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- Author or Editor: Kaicun Wang x
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Abstract
Since the 1950s, precipitation has been measured at national weather stations in China using national standard precipitation gauges. Gauges without wind fence can significantly underestimate precipitation amounts, while this undercatch bias is closely related to surface wind speed and precipitation type. The observed surface wind speed across China has substantially declined during the past decades. Therefore, this study investigated the wind-induced error of the observed precipitation and its impact on regional and national mean trends in precipitation over China due to the reduction in surface wind speed. It was found that the wind-induced error for the mean annual precipitation nationwide was 29.28 mm/year, accounting for 3.92% of total precipitation amount. The variation of precipitation at the regional scale was large but the trends were both positive and negative, approximately cancelling at the national level and resulting in a small national mean trend. The raw observation data showed that the national mean precipitation increased at a rate of 1.85 mm/year/10a from 1960 to 2018, which was reduced to 0.33 mm/year/10a after correction, demonstrating that the correction of wind-induced error had an important impact on the trend of annual precipitation. Meanwhile, the reduction of surface wind speed was consistent at both the regional and national levels. On average, the wind-induced errors decreased at rates of -1.52 mm/year/10a, -1.34 mm/year/10a, and -0.14 mm/year/10a for total precipitation, rainfall , and snowfall, respectively. It illustrates that the decreases of the wind-induced error result in the increasing precipitation of raw observation.
Abstract
Since the 1950s, precipitation has been measured at national weather stations in China using national standard precipitation gauges. Gauges without wind fence can significantly underestimate precipitation amounts, while this undercatch bias is closely related to surface wind speed and precipitation type. The observed surface wind speed across China has substantially declined during the past decades. Therefore, this study investigated the wind-induced error of the observed precipitation and its impact on regional and national mean trends in precipitation over China due to the reduction in surface wind speed. It was found that the wind-induced error for the mean annual precipitation nationwide was 29.28 mm/year, accounting for 3.92% of total precipitation amount. The variation of precipitation at the regional scale was large but the trends were both positive and negative, approximately cancelling at the national level and resulting in a small national mean trend. The raw observation data showed that the national mean precipitation increased at a rate of 1.85 mm/year/10a from 1960 to 2018, which was reduced to 0.33 mm/year/10a after correction, demonstrating that the correction of wind-induced error had an important impact on the trend of annual precipitation. Meanwhile, the reduction of surface wind speed was consistent at both the regional and national levels. On average, the wind-induced errors decreased at rates of -1.52 mm/year/10a, -1.34 mm/year/10a, and -0.14 mm/year/10a for total precipitation, rainfall , and snowfall, respectively. It illustrates that the decreases of the wind-induced error result in the increasing precipitation of raw observation.
Abstract
The recent severe and frequent PM2.5 (i.e., fine particles smaller than 2.5 µm) pollution in China has aroused unprecedented public concern. The first two years of PM2.5 measurements in China are reported and compared with those of Europe and the United States. The average PM2.5 concentration in China is approximately 5 times that over Europe and America. The contribution of atmospheric dispersion to such air quality is evaluated in this study. Air stagnation or its absence is a good indicator of the atmosphere’s capability to disperse its pollutants, but the NOAA definition of an air stagnation event is found to not be applicable to China since it depends on vertical mixing that is weakened in China by the effects of terrain. To address this deficiency, a new threshold for air stagnation events is proposed that depends on the 10-m wind speed, boundary layer height, and occurrence of precipitation. This newly defined air stagnation closely tracks the day-to-day variation of PM2.5 concentrations. Such events are more frequent over China than over Europe and the United States during autumn and winter, especially over the Sichuan basin and Jing-Jin-Ji region of China. If China had the same frequency of air stagnation as the United States or Europe, 67% and 82% of its stations would improve their current air quality during autumn and winter (e.g., an average of 12% decrease in PM2.5 concentrations for the Jing-Jin-Ji region in wintertime). Its severe pollution and frequent air stagnation conditions make controls on emission less effective in China than elsewhere.
Abstract
The recent severe and frequent PM2.5 (i.e., fine particles smaller than 2.5 µm) pollution in China has aroused unprecedented public concern. The first two years of PM2.5 measurements in China are reported and compared with those of Europe and the United States. The average PM2.5 concentration in China is approximately 5 times that over Europe and America. The contribution of atmospheric dispersion to such air quality is evaluated in this study. Air stagnation or its absence is a good indicator of the atmosphere’s capability to disperse its pollutants, but the NOAA definition of an air stagnation event is found to not be applicable to China since it depends on vertical mixing that is weakened in China by the effects of terrain. To address this deficiency, a new threshold for air stagnation events is proposed that depends on the 10-m wind speed, boundary layer height, and occurrence of precipitation. This newly defined air stagnation closely tracks the day-to-day variation of PM2.5 concentrations. Such events are more frequent over China than over Europe and the United States during autumn and winter, especially over the Sichuan basin and Jing-Jin-Ji region of China. If China had the same frequency of air stagnation as the United States or Europe, 67% and 82% of its stations would improve their current air quality during autumn and winter (e.g., an average of 12% decrease in PM2.5 concentrations for the Jing-Jin-Ji region in wintertime). Its severe pollution and frequent air stagnation conditions make controls on emission less effective in China than elsewhere.
Abstract
With urbanization occurring around weather stations, its impact on the observed air temperature has been widely recognized. However, its assessments were varied partially due to the poor understanding of its underlying mechanism. Here, we analyzed the effect of urbanization using observations obtained from ~2200 weather stations in China from 1960 to 2014. The results showed that the urbanization effect increased from 1960 to 1984 but slowed after 1995 with rapid urbanization in China, particularly in terms of the daily minimum temperature T min and daily mean air temperature T mean. The urbanization effect is nearly linearly related to the urban–rural contrast of effective cloud cover (including the impact of atmospheric aerosols) derived from the observed sunshine duration. Aerosols increase atmospheric downward longwave radiation L d through their absorption of solar radiation during the daytime, and they trap longwave radiation emitted from the surface during the nighttime. Increased anthropogenic aerosols caused the urban–rural contrast of effective cloud cover to increase from 1960 to 2014. However, the urban–rural contrast of cloud cover remained stable from 1960 to 1984 but substantially decreased due to the “urban dry island effect” after 1995, which compensated for the impact of anthropogenic aerosols on L d and resulted in a stable urbanization effect after 1995. The urban–rural difference in L d increased by 0.57 W m−2 (10 yr)−1, which resulted in warming of 0.074°C (10 yr)−1 (78.2%) for T min, 0.037°C (10 yr)−1 (151.5%) for T max, and 0.056°C (10 yr)−1 (96.6%) for T mean.
Abstract
With urbanization occurring around weather stations, its impact on the observed air temperature has been widely recognized. However, its assessments were varied partially due to the poor understanding of its underlying mechanism. Here, we analyzed the effect of urbanization using observations obtained from ~2200 weather stations in China from 1960 to 2014. The results showed that the urbanization effect increased from 1960 to 1984 but slowed after 1995 with rapid urbanization in China, particularly in terms of the daily minimum temperature T min and daily mean air temperature T mean. The urbanization effect is nearly linearly related to the urban–rural contrast of effective cloud cover (including the impact of atmospheric aerosols) derived from the observed sunshine duration. Aerosols increase atmospheric downward longwave radiation L d through their absorption of solar radiation during the daytime, and they trap longwave radiation emitted from the surface during the nighttime. Increased anthropogenic aerosols caused the urban–rural contrast of effective cloud cover to increase from 1960 to 2014. However, the urban–rural contrast of cloud cover remained stable from 1960 to 1984 but substantially decreased due to the “urban dry island effect” after 1995, which compensated for the impact of anthropogenic aerosols on L d and resulted in a stable urbanization effect after 1995. The urban–rural difference in L d increased by 0.57 W m−2 (10 yr)−1, which resulted in warming of 0.074°C (10 yr)−1 (78.2%) for T min, 0.037°C (10 yr)−1 (151.5%) for T max, and 0.056°C (10 yr)−1 (96.6%) for T mean.
Abstract
Surface incident solar radiation G determines our climate and environment, and has been widely observed with a single pyranometer since the late 1950s. Such observations have suggested a widespread decrease between the 1950s and 1980s (global dimming), that is, at a rate of −3.5 W m−2 decade−1 (or −2% decade−1) from 1960 to 1990. Since the early 1990s, the diffuse and direct components of G have been measured independently, and a more accurate G has been calculated by summing these two measurements. Data from this summation method suggest that G increased at a rate of 6.6 W m−2 decade−1 (3.6% decade−1) from 1992 to 2002 (brightening) at selected sites. The brightening rates from these studies were also higher than those from a single pyranometer. In this paper, the authors used 17 years (1995–2011) of parallel measurements by the two methods from nearly 50 stations to test whether these two measurement methods of G provide similar long-term trends. The results show that although measurements of G by the two methods agree very well on a monthly time scale, the long-term trend from 1995 to 2011 determined by the single pyranometer is 2–4 W m−2 decade−1 less than that from the summation method. This difference of trends in the observed G is statistically significant. The dependence of trends of G on measurement methods uncovered here has an important implication for the widely reported global dimming and brightening based on datasets collected by different measurement methods; that is, the dimming might have been less if measured with current summation methods.
Abstract
Surface incident solar radiation G determines our climate and environment, and has been widely observed with a single pyranometer since the late 1950s. Such observations have suggested a widespread decrease between the 1950s and 1980s (global dimming), that is, at a rate of −3.5 W m−2 decade−1 (or −2% decade−1) from 1960 to 1990. Since the early 1990s, the diffuse and direct components of G have been measured independently, and a more accurate G has been calculated by summing these two measurements. Data from this summation method suggest that G increased at a rate of 6.6 W m−2 decade−1 (3.6% decade−1) from 1992 to 2002 (brightening) at selected sites. The brightening rates from these studies were also higher than those from a single pyranometer. In this paper, the authors used 17 years (1995–2011) of parallel measurements by the two methods from nearly 50 stations to test whether these two measurement methods of G provide similar long-term trends. The results show that although measurements of G by the two methods agree very well on a monthly time scale, the long-term trend from 1995 to 2011 determined by the single pyranometer is 2–4 W m−2 decade−1 less than that from the summation method. This difference of trends in the observed G is statistically significant. The dependence of trends of G on measurement methods uncovered here has an important implication for the widely reported global dimming and brightening based on datasets collected by different measurement methods; that is, the dimming might have been less if measured with current summation methods.
