1. 中国科学院 东亚区域气候 - 环境重点实验室 Effects of Site-Change and Urbanisation in the Beijing Temperature Series 1977-2006 Li Zhen Key Laboratory of Regional Climate-Environment in Temperate East Asia Institute of Atmospheric Physics, Chinese Academy of Sciences 26 – 30 May 2008, Budapest, Hungary

2. Outline Introduction Data sets Methods Results Conclusions

3. 1. Introduction The Beijing station was moved from its suburban location (Daxing district) to an urban location (Beiwa road in Haidian district ) on 1/1/1981 and back on 1/7/1997. The aims of the present study are: Analyze and adjust site change effects in BJ temperature series Estimate an urbanisation-related warming bias.

4. 2.Datasets Figure 1. Locations of 19 stations in the larger Beijing area used in the present study. The bold are 4 core reference stations (CRS), the Italic fonts indicate 5 rural stations.

5. Table1. Basic information of Beijing (BJ) and 18 nearby stations with daily temperature data back to 1977 or earlier. Italic are rural stations with less-than-18K population and bold are 4 core reference stations for site-change adjustments at BJ. Correlation coefficients with BJ for 3 sub-periods (c1 for 1977-1980, c2 for 1981-1997 and c3 for 1997-2006) are calculated based on seasonal mean temperature anomalies. Abbrev. (name) Lon.E / Lat.N / Elev.m Distance to BJ (km) Population (K) Correlation with BJ ( c1/c2/c3) Relocation records BJ (Beijing) 116.28° / 39.93° / 54.7 - 1556-Moved to Haidian on 01/01/1981 Back to Daxing on 01/07/1997 FS (Fangshan) 116.00° / 39.7° / 48.9 33.093470.98/0.96/0.95Moved within Fanshan District on 01/01/1965 TZ (Tongzhou) 116.63° / 39.92° / 26.9 38.902690.99/0.96/0.98Moved within Tongzhou District on 01/01/1965 MY (Miyun) 116.87° / 40.38° / 73.1 69.271510.97/0.97/0.94No change since 01/01/1957 but data are only available from 1971 PG (Pinggu) 117.10°/40.15° / 29.4 91.784.10.98/0.95/0.90Moved within Pinggu District on 01/01/1977 TH (Tanghekou) 116.63° / 40.73° / 333.7 55.517.10.95/0.94/0.96No change since 01/01/1974 in Huairou District. XY (Xiayunling) 115.73° / 39.73° / 409.1 61.8910.30.96/0.89/0.89No change since 01/02/1959 at Xiayunling, Fangshan District ZT (Zhaitang) 115.68° / 39.97° / 441.1 66.7017.20.97/0.86/0.90No change since 01/01/1974 at Zhaitang, Mentougou District MS (Miyun Shangdianzi) 117.12° / 40.65° / 286.5 100.0217.70.95/0.94/0.88No change since the beginning on 01/01/1958

6. HD (Haidian) 116.28° / 39.98° / 46.3 2.4618070.98/0.99/0.91Moved within Haidian District on 01/04/1974 FT (Fengtai) 116.25° / 39.87° / 56.3 4.458420.96/0.97/0.93Moved within Fengtai District on 01/01/1978 DX (Daxing) 116.33° / 39.75° / 41.3 10.462270.96/0.99/0.80Moved to Huangcun town, Daxing Disrict on 01/01/1978 SJ (Shijingshan) 116.18° / 39.93° / 70.8 11.113500.97/0.98/0.96Moved within Shijingshan District on 01/01/1998 CP (Changping) 116.22° / 40.22° / 79.7 15.742570.96/0.96/0.96Moved within Changping District on 01/01/1980 MT (Mentougou) 116.12° / 39.92° / 93.6 17.791770.98/0.97/0.95Moved within Mentougou District on 01/01/1978 CY (Chaoyang) 116.48° / 39.95° / 36.5 22.2515560.98/0.98/0.96Moved within Chaoyang District on 01/04/1961 SY (Shunyi) 116.63° / 40.12° / 39.5 40.012070.98/0.99/0.90Moved within Shunyi District on 01/01/1999 YQ (Yanqing) 115.97° / 40.45° / 489 42.83970.95/0.94/0.96Moved within Yanqing District on 01/01/1988 and 01/05/2000 HR (Huairou) 116.63° / 40.32° / 60.6 43.421070.97/0.97/0.94Moved within Huairou on 01/07/1996

7. 3.Methods denotes a daily temperature anomaly at BJ; denotes a (mean) daily anomaly at a (set of) reference station (s); is the difference between the average of daily anomalies of a n-day- span after day L and that of the previous n-day-span at BJ; is the same but at the reference station (s). The idea here is similar to that of Alexandersson (1986; 1995). The ξ variable indicating possible site-change- induced bias is expressed as:

8. Adjustment scale: Figure 2. Daily ξ series of (BJ vs mean of MT, MY and TH), from 1/1/1980 to 31/12/1981 (centred at the relocation date for BJ, 1/1/1981, as marked in the series), for n = 1, 30, 91, 366, 732, 1098 days. A site-change effect may be possibly estimated at the seasonal scale.

9. Adjustment variable: denotes the difference between the mean of daily anomalies during the 3 months of a certain season after the relocation day and that of the previous 3 months of the same season at BJ. is the same but at a (set of) reference station (s). denotes the i th year after/before the relocation (1≤i≤m). is the number of years for calculating the multi-year mean adjustment.

10. 4.Results Multi-year mean seasonal adjustments Figure3. The m-year mean seasonal adjustment variables for BJ temperature records changing with m (MY and MS). In the present analysis m is set as 3.

11. Core reference stations (CRS) for site-change adjustments Figure 4. Three-year mean seasonal adjustments for BJ temperature records due to relocations in 1981/1997, based on records of different reference stations. Eliminate obvious and less obvious ‘outliers’ stations, MY, PG, TZ and FS were chosen as the CRS.

12. Site-change adjustments for BJ temperature records during 1981-1997 Table 2. Seasonal adjustments based on 4 CRS records and comparisons of three- year mean seasonal temperature anomalies before and after the relocation dates. Relocation date SpringSummerAutumnWinterAnnual I January 1981 -0.95-0.69-0.73-0.86-0.81 1 July 1997 -0.73-0.43-0.72-0.87-0.69 Difference 0.220.250.01-0.020.12 The two relocations induced larger biases for winter and spring than for summer and autumn. The absolute values of biases for spring(0.22), summer (0.25) and annual(0.12) for 1981 are larger than for 1997 relocation.

13. A liner trend was used for adjusting the daily data at BJ during 1981-1997, for 4 seasons, respectively: -0.95 + 0.22 * n / N -0.69 + 0.25 * n / N -0.73 + 0.01 * n / N -0.86 – 0.02 * n / N

14. Figure 5. Seasonal mean temperature series at BJ during 1977-2006 (adjusted: solid line; original: dashed line). The annual plot includes for comparison the adjusted series from Li et al (2004) and Yan et al (2001). The effects of site-changes on trends of BJ series

15. Table 3. Linear trends in the adjusted and original annual and seasonal temperature series during 1977-2006 (Unit: ℃ /decade) SpringSummerAutumnWinterAnnual Original0.6850.530.5410.7360.618 Adjusted0.8680.6720.6970.8850.781 Difference0.1830.1420.1560.1490.163 After adjustments, the warming trends in the seasonal and annual series are enhanced. Note that the adjusted series here retain an urban-related trends.

16. Urban-related warming bias at BJ Figure 6. BJ adjusted annual temperature series in 1977-2006, compared with those of 5 rural stations. The likely urban-related warming at BJ is of 0.54 ℃ /decade, the possible upper limit of urban warming bias, about 69% contribution of the overall warming. This result coincided with Ren et al (2007). PG with a similar elevation to BJ presents 0.58 ℃ /decade warming trend, and the firm bottom limit of urban warming bias is 0.20 ℃ /decade. 0.78 ℃ /decade0.24 ℃ /decade0.58 ℃ /decade

17. Table 4. Linear trends ( ℃ /decade) in annual temperature series for different periods. Bracketed values are the differences of trend between BJ and the station for comparison. The numbers by each site name are elevation (m) and population of the site (K). Period Station 1965-20061971-20061981-20061991-2006 BJ, 55m, 1556K0.610.650.840.91 XY, 409m, 10K0.05 (0.56)0.05 (0.60)0.12 (0.72)0.15 (0.76) MS, 286m, 18K0.19 (0.42)0.21 (0.44)0.33 (0.51)0.25 (0.66) FS, 49m, 347K0.34 (0.27)0.37 (0.28)0.50 (0.34)0.72 (0.19) TZ, 27m, 269K0.70 (-0.09)0.81 (-0.16)1.06 (-0.22)1.06 (-0.15) Compared with FS, the urban warming bias at BJ is likely about 0.3 ℃ /decade. This estimate coincides with what Portman (1993) concluded for larger cities in China.

18. 5.Conclusion Based on comparisons between 3-year- mean seasonal temperature anomalies at BJ and the mean of 4 CRS, the site-change-induced biases were between 0.43-0.95. The adjusted annual and seasonal mean series exhibited a slightly enhanced long-term warming trend in 1977-2006. The annual mean UHI effect around the site change in 1981 was 0.12 ℃ larger than that in 1997. Urban-related warming bias was likely about 0.3 ℃ /decade, accounting for about 40% of the overall warming at Beijing for the last 3-4 decades.