.38 yr -1, 9.5 days yr -1, and 7.36 C yr -1 in 1960 1980, while they were 0.30 yr -1, 8.4 days yr -1 and 6.46 C yr -1, respectively, in 1985 2008. Considering the transition points detected in Fig. 7e, all regional PLT events for the two periods are compared in Fig. 9. During 1960 1980 (Fig. 9a), the annual frequencies in many stations were >0.8 times per year, while some stations with frequencies >1.5 times per year were scattered in Xinjiang, Northeast China, and southern China. After climatic transition (Fig. 9b), the stations with annual frequencies for >0.8 times per year were mainly located south of 32 N. The differences between the two periods showed that only 13 stations experienced an increasing frequency, while in many other stations, partly in Xinjiang and partly in eastern coastal regions, the annual frequency decreased by >0.75 times per year (Fig. 9c). 6. Conclusion and discussion Using the daily homogeneous minimum temperature data, this study presented an overview of the climatology of regional PLT events in China for the period 15

1960 2008. The climatology of 552 regional PLT events was identified, and their temporal-spatial variations were studied. The main conclusions of the study are highlighted as follows: (1) The site and regional PLT events were defined. In this study, a site PLT event was defined if the minimum temperature is below the 10th percentile threshold lasting at least 5 days at a station without limitation of <10 C. A regional PLT event was defined if at least five adjacent stations (sites) exhibited site PLT events in the same period. After the comparison with NCC s definition, the present definition was found to be suitable to select regional PLT events in different seasons and regions. Relying on this definition, 552 regional PLT events were identified for 1960 2008, with mean frequency of ~11.3 times per year. (2) Regional PLT events can be indicated by several indices, such as duration, geographic extent and intensity, as well as comprehensive strength. To judge how extreme the regional PLT events were, four criteria were developed to quantify all 552 events. Furthermore, these events were ranked according to each index, and four special extreme events were identified, with the longest duration of 46 days from November in 1967 to January in 1968, the largest coverage of 290 grid cells in November 1976, the lowest intensity of 18.21 C in December 1966, and the severest comprehensive strength in November 1967. The most impressive event, the early 2008 persistent freezing-rain and PLT event in southern China, lasting for 22 days, influencing 61 geographic grid cells and reaching minimum temperature anomaly 6.44 C. This event was ranked as the fourth most severe regional PLT event in 16

southern China during 1960 2008. During summertime, severe regional PLT events also occurred. (3) All 552 events were further classified into weak, moderate, and strong groups according to CI. In the last 49 years, there were 126 strong events, 230 moderate events, and 196 weak events. Strong events occurred mainly in September February and were scattered mainly in Xinjiang and along the Yangtze River to south of the Yangtze River. Moderate events appeared mainly in October January and were frequently distributed in Xinjiang, south of the Yangtze River. Weak events occurred frequently in July November and were often found in the south of the Yellow River. (4) Significant negative trends of regional PLT events and an inter-decadal transition point in the 1980s from the annual frequency were detected. The annual frequency of regional PLT events was ~13.9 times per year during 1960 1980, and this frequency decreased to 8.2 times per year during 1990 2008. Spatially, the differences between two periods showed that only 13 stations had experienced an increasing frequency, while in many other stations, especially in Xinjiang and east coastal regions, the annual frequency decreased by >0.75 times per year. The variations of annual CI, annual duration and annual extent showed similar trend, high in the 1960s 1970s and low in the 1990s. Besides these identifications, questions about what caused regional PLT events are useful for practices. In cold season, regional PLT events were often related to strong cold waves. During the winter period, extreme low temperature events are usually caused by the cold air coming down from high-latitude areas, like Siberia or 17

Mongolia. The cold wave mainly impacts the weather and climate over China in winter through three paths (Tao, 1957). These cold waves move in northern China and have some impacts on temperature and precipitation over southern China. The change in cold waves contributed to the variation of regional PLT events. In the past 4 5 decades, the activity of cold waves weakened in China (Wang and Ding, 2006; Qian and Zhang, 2007; Ding et al., 2009). Possible reasons for the decreasing of both cold waves and regional PLT events in China could be the weakened winter Siberian high and East Asian winter monsoon (Wang and Ding, 2006). In summer, PLT events have relationships with rainfall in some regions. Huang (1991) pointed out that the significant negative correlation between temperature and precipitation was found in the mid-lower reaches of the Yangtze River, southern China and North China. On the other hand, large-scale, sea-surface temperature anomalies (SSTA) also played an important role. For example, SSTA in the warm pool of tropical West Pacific and mid-latitude of West Pacific were in correlation to low temperature in the Northeast China (Zheng and Ni, 1999). The three PLT events in Northeast China occurred in 1969, 1972, and 1976, which were also El Niño years. A cold vortex in Northeast China was a direct cause of regional PLT events. In future studies it will be important to investigate the PLT events, trends, and changes in individual regions and their respective linkages to large-scale atmospheric and oceanic circulations. Acknowledgments. The authors thank the anonymous reviewers and the editor for their helpful comments and suggestions. This research was supported jointly by the 18

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Table captions Table 1. The regional PLT events in Northeast China in summer 1969, 1972, and 1976 by the present definition. Table 2. The top ten regional PLT events ranked by durations for the period 1960 2008. Table 3. The top ten regional PLT events ranked by extents for the period 1960 2008. Table 4. The top ten regional PLT events ranked by intensities for the period 1960 2008. Table 5. The top ten regional PLT events ranked by CI index for the period 1960 2008. Table 6. The top ten regional PLT events in the south of 35 N ranked by CI index for the period 1960 2008. Table 7. Classifications of regional PLT events and their characteristics. σ = standard deviation. 24

Figure captions Fig. 1. Stations without records of daily minimum temperature <10 C in (a) June August, (b) May September for 1960 2008. Fig. 2. Site PLT event distributions (yr -1 ) for the period of 1960 2008 in China (a) the NCC s definition, and (b) the present definition. Fig. 3. Total number of regional PLT events based on two definitions (present & NCC) for each calendar month. Fig. 4. Frequency distributions of regional PLT events during 1960 2008 for (a) duration (1-day interval), (b) extent (5 grid-unit interval), (c) intensity (0.2 C interval), and (d) CI index (0.2 interval). Fig. 5. Monthly mean indices and total times of events with various indices separated by (a) duration, (b) extent, (c) intensity, (d) CI index, and (e) total monthly times of three groups of strong, moderate, and weak events. The vertical bars in (a) (d) refer to the monthly mean indices. The curves in (a) (c) indicate the total times of regional PLT events with different indices. Fig. 6. Central positions of three groups of strong, moderate, and weak events and total times affected by three group events for the period 1960 2008 in China, separated by (a, b) strong, (c, d) moderate, and (e, f) weak regional PLT events. Fig. 7. Variations and trends of annual frequency of regional PLT events in China during 1960 2008 (left panel) and the results of scanning t-test calculation with different timescales from 3 to 24 years for each series (right panel), separated by (a, e) all events, trend = 1.99 (10 yr) -1 ***; (b, f) strong events, trend = 0.68 (10 yr) -1 ***; (c, g) moderate events, trend = 25

1.06 (10 yr) -1 ***; and (d, h) weak events, trend = 0.25 (10 yr) -1. In (a), (b), (c) and (d), the dashed line indicates the linear trend of time series, the smoothed curves shows the 5-order polynomial fitting trends, and the thick line denotes the mean of annual times in the corresponding periods. Asterisks *, **, and *** denote the trend reaching the 0.1, 0.05, and 0.01 significant levels, respectively. The shaded area in (e), (f), (g) and (h) indicates that the t-test reached the 0.01 level of significance at a determined time point for different timescales. Fig. 8. As in Fig.7, except for annual-mean variations and their trends of four indices for scanning t-test calculations with different timescales from 3 to 24 years, separated by (a, e) annual CI, trend = -0.16 (10 yr) -1 *; (b, f) annual duration, trend = -0.14 days (10 yr) -1 ; (c, g) annual extent, trend = -1.56 grid cells (10 yr) -1 ; and (d, h) annual intensity, trend = 0.25 C (10 yr) -1 ***. Fig. 9. Annual frequencies (yr -1 ) affected by regional PLT events during (a) 1960 1980, (b) 1990 2008, and (c) frequency differences (yr -1 ) by two periods 1990 2008 minus 1960 1980. 26

Table 1. The regional PLT events in Northeast China in summer 1969, 1972, and 1976 by the present definition. Year 1969 1972 1976 Starting date (day month year) Ending date (day month year) 27 Jun 1969 2 Jul 1969 31 Jul 1969 4 Aug 1969 15 Aug 1972 23 Aug 1972 25 Aug 1972 4 Sep 1972 1 Jul 1976 7 Jul 1976 18 Aug 1976 26 Aug 1976 Duration (Days) 6 5 9 11 7 9 27

Table 2. The top ten regional PLT events ranked by durations for the period 1960 2008. Rank Starting date (day month year) Ending date Central Position (day month year) Lat ( N) Lon ( E) Duration (days) 1 18 Nov 1967 2 Jan 1968 37.29 110.49 46 2 17 Dec 1966 22 Jan 1967 42.91 85.26 37 3 1 Sep 1966 3 Oct 1966 30.20 115.92 33 4 24 Sep 1979 25 Oct 1979 26.92 115.83 32 5 3 Dec 1975 2 Jan 1976 31.17 106.85 31 6 3 Nov 1976 30 Nov 1976 34.47 109.68 28 7 25 Nov 1962 21 Dec 1962 41.07 82.29 27 7 19 Jan 2008 14 Feb 2008 39.28 94.49 27 9 17 Jan 1968 11 Feb 1968 41.59 112.96 26 9 2 Dec 1974 27 Dec 1974 41.48 87.79 26 28

Table 3. The top ten regional PLT events ranked by extents for the period 1960 2008. Rank Starting date (day month year) Ending date Central Position (day month year) Lat ( N) Lon ( E) Extent (grid cells) 1 3 Nov 1976 30 Nov 1976 34.47 109.68 290 2 23 Nov 1987 11 Dec 1987 35.07 110.03 283 3 31 Oct 1981 12 Nov 1981 37.12 113.36 269 4 3 Dec 1975 2 Jan 1976 31.17 106.85 248 5 20 Dec 1976 7 Jan 1977 38.73 111.78 230 6 4 Dec 1985 17 Dec 1985 37.48 117.76 220 7 12 Dec 1984 1 Jan 1985 35.91 102.3 197 8 14 Nov 1993 26 Nov 1993 36.88 111.87 191 9 26 Jan 1977 5 Feb 1977 31.64 115.02 186 10 18 Nov 1967 2 Jan 1968 37.29 110.49 184 29

Table 4. The top ten regional PLT events ranked by intensities for the period 1960 2008. Rank Starting date (day month year) Ending date Central Position (day month year) Lat ( N) Lon ( E) Intensity ( ) 1 17 Dec 1966 22 Jan 1967 42.91 85.26 18.21 2 21 Feb 1974 26 Feb 1974 45.20 86.89 17.21 3 25 Feb 1988 6 Mar 1988 45.28 86.05 17.18 4 11 Feb 1969 23 Feb 1969 45.46 85.78 15.59 5 19 Nov 1960 27 Nov 1960 44.29 85.51 15.40 6 25 Jan 1969 1 Feb 1969 45.41 85.06 15.12 7 15 Apr 2003 19 Apr 2003 45.98 85.14 14.91 8 11 Feb 1988 18 Feb 1988 45.28 86.22 14.53 9 3 Mar 1971 14 Mar 1971 41.85 120.61 14.35 10 12 Dec 1974 27 Dec 1974 41.48 87.79 14.03 30

Table 5. The top ten regional PLT events ranked by CI index for the period 1960-2008. Starting date Central Position Duration Extent Intensity Rank (day month Lat Lon CI Rank Days Rank Grid cells Rank year) ( N) ( E) 1 18 Nov 1967 37.29 110.49 1 46 10 184 53 10.59 12.85 2 3 Nov 1976 34.47 109.68 6 28 1 290 71 10.01 11.51 3 3 Dec 1975 31.17 106.85 5 31 4 248 128 8.70 10.60 4 17 Dec 1966 42.91 85.26 2 37 116 45 1 18.21 10.39 5 23 Nov 1987 35.07 110.03 21 19 2 283 28 11.83 10.14 6 12 Dec 1984 35.91 102.3 13 21 7 197 14 13.51 9.06 7 20 Dec 1976 38.73 111.78 22 19 5 230 40 11.07 8.55 8 11 Feb 1964 30.85 112.98 25 18 12 170 22 12.38 7.35 9 31 Oct 1981 37.12 113.36 63 13 3 269 208 7.55 6.95 10 2 Dec 1974 41.48 87.79 9 26 95 53 10 14.03 6.73 Table 6. The top ten regional PLT events in the south of 35 N ranked by CI index for the period 1960-2008. Rank Starting date Central Position Extent Duration Intensity (day month Lat Lon (grid (days) ( ) year) ( N) ( E) cells) CI 1 24 Sep 1979 26.92 115.83 32 67 6.53 5.52 2 22 Dec 1973 26.23 110.91 20 70 8.32 3.77 3 13 Nov 1971 24.79 109.03 17 101 7.34 3.55 4 13 Jan 2008 27.16 108.95 22 61 6.44 3.26 5 6 Jan 1963 26.92 112.53 15 75 8.06 2.76 6 11 Oct 1971 25.60 111.42 11 105 7.33 2.39 7 17 Feb 1996 25.87 110.32 10 102 7.80 2.29 8 27Oct 1978 25.39 111.17 9 84 8.46 1.88 9 3 Feb 1972 27.85 110.78 9 73 9.11 1.85 10 23 Aug 1974 28.49 114.72 15 94 4.18 1.78 31

Table 7. Classifications of regional PLT events and their characteristics. σ = standard deviation. Classification Strong (>+0.5σ) Moderate [ 0.5σ, +0.5σ] Weak (< 0.5σ) Mean Mean Total Mean Standard Duration Extent Times CI (days) (grid) Mean Intensity ( ) 1.13 < CI 12.85 126 3.23 14.7 78.5 9.28 1.13 CI 1.13 230 0.16 7. 8 25.4 7.57 3.12 CI < 1. 13 196 1.89 6.0 11.7 4.80 32

50N (a) 50N (b) 40N 40N 30N 30N 20N 20N 80E 90E 100E 110E 120E 130E 80E 90E 100E 110E 120E 130E Fig. 1. Stations without records of daily minimum temperature <10 C in (a) June August, (b) May September for 1960 2008. 50N (a) 50N (b) 40N 40N 30N 20N 0.02 to 0.7 0.7 to 0.9 0.9 to 1.1 1.1 to 1.4 80E 90E 100E 110E 120E 130E 30N 20N 0.02 to 0.7 0.7 to 0.9 0.9 to 1.1 1.1 to 1.4 1.4 to 2.1 80E 90E 100E 110E 120E 130E Fig. 2. Site PLT event distributions (yr -1 ) for the period of 1960 2008 in China (a) the NCC s definition, and (b) the present definition. 100 80 Present NCC Times 60 40 20 0 JAN MAR MAY JUL SEP NOV Month Fig. 3. Total number of regional PLT events based on two definitions (present & NCC) for each calendar month. 33

Number of Events 120 80 40 (a) Number of events 120 80 40 (b) Number of Events 0 25 20 15 10 5 0 10 20 30 40 50 Duration (days) (c) Number of Events 0 40 30 20 10 0 50 100 150 200 250 300 Extent (grids) (d) 0 0-19 -17-15 -13-11 -9-7 -5-3 -1 Intensity ( o C) -4-2 0 2 4 6 8 10 12 14 CI Fig. 4. Frequency distributions of regional PLT events during 1960 2008 for (a) duration (1-day interval), (b) extent (5 grid-unit interval), (c) intensity (0.2 C interval), and (d) CI index (0.2 interval). 34

Mean Duration (days) 16 12 8 4 0 Mean duration 7~14 days >14 days (a) 50 60 40 30 20 10 0 Total Times Mean Extent (grids) 40 20 0 Mean Extent 25~78 grids >78 grids (b) 40 30 20 10 0 Total Times Jan Mar May Jul Sep Nov Month Jan Mar May Jul Sep Nov Month Mean Intensity ( o C) -16-12 -8-4 Mean Intensity -9.28~-7.57 oc <-9.28 oc (c) 40 2 30 20 10 Total Times Mean CI 1 0-1 (d) 0 0-2 Jan Mar May Jul Sep Nov Month Jan Mar May Jul Sep Nov Month 50 40 (e) 30 20 10 0 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC strong moderate weak Fig. 5. Monthly mean indices and total times of events with various indices separated by (a) duration, (b) extent, (c) intensity, (d) CI index, and (e) total monthly times of three groups of strong, moderate, and weak events. The vertical bars in (a) (d) refer to the monthly mean indices. The curves in (a) (c) indicate the total times of regional PLT events with different indices. 35

50N (a) 50N (b) 40N 40N 30N 30N 20N Central Position 20N ( 8, 24 ] ( 24, 30 ] ( 30, 51 ] 80E 90E 100E 110E 120E 130E 80E 90E 100E 110E 120E 130E 50N (c) 50N (d) 40N 40N 30N 30N 20N Central Position 20N ( 3, 12 ] ( 12, 18 ] ( 18, 36 ] 80E 90E 100E 110E 120E 130E 80E 90E 100E 110E 120E 130E 50N (e) 50N (f) 40N 40N 30N 30N 20N Central Position 20N ( 0, 4 ] ( 4, 7 ] ( 7, 23 ] 80E 90E 100E 110E 120E 130E 80E 90E 100E 110E 120E 130E Fig. 6. Central positions of three groups of strong, moderate, and weak events and total times affected by three group events for the period 1960 2008 in China, separated by (a, b) strong, (c, d) moderate, and (e, f) weak regional PLT events. 36

Times Times Times Times 32 24 16 9 6 3 0 12 8 0 All 1960 1970 1980 1990 2000 2010 Year Strong 1960 1970 1980 1990 2000 2010 Year 8 4 0 1960 1970 1980 1990 2000 2010 Year 10 8 6 4 2 0 (c) (a) (b) Moderate (d) Weak 1960 1970 1980 1990 2000 2010 Year Timescale Timescale Timescale Timescale 25 20 15 10 25 20 15 10 5 25 20 15 10 5 5 25 20 15 10 5 All 1970 1980 1990 2000 Year Strong 1970 1980 1990 2000 Year Moderate 1970 1980 1990 2000 Year Weak 1970 1980 1990 2000 Year Fig. 7. Variations and trends of annual frequency of regional PLT events in China during 1960 2008 (left panel) and the results of scanning t-test calculation with different timescales from 3 to 24 years for each series (right panel), separated by (a, e) all events, trend = 1.99 (10 yr) -1 ***; (b, f) strong events, trend = 0.68 (10 yr) -1 ***; (c, g) moderate events, trend = 1.06 (10 yr) -1 ***; and (d, h) weak events, trend = 0.25 (10 yr) -1. In (a), (b), (c) and (d), the dashed line indicates the linear trend of time series, the smoothed curves shows the 5-order polynomial fitting trends, and the thick line denotes the mean of annual times in the corresponding periods. Asterisks *, **, and *** denote the trend reaching the 0.1, 0.05, and 0.01 significant levels, respectively. The shaded area in (e), (f), (g) and (h) indicates that the t-test reached the 0.01 level of significance at a determined time point for different timescales. (f) (g) (e) (h) 37

CI Duration (days) Extent (grids) Intensity (oc) 3 2 1 0-1 -2 17 15 13 11 9 7 5 90 70 50 30 10-4 -5-6 -7-8 -9 (a) 1960 1970 1980 1990 2000 2010 Year (b) 1960 1970 1980 1990 2000 2010 Year (c) 1960 1970 1980 1990 2000 2010 Year (d) 1960 1970 1980 1990 2000 2010 Year Timescale Timescale Timescale Timescale 25 20 15 10 5 25 20 15 10 5 25 20 15 10 5 25 20 15 10 5 (e) 1970 1980 1990 2000 Year (f) 1970 1980 1990 2000 Year (g) 1970 1980 1990 2000 Year (h) 1970 1980 1990 2000 Year Fig. 8. As in Fig.7, except for annual-mean variations and their trends of four indices for scanning t-test calculations with different timescales from 3 to 24 years, separated by (a, e) annual CI, trend = -0.16 (10yr) -1 *; (b, f) annual duration, trend = -0.14 days (10yr) -1 ; (c, g) annual extent, trend = -1.56 grid cells(10yr) -1 ; and (d, h) annual intensity, trend = 0.25 C(10yr) -1 ***. 38

50N (a) 40N 30N 20N 50N ( 0.05, 0.6 ] ( 0.6, 0.8 ] ( 0.8, 1.5 ] ( 1.5, 2.2 ] (b) 80E 90E 100E 110E 120E 130E 40N 30N 20N 50N ( 0.05, 0.6 ] ( 0.6, 0.8 ] ( 0.8, 1.4 ] (c) 80E 90E 100E 110E 120E 130E 40N 30N 20N ( -1.3, -0.75 ] ( -0.75, -0.5 ] ( -0.5, 0 ] ( 0, 0.3201 ] 80E 90E 100E 110E 120E 130E Fig. 9. Annual frequencies (yr -1 ) affected by regional PLT events during (a) 1960 1980, (b) 1990 2008, and (c) frequency differences (yr -1 ) by two periods 1990 2008 minus 1960 1980. 39