三种霾日统计方法的比较分析

为了研究单次值法、日均值法、14时值法3种常用的霾日和轻雾(雾)日统计结果的异同,以环首都圈京津冀晋四省市为例进行了比较.华北地区霾日用3种方法统计的过去60余年霾日的区域分布表明,3种方法统计的霾日是单次值法>日均值法>14时值法,大致是1:0.54:0.45的关系,但区域分布趋势比较相似.典型城市霾日的长期变化趋势大都十分相似.而轻雾(雾)日的长期变化趋势表明,用单次值法统计的明显偏多,且有长期下降趋势;而用日均值法与14时值法统计的轻雾(雾)日无大差别,且没有明显的长期变化趋势,反映了年季和年代季的气候波动.从长期季节变化趋势来看,3种统计方法的结果除日数的差别外,季节分布特征比较类似.一个突出的特点是除去采暖季有较多的霾日外,在盛夏季节霾日也明显多,集中出现在6~9月,尤其是7~8月,与桑拿天同期出现,这与全国大部分城市的变化趋势完全不同.是华北地区的特有现象.用单次值法统计霾日,将包括所有的霾过程,即大范围持续时间长,且与一定天气系统与近地层扩散条件相关连的霾,及在稳定的晴朗夜间由于辐射降温,使相对湿度升高而导致能见度下降形成的霾.用日均值法,则可能更多的显示长时间大范围的霾天气过程;而使用14时法,则对早晚因湿度增加降低能见度出现的霾天气漏记,突出长时间大范围的霾天气过程.

Comparison of three statistical methods on calculating haze days-taking areas around the capital for example

This study set Beijing (BJ), Tianjin (TJ), Hebei (HB) and Shanxi (SX) among the areas around the capital as example to compare three methods on calculating haze or mist (fog) days. The three methods include single value method, daily mean method and 14:00PM moment method. The main results are as below: the regional haze days’ distribution during the past 60years in North China calculated by the three methods showed difference. The rank of the haze day numbers was single value method, daily mean method and 14:00PM moment method, with the ratio of about 1:0.54:0.45. But the regional trends appeared similar for the three methods. The long term trends of haze days for the typical cities were quite similar. The long term trends of mist (fog) days calculated by three methods turns out that single value method counted obviously more fog days and presented a long term decreasing trend, while the values gained by daily mean method and 14:00PM moment method had no significant difference, both with no significant long term trends and showed clear the interannual and inter-decadal variation. The long term trends calculated by the three methods presented obvious difference in some cities in North China. For the long term trends of seasonal variation, the seasonal distributions obtained by the three methods were similar although the difference in numbers. One outstanding feature was that there were obviously more haze days in summer besides in the heating season, mainly concentrated in June to September, especially in July to August, occurring at the same time as the sultry weather. This distribution was a unique phenomenon in North China, which was quite different from most of the other cities in China. Calculating by the single value method will include all haze processes, i.e. wide and lasting haze, which is related to the specific synoptic systems and near-surface diffusion conditions, or some other formed by decreased visibility under higher relative humidity caused by radiative cooling at the stable clear night. Calculating by the daily mean method will result in more wide and lasting haze processes, while using 14:00PM moment method can highlight the wide and lasting haze processes but neglect the haze with poor visibility caused by humidity rising in the morning and at night.