MULTI-SCALE CHARACTERISTICS STUDY ON THE FREQUENCY OF FOGGY DAYS OCCURRING IN NANJING IN DECEMBER 2007

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1 Vol.21 No.4 JOURNAL OF TROPICAL METEOROLOGY December 2015 Article ID: (2015) MULTI-SCALE CHARACTERISTICS STUDY ON THE FREQUENCY OF FOGGY DAYS OCCURRING IN NANJING IN DECEMBER 2007 LIU Peng ( ) 1, YU Hua-ying ( ) 2, NIU Sheng-jie ( ) 2 (1. Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing China; 2. Jiangsu Key Laboratory of Atmospheric Environment Monitoring & Pollution Control, Nanjing University of Information Science and Technology, Nanjing China) Abstract: Based on the number of foggy days in Nanjing in December from 1980 to 2011, we analyzed the surface temperature and atmospheric circulation characteristics of foggy years and less-foggy years. Positive anomalies of the Arctic Oscillation (AO) were found to weaken the East Asian trough, which is not conducive to the southward migration of cold air. Simultaneously, this atmospheric condition favors stability as a result of a high-pressure anomaly from the middle Yangtze River Delta region. A portion of La Ni 觡 a events increases the amount of water vapor in the South China Sea region, so this phenomenon could provide the water vapor condition required for foggy days in Nanjing. Based on the data in December 2007, which contained the greatest number of foggy days for the years studied, the source of fog vapor in Nanjing was primarily from southern China and southwest Taiwan Island based on a synoptic scale study. The water vapor in southern China and in the southwestern flow increased, and after a period of 2-3 days, the humidity in Nanjing increased. Simultaneously, the water vapor from the southwestern of Taiwan Island was directly transported to Nanjing by the southerly wind. Therefore, these two areas are the most important sources of water vapor that results in heavy fog in Nanjing. Using the bivariate Empirical Orthogonal Function (EOF) mode on the surface temperature and precipitable water vapor, the first mode was found to reflect the seasonal variation from early winter to late winter, which reduced the surface temperature on a large scale. The second mode was found to reflect a large-scale, northward, warm and humid airflow that was accompanied by the enhancement of the subtropical high, particularly between December 15-21, which is primarily responsible for the consecutive foggy days in Nanjing. Key words: foggy days; frequency; multi-scale characteristics; precipitable water vapor; surface temperature CLC number: P426.4 Document code: A doi: /j INTRODUCTION The international definition of fog is as follows: a collection of suspended water droplets or ice crystals near the earth s surface that leads to a reduction of horizontal visibility below 1 km (Wu et al. [1] ; Yang et al. [2] ). Fog is frequently blamed for traffic disasters, poor air quality and poor visibility weather and has been extensively studied for more than a century (Niu et al. [3] ; Gultepe et al. [4] ). Water droplets and ice crystals, typically 2 to 50 μm in diameter, form as a result of super-saturation generated by cooling, moistening and/or mixing of near-surface air parcels of contrasting temperatures. The presence of suspended droplets and/or crystals can render an object undistinguishable to a distant observer, Received ; Revised ; Accepted Foundation item: China Meteorological Special Program (GY- HY ); National Nature Science Foundation of China ( , , ); Qing-Lan Project of Jiangsu Province; Natural Science Foundation of Jiangsu Province (SBK ). Biography: LIU Peng, Associate Professor, Ph.D., primarily undertaking research on climate change. Corresponding author: LIU Peng, liupeng1998@nuist. edu.cn causing poor visibility conditions. The local climate and weather are important factors influencing fog events, although fog also has regional characteristics. The fog distribution in China is extremely irregular; based on the average number of foggy days, there is a greater fog distribution in the southeast than in the northwest (Wu et al. [1] ; Niu et al. [3] ; Wu et al. [5] ). The average number of foggy days in the coastal region of Jiangsu and Zhejiang provinces is greater than 60 d (Liu et al. [6] ), whereas that of Nanjing is relatively high (at d). Wang et al. [7, 8] divided the foggiest areas of China into six main divisions and noted that fog mostly occurs during the fall and winter in these regions. Other scholars have conducted statistical analyses of the regional variation of heavy fog and proposed a possible mechanism (Shen et al. [9] ; Niu et al. [10] ; Tardif and Rasmussen [11] ; Tong et al. [12] ; Wu et al. [13] ; Yu and Sun [14] ; Sun et al. [15] ). However, the complexity of the fog process lies in multiple time and space scale factors, such as different scales of the dynamics, droplet microphysics, aerosol chemistry, radiation, turbulent mixing and surface conditions. Over the years, numerous observations have been made of the different regions of fog, and the subject of these case studies has ranged from the physical structure of fog (Niu et al. [3] ;

2 No.4 LIU Peng ( ), YU Hua-ying ( ), et al. 429 Li [16] ) to macro- and micro-structures and physical mechanisms of local fog; in addition, numerical simulations have been performed (Lu et al. [17] ; Liu et al. [18] ; He et al. [19] ; Li et al. [20] ; Niu et al. [21] ; Liu [22] ). However, because of the lack of long sequences of high-resolution fog observations, the study of large-scale circulation fog has not been fully discussed. Fog is a phenomenon that occurs over a long spatiotemporal span in the near-surface atmosphere. The frequency of foggy days is determined by the water vapor condition, which is affected by circulation and the mechanism of vapor condensation [1, 2]. Since 1980, the greatest number of foggy days in Nanjing has occurred in December 2007, for a total of 8 d; in contrast, the number of foggy days in December 2008 was only 1 d. In these two consecutive years, there were only slight variations in pollution, topography and other non-meteorological factors, so the occurrence of fog was more related to the meteorological and climatic conditions (Wu et al. [1] ; Yang et al. [2] ; Wu et al. [23] ). Therefore, this article will discuss the causes of the foggy December in 2007 in Nanjing using three time scales: inter-annual, seasonal and daily variations. These scales provide a reference to the study of climate change and short-term climate prediction of fog. 2 DATA AND METHODS Monthly reports from the meteorological weather station of the Pukou District in Nanjing were used to analyze the daily lowest visibility, average daily temperature, daily total precipitation, daily relative humidity at 08:00 and average daily wind speed. Simultaneously, the U.S. National Center for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) provided monthly grid data ( ) for the Decembers between 1980 and 2009, a total of 30 years, and daily reanalysis data (1 1 ) for the entire month of December 2007, all of which were used in the analysis. The December monthly average Arctic Oscillation (AO) index and ENSO indices for the period from 1980 to 2009 were acquired from NOAA. Initially, the climate characteristics of more-foggy years and less-foggy years in Nanjing were compared based on the method of composite analysis. By analyzing the correlation between the meteorological elements and AO and ENSO indices, we discussed the relationship between the number of December foggy days in Nanjing and the AO and ENSO. In addition, with a focus on December 2007, which provided the foggiest days over the last 30 years, an analysis of the influencing factors of fog formation in Nanjing was performed based on the climate and weather circulation background field. The duration of general fog is typically several hours. During the fog development process, the relative humidity and visibility are inversely correlated [1, 2]. In the weather scale analysis, a bivariate EOF using the surface temperature and precipitable water vapor in East Asia was applied. To illustrate the influence of synoptic-scale circulation on foggy days in Nanjing, we discuss the impact of large-scale atmospheric circulation on the relative humidity in the Nanjing area according to the bivariate EOF analysis. 3 ANALYSIS OF CLIMATE BACKGROUND RELATED TO THE OCCURRENCE OF FOG- GY DAYS IN NANJING Based on the number of foggy days from 1980 to 2011, which were obtained from the Jiangsu Province Climate Center, the distribution of foggy days in each December was analyzed. Generally, the foggy day trend was not obvious. The number of foggy days in December 2007 was the greatest over the last 32 years (8 days, Fig.1). In this work, a year is defined as foggy when the number of foggy days in December in the Nanjing area is equal to or greater than 6 d (in a total of 7 years), whereas a year is defined as less foggy if the number of foggy days in December in the Nanjing area is equal to or less than 1 d (in a total of 10 years). Based on a comparison between the climate characteristics of a foggy year and a less-foggy year, the impact of AO and ENSO on the East Asian climate conditions was studied using the December 2007 data. Thus, using data from the foggiest days in Nanjing, an attempt was made to determine the relationship between the occurrence of foggy days in Nanjing and the climate background. Figure 1. Anomalous foggy days in December in Nanjing. A composite method was used to create a surface temperature map for a foggy year and a less-foggy year (Fig.2). A foggy year in Nanjing tends to exhibit a warmer surface temperature (Fig.2a), whereas a less-foggy year in Nanjing tends to exhibit a cooler surface temperature (Fig.2b). The reduction of surface temperature induces the formation of fog, which reflects a small time scale, whereas an increase in surface temperature on a large scale increases the moisture in the Nanjing area, which provides appropriate water vapor conditions for ground fog. Fig.2c and 2d show the relationship between the AO and La Ni 觡 a (negative ENSO) indices and surface temperature in December. It can be

3 430 Journal of Tropical Meteorology Vol.21 Figure 2. The surface land temperature composition of foggy years (a) and less-foggy years (b), with deep shaded area >-1 and shallow <-1. The correlation coefficients between the AO index and surface land temperature (c) and the correlation coefficients between the negative ENSO index and surface land temperatures (d), with shaded areas denoting the 90% confidence level. observed that the positive phase of AO results in abnormally high surface temperatures in the Nanjing area, whereas the La Ni 觡 a years do not provide this condition. In terms of the precipitable water vapor of a foggy year and a less-foggy year in December in the Nanjing area (such as Fig.3a and 3b), the precipitable water vapor from southwest China to east Japan in a foggy year is a positive anomaly that ensures adequate moisture and provides a good condition for foggy days, whereas in less-foggy years, the precipitable water vapor in East China is a negative anomaly. The AO positive phase warms northern China (Fig.2c) and provides an adequate water vapor condition (Fig.3c), whereas in La Ni 觡 a years, the low temperature of the Nanjing area causes the amount of water vapor to decrease in these areas. However, the large amount of water vapor near the South China Sea (SCS) may be transported to help form fog in Nanjing. The occurrence of fog requires proper surface temperatures, adequate water vapor and a more stable atmospheric structure. Fig.4a and 4b show a 500-hPa map for a foggy year and a less-foggy year in December, respectively. During the foggy years, the height field of Nanjing is a positive anomaly, with the positive abnormal AO increasing the positive anomaly height field. In less-foggy years, the height field of Nanjing is a negative anomaly; La Ni 觡 a lowers the 500-hPa height field, which is not conducive to a stable atmospheric structure (Fig.4d). The positive anomaly AO is conducive to foggy days in Nanjing; however, the La Ni 觡 a event does not provide the correct conditions of temperature and atmospheric stability for foggy days. In this article, December 2007, which had the greatest number of foggy days in the last 30 years, was used as an example for a climate circulation background. Fig.5a shows the climate anomaly of precipitable water vapor of December 2007 (subtracting the climate state from 1980 to 2009). The Yangtze River Delta (YRD) region contained positive regional precipitable water vapor anomalies, and the western Pacific (east of 120 E) and SCS region ( E) had clear abnormalities in the vapor flux transport, which is the primary source of an abnormal increase in water vapor in the YRD. Fig.5b shows that the surface temperature of December 2007 in the Nanjing area was less than 5 C, which was less than that of the eastern region (Shanghai). Regarding the temperature anomalies, the temperature north of 45 N was higher in 2007 than in other years, and this situation was the same for the south of China and the YRD area. The foggy weather process depends much on the temperature gradient, or the temperature difference between the foggy area and surrounding area. Even if the

4 No.4 LIU Peng ( ), YU Hua-ying ( ), et al. 431 Figure 3. Nanjing area during a foggy year (a) and a less-foggy year (b) showing the composite of precipitable water vapor in December (units: dark color represents amounts more than 2 mm and light color represents amounts less than 2 mm); AO index (c) and negative ENSO (d) associated with the precipitable water vapor map, with shaded areas denoting the 90% confidence level. Figure 4. The 500-hPa geopotential height composition in foggy years (a) and less-foggy years (b); the correlation coefficients between the AO index and 500-hPa geopotential height (unit: gpm) (c); the correlation coefficients between the negative ENSO index and 500-hPa geopotential height (d). Shaded areas denote the 90% confidence level.

5 432 Journal of Tropical Meteorology Vol.21 climate state of the Nanjing area is warm, the lower temperatures in Nanjing, compared to the temperatures on its eastern side, causes the east wind from a low height (below 850 hpa) to bring warm and moist air to Nanjing (Fig.5a), which is conducive to forming fog. Fig.6 shows a vertical section map of the streamlines and temperatures along 32 N and along E in December 2007, which indicates that there was a cyclonic circulation at the southwest side of Nanjing (110 E, 30 N). The height of the center of this cyclone was approximately 850 hpa, and a downdraft was in the upper and middle atmosphere in Nanjing. The lower atmosphere of Nanjing is cooler than that in the area on the same latitude on the eastern side, which is conducive to fog formation. The accompanying inversion occurs when fog is present at a height range of m from the ground [18], where the vertical range is small; the fog is present for short durations, which indicates that the inversion is not present in the climate data. Figure 5c presents the positive anomaly of 500 hpa in the areas north of 45 N; this anomaly weakens the East Asian trough and is not conducive to the cold air flowing south. The positive anomaly also appears in the south of China and YRD, a location less conducive to an ascending atmosphere, so it provides a more stable atmospheric structure for developing fog [14]. In the region west of 110 E, 35 N, the height field is a negative anomaly. In conjunction with Fig.5d, it was found that consistent cyclonic anomalies of pressure occur from sea level to 500 hpa, which indicates a strong southwest vortex that is able to transport additional water vapor to China by the southwesterly flow. The average sea level pressure in December 2007 in the Nanjing area was 1024 hpa, which is less than that in other years and the entire region of the south of China. This sea level pressure creates underlying cyclonic circulation anomalies and provides proper conditions for moisture conver gence in foggy days in Nanjing. Figure 4c shows the AO at 500 hpa, which indi cates that the relatively large values of the area are primarily in the high latitude regions; the AO and WPSH (West Pacific subtropical high) also have a good positive correlation. In December 2007, the AO was a strong positive anomaly (figure not shown) that enhanced the blocking high and weakened the East Asian trough, which is not conducive to cold air flowing southward. The enhanced subtropical high favors the transportation of warm air into East China from the Pacific. Fig.2c shows the correlation of AO to surface temperature, which indicates that the AO is well corre- Figure 5. (a) Anomalies of vertically integrated moisture (deep shaded area >0.7 mm and shallow shaded area <-0.7 mm) and vectors are integrated moisture transport from the surface to 300 hpa (kg m -1 s -1 ); (b) Surface land temperature (units:, solid) and its anomalies (deep shaded area >1.2 and shallow <-1.2 ); (c) the 500-hPa geopotential height (units: gpm, solid) and its anomalies (deep shaded area >2 gpm and shallow <-2 gpm); (d) sea level pressure (units: hpa, solid) and its anomalies (deep shaded area >3 hpa and shallow <-3 hpa).

6 No.4 LIU Peng ( ), YU Hua-ying ( ), et al. 433 Figure 6. Vertical cross section of the streamline field (vector) and temperature anomalies (unit:, deep shaded area >7 and shallow 2-7 ) at 32 N (a) and E (b). Figure 7. Dececmber 2007 in Nanjing. (a) The precipitation (column, unit: mm), land air temperature (dotted line, ) and visibility (solid line, km). (b) Surface wind (dotted line, unit: m/s) and 08:00 relative humidity (unit: %). lated with high-latitude ground temperature, resulting in a warm winter when the AO is strong. The increased temperature in December 2007 for most of China may be a result of the impact of the AO (Fig.2c). In the winter of 2007/2008, a La Ni 觡 a event occurred; during this event, the surface temperature of the eastern equatorial Pacific was relatively cold. In conjunction with Fig.2d and 4d, La Ni 觡 a years tend to weaken the 500-hPa subtropical high, but the surface temperatures of the south of China and East China remain relatively cold. However, in December 2007, the subtropical high was relatively strong because the ground temperature was relatively warm, which resulted from the positive AO. The primary reason may have been a delayed impact of ENSO s ocean signals, so the impact of the La Ni 觡 a event was more prominent in Jan uary 2008 (Yang and Li [24] ; Zhang et al. [25] ). Because the AO is an atmospheric signal that essentially occurs simultaneously with local atmospheric changes, the cli mate anomalies of East China in December 2007 were primarily affected by the AO with a strong subtropical high and warm ground temperatures. 4 FOGGY DAYS IN DECEMBER 2007 AND DISTRIBUTION OF METEOROLOGICAL EL- EMENTS To better illustrate the synoptic scale characteristics of the frequency of foggy days in Nanjing, the daily meteorological elements and foggy days in December 2007 were analyzed. According to the weather station of Pukou (at an observation altitude of 8.9 m above sea level), the average temperature in December 2007 was 6.2, which was 1.5 higher than in other years; the amount of precipitation was 42.4 mm, which was 18.5 mm greater than in other years; and the total number of sunshine hours was 65.9 h, which was 88.2 less than in other years and broke the record for the least amount sunshine for December. Based on the data provided by the Pukou meteorological monthly reports, which included the daily temperature, daily wind speed, 24-h precipitation and visibility, the lowest visibility was selected for analysis at 08:00, 14:00 and 20:00 (BST, same below); the lowest

7 434 Journal of Tropical Meteorology Vol.21 foggy visibility was recorded at night except for December 18, when the lowest visibility appeared at 08:00. Therefore, only the relative humidity at 08:00 was listed (Fig.7b). As shown in Fig.7a, the foggy days were on December 3, 15, 17, 18, 19, 20, 21 and 27, with December 2 having a small amount of precipitation with cooling. The relative humidity of December 3 reached 94% at 08:00, and the visibility was 0.2 km. On December 4, the wind speed increased and the fog disappeared. December 12 had significant precipitation, and the decrease in temperature on December 13 and 14 resulted in heavy fog on Day 15, where the minimum visibility was only 10 m. December 16 had a small amount of precipitation, which was accompanied by an increase in temperature and a wind speed greater than 3 m/s, resulting in a brief rebound in visibility. December 16 and 17 had a small amount of precipitation. From Day 17 to Day 20, the temperature continued to decrease (T Day 20 - T Day 17 = -3.1 ), which resulted in five consecutive foggy days from Day 17 to Day 21. Precipitation occurred on December 21 and 22; however, because the temperature increased to 8.4, the visibility improved. The fog on Day 27 also occurred with a decrease in temperature after precipitation occurred. On December 7, 11, 14 and 29, fog occurred because of the decrease in temperature after precipitation, and the visibilities were all approximately 2 km. In conjunction with Fig.7b, all of the foggy days had a ground wind speed less than 3 m/s and a relative humidity greater than 93%. In summary, when there was a small amount of precipitation, fog formed because of a decrease in temperature. Precipitation is deeply affected by atmospheric circulation, including the horizontoal and vertical water vapor transport, and decreases in temperature (cooling down) are affected by multiple factors [1]. In the following sections, the effects of atmospheric circulation on foggy days in Nanjing will be discussed. 5 CHARACTERISTIC OF SYNOPTIC SCALE VAPOR TRANSPORTATION IN DECEMBER 2007 The relative humidity in the lower atmosphere is a direct physical parameter related to changes in fog, and changes in relative humidity are also a result of the interaction between water vapor at the boundary layer and the land surface temperature. Large-scale cold and warm air activity have an important impact on the water vapor of the boundary layer and land surface temperature. Therefore, we discuss the precipitable water vapor and surface temperature as two physical parameters of fog that underlie the water vapor conditions and surface thermodynamic conditions. Based on the evolution of synoptic scale atmospheric circulation, the characteristics of water vapor transportation in Nanjing were analyzed. The correlation between the relative humidity measured every 6 hours (rh2m) in Nanjing, vertically integrated moisture and 850-hPa wind field was determined (Fig.8a), and it showed that there was a good correlation between the water vapor of the entire region of the south of China and YRD area and the relative humidity of Nanjing. This result indicates that changes in water vapor during fog formation in Nanjing may be affected by the south of China and the YRD. Compared to the Figure 8. (a) The correlation coefficient between relative humidity at 2 m from the surface and vertically integrated moisture (shaded) and wind (vector) in December 2007, (b) the maximum correlation coefficient between the relative humidity at 2 m from the surface and advancing vertically integrated moisture and (c) the advancing day of maximum correlation coefficient.

8 No.4 LIU Peng ( ), YU Hua-ying ( ), et al. 435 correlation with the 850-hPa wind field, the southwest airflow and southeast airflow from the northwest Pacific were the primary sources of water vapor for fog formation in Nanjing. Further research is required to determine how far in advance these areas affect fog formation in Nanjing and how much water vapor is provided. Fig.8b shows the 0~3 day leading maximum correlation coefficient, indicating that the correlation coefficient is largest for the south of China and YRD, which suggests that these areas provide water vapor for Nanjing fog. Fig.8c shows the advancing time according to the maximum correlation coefficient, which indicates that the changes in Nanjing and surrounding areas occur simultaneously and that there is a positive correlation with southern China along 25 N, which is shown in Fig.8b. This result indicates that the increase in water vapor in these areas results from the southwest airflow, which is consistent with the case study by Liu et al. [18]. In addition, the formation of fog is directly related to the underlying surface. Fig.9 shows the 6-hour NCEP data for the bivariate EOF using the surface temperature and precipitable water vapor. The results show that the first mode has a variance of 19% and second mode has a variance of 13%, which indicates that the amount of precipitable water vapor in the YRD and SCS is significant (Fig.9a). The temperature in all of East China is positive (Fig.9b), and as shown in Fig.9c, temperature Figure 9. The EOF analysis of the two vertically integrated variables, moisture and surface land temperature, in Nanjing during December The first mode of vertically integrated moisture (a), surface land temperature (b) and temporal coefficients(c); and the second mode of vertically integrated moisture (d), surface land temperature (e) and temporal coefficients (f).

9 436 Journal of Tropical Meteorology Vol.21 has the greatest positive value and decreases to the largest negative value for December 1 to 31, which indicates a huge seasonal change. Combined with evolution over time and the space-phase mode, the water vapor content in early December in the YRD has increased and the surface temperature is higher. Accompanied with a time-varying low-level wind divergence (Fig.10a), as the water vapor of the YRD gradually decreases, the surface temperature decreases and reaches its smallest value in late December, which is the evolution process over time from early winter to late winter. This result explains why there is a huge amount of water vapor in early December in the YRD; this vapor tends to cause the Nanjing fog because of the decreasing temperature. Five days later, the YRD, in the first mode, turns dry and is not conducive to forming fog; on Day 27, this mode significantly decreases the temperature, and Nanjing becomes foggy again. The space distribution of the second mode is shown in Fig.9d, which indicates that the precipitable water vapor in the south of China contains a positive region for water vapor and is greater than the first mode because Taiwan Island is a large negative value area to the south. As shown in Fig. 9e, there is a positive temperature area south of 40 N in China. The time factor of the second mode in Fig.9f was positive after December 10, and a significant peak was found between December 15 and 20. Combined with the spatial distribution, it was found that large-scale warm moist air activity occurred after December 10, and it caused precipitation on December 10 and 12 in Nanjing. As the warm moist air activities became more significant between December 15 and 21, the increasing surface temperature in the YRD and the south of China caused convergence of a low-level wind field, increase in humidity and enhancement of the subtropical high (Fig.10b). During the strongest period of warm moist air activities on December 17, the precipitable water vapor center in the second mode located in the south of China became the maximum area of water vapor, which affected the relative humidity in Nanjing 2 days later (Fig.7c), with Nanjing experiencing low visibility on December 19. After December 21, the warm moist air activities essentially ended, and the consecutive foggy days ended. Figure 10. Linear regression coefficient of 850-hPa divergence (units: deep shaded area <-10-6 s -1 and light shaded area >10-6 s -1 ) and 500-hPa height (contour line, units: gpm) against the time series of EOF1 (a) and EOF2 (b). The spatial and temporal variations of the corresponding mode can be obtained by the multiplication of the EOF space mode and the time coefficient. For a single point in Nanjing, the evolution of the corresponding variables to each mode and values of each variable in this mode at a certain period can be calculated. Table 1 shows the average results from December 15 to 21, from which the December monthly mean was subtracted. It can be observed that the EOF1 mode decreased the Nanjing precipitable water vapor by 0.18 mm, whereas the surface temperature decreased by 0.18 K. The EOF2 mode increased the Nanjing precipitable water vapor by 0.8 mm, whereas the surface temperature increased by 0.06 K. The combination of these two modes increased the Nanjing water vapor by 0.62 mm, which is comparable to the 0.58 mm anomaly of the actual precipitable water vapor. In fact, the surface temperature decreased by 0.4 K, which is greater than the decrease by 0.12 K of the surface temperatures by the combination of these 2 modes and indicates that the primary reason for changes in the surface temperature during this period in Nanjing was the contribution of the other EOF mode. In summary, the increase of water va- Table 1. Average of December with the December mean subtracted. Physical quantities FNL data EOF1 EOF2 EOF1+EOF2 Precipitable water vapor/mm Surface temp./

10 No.4 LIU Peng ( ), YU Hua-ying ( ), et al. 437 por by EOF2 was the primary reason for consecutive foggy days in Nanjing. 6 CONCLUSIONS (1) During foggy years in Nanjing, the AO is mostly a positive anomaly and the East Asian trough weakens, which is not conductive to the southward migration of cold air. However, the atmosphere is conductive to stability because of the high-pressure anomaly of the middle YRD. In addition, the La Ni 觡 a event increases the water vapor in the SCS region, which could provide the water vapor conditions. All of these conditions result in foggy days in Nanjing. In December of 2007, the presence of cyclonic circulation in southwest China provided large amounts of water vapor southwest of the YRD. Simultaneously, the AO positive anomaly also provided an important climate background for consecutive foggy days in Nanjing. (2) In terms of the synoptic scale, the water vapor source for the fog in Nanjing in December of 2007 primarily came from the south of China and southwest Taiwan Island. Two days earlier, the water vapor increased in the south of China, and along with the southwesterly flow, the humidity increased in the Nanjing area. The water vapor from southwest Taiwan Island was directly transported to the Nanjing area via the southerly wind. These two regions are the most important sources of water vapor for the fog in Nanjing. (3) Using surface temperature and precipitable water vapor datasets, we conducted a bivariate EOF analysis. The first mode reflects the seasonal variation from early winter to late winter, which is favorable to a large-scale decrease in surface temperature, whereas the second mode reflects a large-scale, northward, warm moist air flow, which is accompanied by the enhancement of the subtropical high. 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