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A Comprehensive Evaluation of Surface Air Temperature Reanalyses over China against Urbanization Bias–Adjusted Observations

12 Feb 2021-Advances in Climate Change Research (Elsevier BV)-

Abstract: The reanalysis datasets (CRA40) have now been developed in China Meteorological Administration. We aim to compare the differences in surface air temperature (SAT) between observational that has been adjusted for urbanization bias and reanalysis data (NCEPV1, NCEPV2, ERA5, CFSR, MERRA, JRA55, 20CRV3 and CRA40) over mainland China during 1961–2015. The main results are presented as follows. The correlation and standard deviation between the reanalysis data and observations exhibit highly consistent interannual variability and dispersion, with the interannual SAT variability in JRA55 being closest to the observations for 1961–2015 and that of ERA5 for 1979–2015; the dispersions of 20CRV3 is most consistent with the observations for 1961–2015 and that of NCEPV1 for 1979–2015. Although annual mean SAT of the reanalysis is generally 0–2.0 °C lower than the observations, the bias in the SAT climatology of 20CRV3 is the least for 1961–2015 in all reanalysis datasets and that of CRA40 is the least for 1979–2015. The trends of NCEPV1 is closer to the observations than other reanalysis for 1961–2015 and that of 20CRV3 for 1979–2015. The biases in terms of interannual variability, dispersion, climatology, and linear trend are increase with altitude. Overall, in terms of the similarity of multiple measures to the urbanization bias-adjusted observations, JRA55 and CRA40 show the best performances for the periods 1961–2015 and 1979–2015 respectively in reproducing various aspects of climatological and climate change features in mainland China.

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A Comprehensive Evaluation of Surface Air
Temperature Reanalyses over China against
Urbanization Bias–Adjusted Observations
Siqi Zhang ( )
National Climate Center
Guoyu Ren
China University of Geosciences (CUG)
Yuyu Ren
National Climate Center, China Meteorological Administration (CMA)
Yingxian Zhang
National Climate Center, China Meteorological Administration (CMA)
Xiaoying Xue
China University of Geosciences (CUG),
Original Paper
Keywords: temperature, reanalysis data, urbanization bias, adjusted observations, comparison,
evaluation, China
Posted Date: February 12th, 2021
License: This work is licensed under a Creative Commons Attribution 4.0 International License. 
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A Comprehensive Evaluation of Surface Air Temperature 1
Reanalyses over China against Urbanizationbiasadjusted 2
Observations 3
Siqi Zhang
Guoyu Ren
Yuyu Ren
Yingxian Zhang
Xiaoying Xue
1. Department of Atmospheric Science, School of Environmental Studies, China University of 7
Geosciences (CUG), Wuhan 430074, China 8
2. Laboratory for Climate Studies, National Climate Center, China Meteorological 9
Administration (CMA), China 10
3 Key laboratory for Cloud Physics of China Meteorological Administration, Beijing 100081, 11
China 12
*Corresponding author: Guoyu Ren 13
(Email:; Tel: +86 010-68406408) 14
Submitted to Theoretical and Applied Climatology for possible publication 16

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The goal of this study is to compare the differences in surface air temperature (SAT) between 19
observational and reanalysis data in mainland China from 1961–2015 for evaluating the reliability 20
and applicability of the reanalysis datasets, based on an observational dataset of 763 stations which 21
has been adjusted for urbanization bias, and 8 reanalysis datasets. The time series, anomaly 22
correlations, standard deviations, climate state, and linear trends of the reanalysis data are evaluated 23
against the observations. The reanalysis data are consistent with the observational climate 24
characteristics to a large extent. The correlation and standard deviation ratio between the reanalysis 25
data and observations exhibited highly consistent interannual variability and dispersion, with the 26
interannual SAT variability of JRA55 and ERA5 the closest to the observations for the periods 27
1961–2015 and 1979–2015, and the dispersions of 20CRV3 and NCEPV1 the most consistent with 28
the observations for the two periods. The annual mean SAT of the reanalyses is generally 0–2.0°C 29
lower than the observations, while the linear trends of all datasets exhibited clear warming. The 30
biases in the SAT climatology of 20CRV3 and CRA40 are lower than other reanalysis datasets, and 31
the linear trends of NCEPV1 and 20CRV3 are closer to the observations. With increasing elevation, 32
the biases of the reanalysis data in terms of correlation, standard deviation, climate state, and linear 33
trend all increased. Overall, in terms of the similarity of multiple measures to the urbanization bias–34
adjusted observations, CRA40 and JRA55 show the best performance of the products in 35
reproducing various aspects of climatological and climate change features in mainland China for 36
the period 1979–2015 and 1961–2015 respectively. 37
KEY WORDS: temperature; reanalysis data; urbanization bias; adjusted observations; comparison; 38
evaluation; China 39
Reanalysis datasets have been widely used in climate change and climatological research. Over 42
the past 30 years, the United States, the European Union, and Japan have produced a series of 43
reanalysis datasets. The fourth generation reanalysis datasets have now been developed, which 44
include NCEP–NCAR (Kalnay et al., 1996), NCEP–DOE (Kanamitsu et al., 2002), CFSR (Saha 45
et al., 2010), NASA–MERRA (Rienecker et al., 2011), 20CR (Compo et al., 2011), ERAinterim 46
(Dee et al., 2011), and JRA55 (Kobayashi et al., 2015). 47
The ERA5 and CRA40 were developed by the European Centre for MediumRange Weather 48
Forecasts (ECMWF) (Hersbach et al., 2020) and China Meteorological Administration (CMA) 49
(Liang et al., 2020), respectively, as the new datasets in 2019. In addition, the NOAACIRES50
DOE Twentieth Century Reanalysis (20CR) project has generated an updated fourdimensional 51
global atmospheric dataset spanning the period 1836–2015 to replace the current 20CRV2 and 52
20CRV2C datasets. Since these reanalyses have the disadvantages of incorporating the errors from 53
the numerical prediction models, the assimilation processes, and the observation systems 54
(Bengtsson et al., 2004a, 2004b; Zhao et al., 2010; Zhao et al., 2015), the reliability of the reanalysis 55
data needs to be proven on global and regional scales. 56
Kistler et al. (2001) showed that the surface air temperatures belonging to class B were 57
generally not as reliable as air pressure. Liu et al. (2008) compared NCEPV2 and ERA–40 to buoy 58
observations and found that the reanalysis had warm biases and underestimated the observed inter–59
annual variability of summer surface air temperature from 1979–1999. Lindsay et al. (2014) 60
showed that the third generation models (CFSR, MERRA, and ERA–Interim) stood out as being 61

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more consistent with independent observations for period 1981–2010 in the Arctic region. Fan and 62
Liu (2013) indicated the consistency of SAT climatology in the Southern Hemisphere for the data 63
derived from 20CR and observations from 1979–2010. Zhou et al. (2016) evaluated the SATs in 64
reanalysis data (NCEPV1, NCEPV2, JRA55, ERA–interim, and MERRA) over global deserts, 65
revealing annual mean values ranging from 20.8–24.5°C and long–term trends varying from 0.10–66
0.14°C/decade. 67
With the newly–developed reanalysis datasets, many researchers have examined the reliability 68
and applicability of reanalysis surface air temperatures in China (Su et al., 1999; Xu et al., 2001; 69
Zhao et al., 2006; Gao et al., 2010; Li et al., 2012; He et al., 2013; Zhu et al., 2015). The biases in 70
inter–annual variability, time series, and spatial distribution characteristics between reanalysis and 71
observational data have been analyzed. The results revealed that the application of reanalysis air 72
temperature in eastern China was more consistent with observation than the application in western 73
China (Zhao et al., 2006; Zhu et al., 2015). The climate characteristics displayed by the datasets 74
had a higher agreement with observations after 1979 than before 1979, reflecting the fact that the 75
new generation reanalysis data were generally better than the old products. 76
Many researchers have confirmed the above conclusions and found that the bias of SAT 77
between reanalysis and observation was affected by elevation (Ma et al., 2008; Zhao et al., 2006; 78
Zhou et al., 2018). The biases in the temperature trends were derived jointly from those in 79
atmospheric downward long–wave radiation and precipitation frequency. For the linear trend bias 80
of temperature from 1909–2010, Zhang et al. (2019) analyzed the linear trend of temperature in 81
eastern China based on the homogenized data of 16 observation stations and two sets of 20th 82
century reanalysis data (20CR and ERA20C), and found that, although ERA20C was generally 83
closer to the observational temperatures than 20CR from 1909–2010, their consistency did not 84
indicate that ERA20C and 20CRV2 were suitable for climate change research due to the systematic 85
bias related to the observational data from the stations. 86
The aforementioned studies revealed the uncertainties of reanalysis datasets in different regions 87
and different periods. These may be due to limited assimilation sources, errors from the numerical 88
prediction models, and observation system changes, which produced different results for different 89
periods and regions (Xu et al., 2001; Thorne and Vose, 2010; Parker, 2016; Lahoz and Schneider, 90
2014; Dee et al., 2014; Zhou et al., 2017). In addition, the previous comparisons of SAT data 91
between reanalysis and observation generally considered the inhomogeneity of the reference data, 92
but ignored the bias of the linear trend caused by the urbanization that has occurred in recent 93
decades. Thus, the question as to how to evaluate the potential of reanalysis data for climate change 94
research has not been given sufficient attention, since the effects of urbanization have not been 95
considered. 96
There have been many recent studies demonstrating that the SAT observation records of urban 97
stations and national meteorological station networks contain a significant urbanization bias, 98
regardless of whether homogenization corrections were applied, especially since the middle of the 99
20th century in China (Ren et al., 2007, 2008; Hua et al., 2008; Li et al., 2010; Ren and Ren, 2011; 100
Wang and Ge, 2012; He et al., 2013; Ren and Zhou, 2014; Shi et al., 2015; Li et al., 2016). Due to 101
the differences in research regions, time periods, and data processing methods, and, in particular, 102
to the differences in the reference stations networks that were applied, estimating of the 103
urbanization effect and contribution varied among the different research groups. It was therefore 104
necessary to develop an actual rural (reference) national station network for assessing and adjusting 105

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the current temperature biases in the national historical SAT datasets widely used in studies of 106
climate change. This national station network has been conducted for mainland China over the last 107
decade by the CMA (Ren et al., 2008, 2015; Zhang et al., 2010; Ren and Zhou, 2014; Wen et al., 108
2019). Wen et al. (2019) recently adjusted the monthly 1961–2015 temperature data of 763 national 109
stations in China for urbanization bias based on previous research. Their study concluded that the 110
trend of the original temperature was 0.048—0.049/10year higher than that of the urbanization 111
bias adjusted temperature series. Therefore, the urbanization bias adjustment removed systematic 112
bias in the quality-controlled and homogenized data, which had caused an overestimate of the 113
annual warming rate of more than 19.6%. In addition, the spatial patterns of the annual mean SAT 114
linear trends also exhibit an obvious difference from those of the previous analyses. Wen et al. 115
(2019) concluded that a weak warming area appear in the small part of southwestern North China, 116
the northwestern Central China and the eastern part of Southwest China. The annual mean warming 117
trends in Northeast and North China obviously decreased, which caused a relatively more 118
significant cooling in Northeast China after 1998. This indicates that the influence of urbanization–119
bias on the estimate of temperature trend is not ignorable. This adjusted dataset is thus considered 120
to be the most suitable one for use in monitoring and studying regional climate change in China 121
than the only homogenized temperature data. 122
It remains necessary, therefore, to evaluate climatology and the long–term trends of the 2 m air 123
temperature of the reanalysis data against the new observational data exempt from urbanization 124
bias. In this study, we evaluate the applicability of the reanalysis data by using the urbanization 125
bias–adjusted observational data from 1961–2015 across mainland China. The results of this work 126
can serve as a reference for both developers of reanalysis data and researchers of climate change. 127
The monthly mean surface air temperature (referred to as SAT) observational dataset (referred 130
to as OBS) of 763 national meteorological stations across China for the period 1961–2015 was 131
developed by adjusting for urbanization bias (Wen et al. 2019) after it had been adjusted for 132
inhomogeneities mainly caused by station and instrumentation relocation (Cao et al., 2016). The 133
distribution of the stations is shown in Figure 1. The method of urbanization bias–adjustment 134
consisted of 3 steps: (1) The SAT reference station networks developed in previous studies (Ren et 135
al., 2010; Ren et al., 2015) were applied, and the reference stations chosen for any target stations 136
were stipulated to be within 500 km (Janis et al., 2004; Ren et al., 2012); (2) The weighted averages 137
of monthly mean temperature for all reference stations were calculated using the correlation 138
coefficients of the detrended monthly mean temperature series between the reference stations and 139
the target station as weights; (3) The linear trend bias of temperature between the target station and 140
the reference series was used as the total correction in order to adjust for urbanization bias at each 141
target station (Zhang et al., 2009). Given the lack of high–quality observational data before 1961, 142
we selected 1961–2015 as the research period for comparing the observational and reanalysis 143
temperatures. 144
Figure 1. Distribution of the elevation distribution and 763 observation stations across 146
mainland China. (The insert shows the number of stations at various elevations, the asl stands for 147
elevation (units: m), the red bar stands for the number of the elevation distribution) 148

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