To analyze the differences between winter GST observations in different parts of China during the parallel observation period, the country is divided into eight regions according to the climate division of China proposed by Wang et al. (2004), combined with the geographical division. The divisions and the stations that they contain are shown in Fig. 3. The results show that during the parallel observation period, the national average deviation of the daily GST in winter is 1.18°C (Table 1)—the GST from automatic observations is 1.18°C higher than that from manual observations. The adjustment of observation systems has the greatest impact in Northeast and Northwest China, where the automatically observed daily GSTs is respectively 5.24 or 2.09°C higher than the manually observed one. The daily deviations in North China and the Tibetan Plateau are both below 1.0°C. The regions of central, East, South, and South-west China are less affected by the change of observation methods, with all the deviations below 0.1°C.
Daily GST GST at 0200 BT GST at 0800 BT GST at 1400 BT GST at 2000 BT China 1.18 1.56 1.73 0.11 1.31 Northeast China 5.24 7.17 5.73 0.47 6.51 Northwest China 2.09 2.91 2.96 0.16 2.33 North China 0.76 1.11 1.48 −0.56 1.01 Tibetan Plateau 0.51 0.38 0.61 0.96 0.10 Central China 0.10 0.06 0.28 0.10 −0.03 East China 0.08 −0.01 0.46 −0.05 −0.08 Southwest China 0.10 0.02 0.18 0.21 −0.04 South China −0.05 −0.05 0.23 −0.30 −0.11
Table 1. Deviations in winter ground surface temperature (GST; °C) between automatic and manual observations averaged in China and its eight divisions during the parallel observation period
From comparison of the daily GST observations at the four fixed times (Table. 1), it can be seen that the adjustment of observation systems has a relatively small impact at 1400 BT, but a greater impact at 0200, 0800, and 2000 BT. In addition, the adjustment has a greater impact on the GST in the north regions than those in the south. In terms of national average, the GST deviation at 1400 BT is only 0.11°C, while those at the other three times are all above 1.0°C. The largest deviation is at 0800 BT (1.73°C), followed by 0200 BT (1.56°C). The deviation at 2000 BT is 1.31°C. For regional average, it can be seen that the GST deviation values are relatively small in low and southern most latitudes in the south (central, East, South, and Southwest China), ranging within 0.5°C. Meanwhile, the differences among the deviations at the four times are also small, especially in Southwest China with the smallest differences between −0.04 and 0.21°C, within a range of 0.3°C. There are obvious differences among the GST deviations at the four times in the northern most and high latitudes of northern areas (especially in Northeast and Northwest China), and the deviation range is significantly larger than that in the southern area. In Northeast China, the GST deviation is only 0.47°C at 1400 BT, significantly smaller than those at the other three times that exceed 5.5°C, leading to an overall difference of more than 5°C among these GST deviations. The nighttime GST deviation in Northwest China also reaches approximately 3°C. In summary, the automatic observations of winter GST in Northeast China and Northwest China are significantly higher than those from manual observations.
From the distribution of winter GST deviations (Fig. 4), it can be seen that the deviation range of the daily average GST is between −2.0 and 15.7°C across the whole country. The largest variations lie in Northeast and Northwest China. The GST deviations are between −0.1 and 15.7°C in Northeast China, and a similar distribution (between −0.4 and 14.8°C) is found in Northwest China. The deviations are between −1.0 and 4.7°C in North China, and between −2 and 2°C in other regions. From the fixed four time results, it can be seen that the characteristics of the GST deviations are largely consistent with those of the daily average. The variations in Northeast China are greater at 0200, 0800, and 2000 BT, with an average deviation of more than 5°C, indicating that the automatic GST observations are significantly higher than those from manual observations. Although the regional average deviations at 0200, 0800, and 2000 BT in Northwest and North China are smaller than those in Northeast China, extreme values of the deviations all tend to be positive, suggesting that positive deviations exist at most stations in these areas. At 1400 BT, the average deviation in each region is close to 0. At that time, the absolute value of the average deviation is the largest over the Tibetan Plateau with the value of only about 0.96°C, showing that the data at 1400 BT are the least affected by the adjustment in observation systems.