上海地区冬季能见度变化特征及区域差异

Variation Characteristics and Regional Difference of Winter Visibility in Shanghai

利用2014年12月2017年2月上海地区冬季11个气象观测站逐小时能见度、气温、相对湿度、风速等气象观测资料,以及徐家汇、浦东和崇明3个站逐小时PM2.5浓度资料,采用了相关分析等统计方法,对上海地区冬季能见度的时空分布特征进行了分析。结果表明:上海地区冬季平均能见度为11.6 km,2月的最好,12月的最差;上海地区能见度值呈现由西北向东南递增的变化特征,市区出现相对低值区;冬季能见度日变化呈单峰型特征,其与气温、相对湿度、风速、PM2.5浓度日变化呈显著相关;日最小能见度各时刻出现的频率大小与对应的平均能见度大小并不完全匹配,06:00最小能见度出现频率大且平均能见度低,需要特别关注;小于1 km低能见度多出现在崇明,可能与海岛充沛的水汽有关,而110 km能见度频次在市区出现了低值中心,这与城区“干岛”现象存在一定的关联;能见度与相对湿度、PM2.5浓度密切相关,在相对湿度较小时,PM2.5浓度对能见度影响较大,在PM2.5浓度较大时,相对湿度则对能见度的影响更大;从能见度小于1 km的持续时间上看,崇明站能见度最小且持续时间相对较长;小于1 km低能见度与相对湿度密切相关,小于0.5 km超低能见度则与气温的相关性更好。

Hourly observation data of visibility,temperature,relative humidity and wind speed collected from 11 observation sites in Shanghai from December 2014 to February 2017,and hourly PM2.5 concentration data collected at the stations in Xujiahui,Chongming and Pudong of Shanghai are used to analyze the temporal and spatial characteristics of winter visibility in Shanghai via the correlation analysis method.The results are as follows:The mean winter visibility in Shanghai is 11.6 km,and it is the best in February but the worst in December,increasing from northwest to southeast.The low values often appear in urban areas.The daily variation of winter visibility shows a unimodal feature,which is highly correlated to the daily changes of temperature,relative humidity,wind speed and PM2.5.The frequency of daily minimum visibility at each moment does not exactly match the corresponding mean visibility.Minimum visibility frequently happens at 06:00 am and the mean visibility value is low,too,which needs special focus.Low visibility less than 1 km occurs more in Chongming,which may be related to the abundant water vapor on the island.The visibility in the range of 110 km has low-value center in the urban,which is correlated to the“arid island”in the urban to some extent.In addition,visibility is closely related to relative humidity and PM2.5 concentration.PM2.5 has a major effect on the visibility if relative humidity is low;relative humidity has much more impact on visibility if PM2.5 concentration is high.According to the low visibility lasting time,visibility at Chongming is the lowest and duration is much longer.The visibility less than 1 km is closely related to relative humidity,while visibility less than 0.5 km has much closer correlations with temperature.