As introduced in section 1, an ECPFE is characterized by cold-air invasion, precipitation, and freezing weather (Tao and Wei, 2008; Yang et al., 2008; Zhao and Sun, 2008). Accordingly, an ECPFE can be defined if low temperature, precipitation, and freezing weather concur in an extensive area. According to the World Meteorological Organization (WMO) Task Team on the Definition of Extreme Weather and Climate Events (TT-DEWCE), the definition of a cold wave and marked precipitation event should be complemented by characterization of its magnitude, duration, and extent (WMO, 2016). As such, the definition of an ECPFE here should reflect the abovementioned metrics. Specifically, the determination of an ECPFE consists of the following four steps:
We first define cold stations, precipitation stations, and freezing stations, respectively. A cold station is defined if its normalized daily mean temperature falls below −0.5 σ. The −0.5 σ is approximately the 31st percentile of daily observations for the period 1951−2013, and is roughly equivalent to a temperature anomaly of −2°C in southern China. Considering the precipitation in the cold area, a precipitation station is defined when daily precipitation is greater than 1 mm d−1 and a negative temperature anomaly is observed. In fact, the threshold of 1 mm d−1 is recommended by the WMO TT-DEWCE for the definition of a precipitation station (World Meteorological Organization, Geneva, 2016). Because freezing weather is represented by the presence of glaze in southern China (Li et al., 2008; Wang, 2014), a freezing station is simply defined if glaze is observed in-situ.
Then, we calculate the areas of cold stations, precipitation stations, and freezing stations, separately. Since the 316 stations are nonuniformly distributed across southern China, it is difficult to quantify the area of cold stations. To measure the cold-station area, we divide southern China into 300 0.5° × 0.5° latitude/longitude grid boxes, as shown in Fig. 1. The cold-station area is represented by the total number of grid boxes that contain at least one cold station. The precipitation-station area and the freezing-station area are calculated in the same manner as the cold-station area.
Here, we define thresholds for the areas of cold stations, precipitation stations, and freezing stations. The area is identified as extensive if it exceeds the threshold. We adopt the 67th percentile (the upper tertile) as thresholds for the cold-station area and precipitation-station area, which are 116 and 43 grid boxes, respectively. It is observed that freezing weather occurs in a considerably smaller area than the cold area and the precipitation area. For example, the 67th percentile of the freezing-station area is only 6 grid boxes. Thus, we use the 85th percentile as the threshold for the freezing-station area, i.e., 10 grid boxes. The thresholds of the cold area, precipitation area, and freezing area account for 38.7%, 14.3% and 3.3% of the total area of southern China, respectively.
Finally, we define an ECPFE, as follows: An ECPFE is identified in southern China if the cold area, precipitation area, and freezing area concurrently exceed their respective thresholds for at least three consecutive days. The duration of three days is approximately the 80th percentile of the duration in which the cold area, precipitation area, and freezing area simultaneously exceed their respective thresholds (including one and two days).
According to the above procedure, a total of 109 ECPFEs are identified from 1951 to 2013 (Table 1, column 2).
Number Starting and ending dates of ECPFEs Duration Date of the largest freezing area 1. 4Dec1954–9Dec1954 6 17Feb1988 2. 29Dec1954–4Jan1955 7 1Mar1988 3. 4Feb1957–6Feb1957 3 16Mar1988 4. 8Feb1957–10Feb1957 3 13Jan1989 5. 12Mar1957–14Mar1957 3 17Jan1989 6. 30Jan1958–2Feb1958 4 7Feb1989 7. 27Feb1958–1Mar1958 3 23Feb1989 8. 7Jan1959–9Jan1959 3 31Jan1990 9. 18Feb1959–24Feb1959 7 27Feb1990 10. 23Dec1959–26Dec1959 4 27Dec1991 11. 22Jan1960–24Jan1960 3 4Mar1992 12. 24Nov1960–26Nov1960 3 16Jan1993 13. 14Feb1961–16Feb1961 3 20Jan1993 14. 8Feb1963–10Feb1963 3 24Feb1993 15. 11Mar1963–14Mar1963 4 20Nov1993 16. 25Jan1964–27Jan1964 3 16Dec1993 17. 14Feb1964–24Feb1964 11 20Jan1994 18. 16Dec1965–19Dec1965 4 2Jan1995 19. 22Feb1966–24Feb1966 3 19Feb1996 20. 9Jan1967–11Jan1967 3 6Feb1997 21. 30Jan1967–2Feb1967 4 20Jan1998 22. 23Feb1967–27Feb1967 5 27Dec2002 23. 29Nov1967–2Dec1967 4 6Jan2003 24. 26Dec1967–28Dec1967 3 24Dec2004 25. 30Jan1968–7Feb1968 9 15Jan2008 26. 31Dec1968–4Jan1969 5 21Jan2008 27. 11Jan1969–16Jan1969 6 26Jan2008 28. 29Jan1969–31Jan1969 3 3Jan2011 29. 2Feb1969–4Feb1969 3 20Jan2011 30. 16Feb1969–20Feb1969 5 28Jan2011 31. 23Feb1969–27Feb1969 5 5Jan2012 32. 16Nov1969–18Nov1969 3 24Jan2012 33. 26Feb1970–7Mar1970 10 4Jan2013 34. 27Jan1971–29Jan1971 3 17Feb1988 35. 3Feb1972–8Feb1972 6 1Mar1988 36. 4Jan1973–6Jan1973 3 16Mar1988 37. 11Jan1974–18Jan1974 8 13Jan1989 38. 24Jan1974–26Jan1974 3 17Jan1989 39. 29Jan1974–6Feb1974 9 7Feb1989 40. 23Feb1974–25Feb1974 3 23Feb1989 41. 12Dec1974–17Dec1974 6 31Jan1990 42. 12Dec1975–14Dec1975 3 27Feb1990 43. 21Jan1976–23Jan1976 3 27Dec1991 44. 18Feb1976–20Feb1976 3 4Mar1992 45. 18Mar1976–21Mar1976 4 16Jan1993 46. 13Nov1976–17Nov1976 5 20Jan1993 47. 7Jan1977–9Jan1977 3 24Feb1993 48. 16Jan1977–18Jan1977 3 20Nov1993 49. 27Jan1977–1Feb1977 6 16Dec1993 50. 16Jan1978–18Jan1978* 3 20Jan1994 51. 10Feb1978–16Feb1978* 7 2Jan1995 52. 13Mar1978–15Mar1978** 3 19Feb1996 53. 2Feb1980–4Feb1980* 3 6Feb1997 54. 6Feb1980–8Feb1980* 3 20Jan1998 55. 11Mar1980–14Mar1980** 4 27Dec2002 56. 27Feb1981–2Mar1981* 4 6Jan2003 57. 7Nov1981–9Nov1981** 3 24Dec2004 58. 5Feb1982–14Feb1982* 10 15Jan2008 59. 25Mar1982–27Mar1982** 3 21Jan2008 60. 6Dec1982–8Dec1982* 3 26Jan2008 61. 9Jan1983–12Jan1983* 4 3Jan2011 62. 27Dec1983–29Dec1983* 3 20Jan2011 63. 17Jan1984–21Jan1984* 5 28Jan2011 64. 27Jan1984–31Jan1984* 5 5Jan2012 65. 2Feb1984-5Feb1984* 4 24Jan2012 66. 9Feb1984–12Feb1984* 4 4Jan2013 67. 16Dec1984–21Dec1984* 6 17Feb1988 68. 6Jan1985–9Jan1985* 4 1Mar1988 69. 17Feb1985–22Feb1985* 6 16Mar1988 70. 24Feb1985–27Feb1985* 4 13Jan1989 71. 3Mar1985–5Mar1985** 3 17Jan1989 72. 8Mar1985–14Mar1985** 7 7Feb1989 73. 9Dec1985–11Dec1985* 3 23Feb1989 74. 27Feb1986–1Mar1986** 3 31Jan1990 75. 19Feb1987–22Feb1987* 4 27Feb1990 76. 9Feb1988–11Feb1988* 3 27Dec1991 77. 16Feb1988–20Feb1988* 5 4Mar1992 78. 27Feb1988–4Mar1988* 7 16Jan1993 79. 16Mar1988–18Mar1988** 3 20Jan1993 80. 11Jan1989–14Jan1989* 4 24Feb1993 81. 16Jan1989–19Jan1989* 4 20Nov1993 82. 6Feb1989–9Feb1989** 4 16Dec1993 83. 21Feb1989–25Feb1989* 5 20Jan1994 84. 31Jan1990–3Feb1990* 4 2Jan1995 85. 23Feb1990–4Mar1990* 10 19Feb1996 86. 25Dec1991–28Dec1991* 4 6Feb1997 87. 3Mar1992–7Mar1992** 5 20Jan1998 88. 14Jan1993–16Jan1993* 3 27Dec2002 89. 20Jan1993–23Jan1993** 4 6Jan2003 90. 23Feb1993–25Feb1993** 3 24Dec2004 91. 19Nov1993–21Nov1993** 3 15Jan2008 92. 14Dec1993–16Dec1993** 3 21Jan2008 93. 18Jan1994–20Jan1994* 3 26Jan2008 94. 1Jan1995–3Jan1995* 3 3Jan2011 95. 17Feb1996–26Feb1996* 10 20Jan2011 96. 4Feb1997–7Feb1997* 4 28Jan2011 97. 20Jan1998–22Jan1998* 3 5Jan2012 98. 25Dec2002–27Dec2002* 3 24Jan2012 99. 5Jan2003–7Jan2003* 3 4Jan2013 100. 23Dec2004–25Dec2004* 3 17Feb1988 101. 13Jan2008–15Jan2008* 3 1Mar1988 102. 17Jan2008–22Jan2008* 6 16Mar1988 103. 2Jan2011–6Jan2011 9 13Jan1989 104. 17Jan2011–20Jan2011 5 17Jan1989 105. 26Jan2011–28Jan2011 4 7Feb1989 106. 4Jan2012–6Jan2012 3 23Feb1989 107. 21Jan2012–24Jan2012 3 31Jan1990 108. 3Jan2013–6Jan2013 4 27Feb1990 109. 4Dec1954–9Dec1954 4 27Dec1991
Table 1. Information on 109 ECPFEs. Units of duration and area are given in days and numbers of grid boxes (0.5° × 0.5°), respectively. An asterisk (*) denotes the events that were also identified by the NMC/CMA, and a double asterisk (**) denotes the events that were not identified by the NMC/CMA (Zong et al., 2013).
To examine the robustness of the ECPFEs identified in this study, we first compare our results of a typical case of great influence in 2008 with those defined in previous studies (Tao and Wei, 2008; Zong et al., 2013). The well-known ECPFE episodes in early 2008, ranking 101st to 103rd in Table 1, are identified to begin on 13 January and end on 2 February 2008. The ending day of the ECPFE episodes in the current study (2 February) is the same as the date in a previous study (Tao and Wei, 2008). The current identification of the starting day of the ECPFE (13 January) basically agrees with the date (10 January) of the previous study (Tao and Wei, 2008). The difference possibly comes from the more critical areal thresholds in our definition.
Figure 3 shows the daily evolutions of cold-station area, precipitation-station area, and freezing-station area, respectively, from 10 January to 10 February 2008. The cold-station area (Fig. 3a) and freezing-station area (Fig. 3c) both exceed their thresholds during the entire period. In contrast with these two areas, the precipitation-station area exhibits a more drastic variation in this period, during which it falls below the threshold three times (16, 23 and 24 January). The variation of precipitation-station area reflects that there are three ECPFEs: 13−15 January, 17−22 January, and 25 January−2 February (Table 1). In previous studies (Sun and Zhao, 2008; Tao and Wei, 2008; Zhao and Sun, 2008), there are four ECPFEs at the same time: 10−14 January, 18−23 January, 25−28 January, and 30 January−2 February. The difference appears in the third period. By examining the daily evolution of the precipitation-station area (Fig. 3b), it can be seen that, although the precipitation area reaches a local minimum on 30 January, it is above its threshold. Meanwhile, precipitation of more than 2 mm d−1 dominates in southern China during 29−31 January 2008 (Fig. 4). Therefore, the period from 25 January to 2 February should not be divided into two synoptic periods, according to the daily evolution of precipitation in southern China. This fact suggests that, on the whole, our identification of ECPFE episodes in 2008 is reasonable and in agreement with previous studies.
Figure 3. Daily evolutions of (a) cold-station area, (b) precipitation-station area, and (c) freezing-station area, respectively, from 10 January to 10 February 2008. Black horizontal lines indicate the areal thresholds.
Figure 4. Distributions of normalized temperature anomalies (units: °C; contours are from −4°C to 0.5°C with intervals of 0.5°C) and precipitation (shaded; units: mm d−1) on (a) 29 January 2008, (b) 30 January 2008, and (c) 31 January 2008. The stations with glaze (freezing weather) are indicated by red dots. Blue lines denote the Yellow River and Yangtze River.
Next, we compare our identified ECPFEs with those identified subjectively by the NMC/CMA (Zong et al., 2013). The NMC/CMA identified 62 ECPFEs in the period 1978−2008 (Zong et al., 2013), while we identify 54 ECPFEs in the same period (50th−103rd in Table 1). 40 out of 54 ECPFEs in our identification overlap with those by the NMC/CMA (Zong et al., 2013), as denoted by an asterisk in Table 1. Meanwhile, as shown in Table 2, a total of 28 ECPFEs identified by NMC/CMA (Zong et al., 2013) are not identified in this study. In five events (16th, 17th, 21st, 24th, and 27th events in Table 2), the three areal threshold criteria and the requirement of three consecutive days are satisfied, but the period during which the cold area, precipitation area and freezing area simultaneously meet the areal threshold criteria is less than 3 days. In four events (11th, 12th, 13th and 22nd events in Table 2), each of the three areal threshold criteria and the requirement of three consecutive days are not satisfied concurrently. Especially, the 11th, 12th and 22nd events (Table 2) lasted for less than 3 days. In six events (5th, 8th, 18th, 19th, 20th and 25th events in Table 2), only two of the three areal threshold criteria are satisfied along with meeting the demand of at least three consecutive days; and in the remaining 13 events, only one of the three areal threshold criteria is satisfied as well as lasting for three consecutive days. Thus, the reason why the 28 ECPFEs identified by NMC/CMA (Zong et al., 2013) have not been identified in this study is primarily due to the strict criteria in our ECPFE definition.
Number Start and end dates of the
ECPFEs identified by
Reason for being unidentified 1. 11Jan1979–16Jan1979 The precipitation area that meets the threshold criterion in our identification persist
for less than three days
2. 29Jan1979–31Jan1979 The cold area that meets the threshold criterion in our identification persist for l
ess than three days
3. 4Jan1981–9Jan1981 The cold area that meets the threshold criterion in our identification persist for
less than three days
4. 20Jan1981–31Jan1981 The cold area that meets the threshold criterion in our identification persist
for less than 3 days
5. 29Jan1982–31Jan1982 Both the cold area and precipitation area that meet the threshold criteria in our identification persist for less than three days 6. 17Jan1983–21Jan1983 The precipitation area that meets the threshold criterion in our identification persist
for less than three days
7. 19Feb1983–22Feb1983 The freezing weather area that meets the threshold criterion in our identification persist
for less than three days
8. 30Jan1986–5Feb1986 Both the cold area and freezing area that meet the threshold criteria in our identification persist for less than three days 9. 16Jan1988–18Jan1988 The cold area that meets the threshold criterion in our identification persists
for less than three days
10. 23Jan1988–26Jan1988 The precipitation area that meets the threshold criterion in our identification persists for less than three days 11. 22Dec1989–23Dec1989 The identified event itself persisted for less than 3 days 12. 15Jan1990–15Jan1990 The identified event itself persisted for less than 3 days 13. 23Jan1991–26Jan1991 Each of three areal threshold criteria and the requirement of three consecutive days
are not satisfied concurrently
14. 31Jan1992–4Feb1992 The cold area that meets the threshold criteria in our identification persisted
for less than three days
15. 7Feb1992–10Feb1992 The freezing weather area that meets the threshold criterion in our identification persists for less than three days 16. 23Feb1994–28Feb1994 The three areal threshold criteria and the requirement of three consecutive days are satisfied, but the period during which the cold area, precipitation area and freezing area simultaneously meet the areal threshold criteria is less than 3 days 17. 12Jan1996–21Jan1996 The three areal threshold criteria and the requirement of three consecutive days are satisfied, but the period during which the cold area, precipitation area and freezing area simultaneously meet the areal threshold criteria is less than 3 days 18. 22Jan1997–24Jan1997 The freezing weather area that meets the threshold criterion in our identification
is less than three days
19. 11Jan1999–14Jan1999 Both the precipitation area and freezing weather area that meet the threshold criteria in our identification persist for less than three days 20. 13Jan2000–17Jan2000 Both the cold area and freezing area that meet the threshold criteria in our identification persist for less than three days 21. 21Jan2000–5Feb2000 The three areal threshold criteria and the requirement of three consecutive days are satisfied, but the period during which the cold area, precipitation area and freezing area simultaneously meet the threshold criteria is less than 3 days 22. 12Dec2000–12Dec2000 The identified area itself persisted for less than 3 days 23. 23Jan2001–27Jan2001 The freezing weather area never meets the area threshold criterion in our identification 24. 10Feb2003–13Feb2003 The three areal threshold criteria and the requirement of three consecutive days are satisfied, but the period during which the cold area, precipitation area and freezing area simultaneously meet the threshold criteria is less than 3 days 25. 18Jan2004–20Jan2004 Both the freezing weather area and the cold area that meet the threshold criterion in our identification persistfor less than three days 26. 9Jan2005–13Jan2005 The freezing weather area that meets the threshold criterion in our identification persists
for less than three days
27. 7Feb2005–13Feb2005 The three areal threshold criteria and the requirement of three consecutive days are satisfied, but the period during which the cold area, precipitation area and freezing area simultaneously meet the threshold criteria is less than 3 days 28. 13Jan2007–19Jan2007 The cold area that meets the threshold criterion in our identification persists
for less than three days
Table 2. The ECPFEs that are identified by the NMC/CMA (Zong et al., 2013) but not identified in the current study.
On the other hand, as denoted by a double asterisk in Table 1, 14 ECPFEs in our identification are absent in the 62 ECPFEs identified by the NMC/CMA (Zong et al., 2013). This results mainly from the different lengths of winter used. As can be seen from Table 1, 10 of these events occur in November or late February to March.
The comparison of our results with an existing ECPFE identification demonstrates that, on the whole, the current identification of ECPFEs can be considered robust and reliable. The discrepancies found in the comparison result primarily from our strict areal threshold criteria in low temperature, precipitation and freezing weather. A longer length of winter also affects the number of ECPFEs identified.
According to the definition of an ECPFE, it is simply known as an extensive cold event with extensive precipitation and freezing weather. To depict the nature of an ECPFE, therefore, it is better to compare it with extensive cold events with less precipitation and less freezing (i.e., ECEs), as well as with precipitation but with less freezing (i.e., ECE-Ps).
In an analogous way to ECPFEs (see subsection 3.1), an ECE is identified with an areal threshold criterion of cold stations and the requirement of three consecutive days in winter. Similarly, an ECE-P is identified with an areal threshold criteria of cold stations and precipitation stations and the requirement of three consecutive days in winter. To reduce the influence of seasonal variation of climatological background circulation, we choose the conventional winter (December−January−February, DJF) cases of ECPFEs, ECEs and ECE-Ps to configure their key features. As such, a total of 87 ECPFEs, 227 ECEs and 125 ECE-Ps are identified for the period 1951−2013.
According to the definitions of these three types of events, it is obvious that the ECEs contain the ECE-Ps, and the ECE-Ps also contain the ECPFEs. The relationships among the three types of events are illustrated in Fig. 5. To depict the key features of these three types of events, we select the total 87 ECPFEs, 127 ECEs excluding all of the ECE-Ps, and 40 ECE-Ps excluding all of the ECPFEs (see Fig. 5) to analyze their near-surface observational features and circulation characteristics. Note that an ECE may contain two or even more ECE-Ps. Hence, the number of ECEs excluding all ECE-Ps is 127, instead of 227 − 125 = 102. Besides, two ECE-Ps that occurred during 16 February to 1 March 1959 and during 23 February to 2 March 1969, are not included in the 125 DJF ECE-Ps. Thus, the number of ECE-Ps excluding all ECPFEs is 40, instead of 125 − 87 = 38.
Figure 5. Schematic diagram of the relationships among the three types of low-temperature events. There are 87 ECPFEs, 125 ECE-Ps and 227 ECEs during the DJF of 1951−2013. According to the definitions of the three types of events, the ECEs contain the ECE-Ps, and the ECE-Ps also contain the ECPFEs. There are 127 ECEs excluding all ECE-Ps, and 40 ECE-Ps excluding all ECPFEs.
Figure 6 shows the composite fields of the normalized temperature anomalies, precipitation anomalies (in percentage), and freezing-weather frequencies averaged during the ECPFEs (left), ECEs (middle), and ECE-Ps (right). The ECPFEs are characterized by significant negative temperature anomalies in China, increased precipitation from the Yangtze River valley to southern China (Fig. 6b), and frequent freezing weather from southwestern China to the regions along the middle and lower reaches of the Yangtze River. The maximum frequency of freezing weather in the middle and lower reaches of the Yangtze River is up to 50% (Fig. 6c).
Figure 6. Composite fields of normalized temperature anomalies (top; contour lines are drawn for every 0.25 σ), precipitation anomalies in percentage (middle; contours are drawn for −0.5, 0.0, 0.5, 1.0, 2.0, 4.0, and 6.0), and the freezing-weather frequency (bottom; contours are drawn for 5%, 10%, 15%, 20% and 40%), averaged during ECPFEs (left), ECEs (middle), and ECE-Ps (right). Red dots denote that the composite anomalies are statistically significant at the 0.05 level.
In contrast to the ECPFEs, the ECEs feature a weaker cold anomaly, decreased rainfall, and considerably less freezing-weather occurrence (Fig. 6d). For the ECEs, a precipitation deficit dominates over China (Fig. 6e) and the freezing-weather frequency is below 5% in most areas (Fig. 6f).
For the ECE-Ps, the amplitude of the cold anomaly is equivalent to that of the ECEs and weaker than that of the ECPFEs (Fig. 6g). The precipitation is above normal in southern China (Fig. 6h). The freezing-weather frequency ranges between 5% and 10% in the middle and lower reaches of the Yangtze River (Fig. 6i).
Figure 7 shows the composite fields of averaged Z500 and the corresponding anomalies during the three groups of extensive cold events. Focusing first on the ECPFEs, a tilted ridge with significant positive height anomalies is situated over Eurasia north of 45°N, with the anomaly center residing around the Ural Mountains. At the same time, a tilted trough with significant negative anomalies is located over the 30°−45°N zone in East Asia. This configuration is reminiscent of the large-scale tilted ridge and trough pairing that is the crucial circulation pattern for extensive and persistent extreme cold events (Bueh et al., 2011a, b, 2018; Peng and Bueh, 2011, 2012; Bueh and Xie, 2015). The tilted ridges and troughs are more quasi-stationary than ordinary ridges and troughs, leading to more persistent cold air invading China (Bueh et al., 2011a, b; Peng and Bueh, 2012). This is consistent with the longer duration of ECPFEs.
Figure 7. Composite fields of Z500 (contours; interval: 50 gpm) and the corresponding anomalies (shading) averaged during the (a) ECPFEs, (b) ECEs, and (c) ECE-Ps. Black dots show the t-test significance at the 0.05 level.
Unlike the ECPFEs, the ECEs are characterized by a zonal wave train circulation pattern (Fig. 7b). The ridge is displaced southeastward from the Caspian Sea to Lake Baikal with respect to that of the ECPFEs, while the East Asian trough is deepened over its climatological position. This zonal wave train pattern is a typical pattern for ordinary cold waves in East Asia (Park et al., 2011; Peng and Bueh, 2012), which exhibit an obvious migrating feature. A pronounced zonal height gradient is observed over eastern China, which results in stronger northerlies that convey cold air into the southeastern coastal region of China.
Regarding the ECE-Ps (Fig. 7c), the circulation features a dipole pattern over eastern Asia, with a positive height anomaly over northeastern Asia and a negative height anomaly band from Tibet to Japan. In comparison with ECPFEs, the significant negative height anomaly extends further south into the Indochina Peninsula and thus favors a stronger northeastward moisture flux and a more active subtropical synoptic disturbance, inducing more precipitation in-situ.
Different mid- and high-latitude circulation anomalies may lead to different pathways of cold air. Hereafter, for brevity, 0 d refers to the starting day of the event and −N (N) d refers to N days before (after) 0 days. Figure 8 shows the evolution of △T24 for the three groups of cold events from −10 d to 1 d. Regarding the ECPFEs (left-hand column), at −10 d, there are significant negative △T24s to the north of the Caspian Sea where cold air accumulates. From −5 d to −3 d, a significant negative △T24 center exceeding −4°C d−1 propagates eastward to the east of Lake Baikal. From −1 d to 0 d, cold air sweeps southward to southern China. Significant cooling occurs in the middle and lower reaches of the Yangtze River and to the south of the Yangtze River. At 1 d, significant negative △T24s of less than −4°C d−1 still occur in southern China. The southward intrusion of cold flow leads to the propagation of the 0°C isotherm (the red lines in Fig. 8) from North China (−3 d) southward to the middle and lower reaches of the Yangtze River (−1 d) and then to the northern part of the Yangtze River (0−1 d). The invading cold air favors a cold layer of below 0°C in the lower troposphere over the increased rainfall region that turns the liquid precipitation to be refrozen when the raindrops fall to the ground and on exposed objects. Studies have shown that low-level cold air is one of the key factors for freezing rain and freezing weather (Li et al., 2008; Tao and Wei, 2008; Zhao and Sun, 2008).
Figure 8. Composite daily evolutions of ΔT24 (units: °C d−1) for the ECPFEs (left), ECEs (middle), and ECE-Ps (right). Black dots show the t-test significance at the 0.05 level. Red lines refer to the 0°C isotherm. Purple dashed lines denote 30°N.
In contrast with the ECPFEs, the negative △T24 of the ECEs intrudes southward from Lake Baikal to southeastern coastal China (middle column of Fig. 8), the so-called eastern route of cold-air intrusion. Although the negative △T24 of the ECEs is more intensive at −1 d than the ECPFEs’ counterpart, it is more transient and then absent at 1 d. Therefore, the troposphere is controlled by the cold air that is unfavorable for precipitation and freezing weather (Figs. 6d-f).
Similar to the ECPFEs, the ECE-Ps also feature a southeastward incursion of negative △T24 from the Caspian Sea with a relatively weak amplitude (right-hand column of Fig. 8). Meanwhile, a moderately negative △T24 is advected southward from Northeast China in conjunction with southeastward negative △T24 incursion over southern China. However, the weaker amplitude of negative △T24 is incapable of producing a below 0°C layer near the ground (indicated by the 0°C isotherm). In this situation, precipitation tends to prevail over southern China with moderate freezing weather (Figs. 6g-i).
Different circulation anomalies over the mid and high latitudes during these three types of cold events correspond to different cold-air sources and pathways. The large-scale tilted ridge and trough facilitate the propagation of cold air from the north of the Caspian Sea to South China during an ECPFE. The zonal wave-like ridges and troughs during an ECE lead to the southward propagation of cold air from Lake Baikal. The negative height-anomaly band extending from Tibet to Japan during an ECE-P corresponds to weak cold-air incursion from the Caspian Sea to South China.
The near-surface cold layer and warm layer aloft are the key factors for freezing rain in China (Sun and Zhao, 2008; Zhao and Sun, 2008; Deng et al., 2012). In January 2008, the warm layer in the middle and lower troposphere was found to be closely related to the abnormal southerly wind field (Zong and Ma, 2011). Next, we turn our attention to examining the subtropical circulation system and vertical structure of wind and temperature during ECPFEs, and compare the results with those of ECEs and ECE-Ps.
Figure 9 shows the composite wind and temperature anomaly fields at three lower-tropospheric pressure levels for the ECPFEs. At 700 hPa (Fig. 9a), there are significant westerly anomalies near 25°N, suggesting an intensification of the southern branch of the westerly jet. Meanwhile, an anticyclonic anomaly stays over the western Pacific Ocean. The enhanced southern branch of the westerly jet and the anticyclonic anomalies lead to significant southerly wind anomalies prevailing to the south of 35°N. The anomalous southwesterly airflow transports warm air from the southeast of the Tibetan Plateau to the Yangtze River basin (Fig. 9d), resulting in a warm layer. In the meantime, the large-scale tilted ridge and trough in the ECPFEs foster significant northerly anomalies over southern China at 850 hPa and 925 hPa (Figs. 9b and c), favoring a southward invasion of cold air (Figs. 9e and f), and hence the formation of a near-surface cold layer.
Figure 9. Composite wind anomalies (left; units: m s−1) and temperature anomalies (right; units: °C) at 700 hPa (top), 850 hPa (middle) and 925 hPa (bottom) averaged during 87 ECPFEs. The dots indicate regions where the composite temperature anomalies are significant at the 0.05 level. Red vectors show that the composite zonal wind anomalies are significant at the 0.05 level. The yellow shading denotes areas where the composite meridional wind anomalies are significant at the 0.05 level. The black shading denotes topography higher than 3000 m in (a, d) and 1500 m in (b, c, e, h).
Figure 10a displays the pressure−latitude cross sections (averaged over 107.5°−120°E) of composite temperature and wind anomalies for 87 ECPFEs on the days of the largest freezing-station area (fourth column of Table 1). Around 25°−30°N, the significant warm flow glides up over shallow cold air, resulting in an inversion layer and hence the frequent freezing rain (Fig. 6c). Thus, during ECPFEs, the enhanced southern branch of westerlies and the anticyclonic anomalies over the western Pacific lead to significant southwesterlies, an inversion layer, and the favorable environment of freezing rain.
Figure 10. Pressure−latitude cross sections (averaged over 107.5°−120°E) of composite temperature anomalies (shaded; units: °C) and wind anomalies (arrows; meridional wind anomalies: m s−1; pressure vertical velocity: −1 hPa h−1, negative values denote upward vertical velocity) on the days of (a) the largest freezing-station area in the ECPFEs, (b) the largest cold-station area in the ECEs, and (c) the largest precipitation-station area in the ECE-Ps. The purple arrows indicate composite vertical motions that are significant at the 95% confidence level. The black dots show the t-test significance of composite temperature anomalies at the 0.05 level.
For the ECEs, significant northerly anomalies prevail over eastern China in the middle and lower troposphere (Figs. 11a-c). Their intensities are stronger than those in ECPFEs (Figs. 9a-c). The northerly anomalies lead to the invasion of cold and dry air from the north (Figs. 11d-f). In addition, there are significant easterly anomalies near 25°N, and the southern branch of the westerly jet is weakened. The vertical cross section of wind anomalies of 107.5°−120°E (Fig. 10b) shows that significant descending air flows control from 20°N to 40°N on the days of the largest cold-station area. The northerly anomalies and descending air flows correspond to little freezing weather and precipitation in the ECEs (Figs. 6e and f).
Figure 11. As in Fig. 9 but for 127 ECEs.
The composite wind and temperature anomalies at 700, 850 and 925 hPa for 40 ECE-Ps (shown in Fig. 12) are similar to those for ECPFEs. At 700 hPa, the southern branch of the westerly jet enhances, and an anomalous anticyclonic center over the east of the Philippines is located to the south of that in ECPFEs (Fig. 9a). Regions to the south of the Yangtze River are controlled by southerly anomalies. At 850 hPa and 925 hPa, northerly anomalies prevail over southern China. The southerly anomalies at 700 hPa and the northerly anomalies at lower levels are weaker than those for ECPFEs (Figs. 9a-c). The weaker cold air cannot help form frozen (below 0°C) layers near the ground. In addition, due to the weaker southerly anomalies at 700 hPa (Fig. 12a), the warm layer for ECE-Ps on the days of the largest precipitation-station area (Fig. 10c) is shallow and shifts southward compared to those in ECPFEs (Fig. 10a). There is a close relationship between the freezing-rain area and the intensity of the inversion layer (Zong and Ma, 2011). Therefore, there is extensive precipitation but less freezing weather in ECE-Ps.
Figure 12. As in Fig. 9 but for 40 ECE-Ps.
These results suggest that the southern branch of the westerlies and the anomalous anticyclonic circulation over the western Pacific are the important subtropical circulation systems for ECPFEs in southern China. The active disturbances in the southern branch of the westerlies and the anomalous anticyclonic circulation over the western Pacific lead to the significant southwesterly anomalies in the mid-tropospheric layer over southern China. The southwesterly anomalies help form the inversion layer, which is crucial for the freezing rain. In ECEs, the southern branch of westerlies is weakened. Strong cold and dry air invades southern China from the north. Northerly anomalies prevail over southern China, resulting in less precipitation and freezing rain. In ECE-Ps, the active subtropical systems and the weak cold air lead to less freezing rain.
|Number||Starting and ending dates of ECPFEs||Duration||Date of the largest freezing area|