杭州湾交通沿线雷达非气象回波的特征和滤除

Characteristics and Removal Technique of a Kind of Non-meteorological Echo along the Hangzhou Bay Expressways

近年来,随着杭州湾大桥、嘉兴沿岸的高速公路和上海洋山港大桥等交通干线的建成和通车,宁波、舟山和上海雷达探测到一种非气象回波,该杂波出现在大桥和高速公路的位置附近。分析发现该杂波与地物杂波类似又有所不同。类似之处在于地理位置相对固定,回波强度在最低层表现明显,随雷达仰角的抬升迅速减弱。不同之处为回波强度在时间和空间上有一定程度的变化,径向速度和谱宽也与通常的地物杂波特征有所不同。此外,杂波范围与低层风向、大气稳定度等气象条件关系较大,反映出气象条件对低层大气折射率的影响。统计2014年1～12月宁波雷达显示,该类杂波白天比夜间强,表明杂波与机动车流量存在一定的对应关系。针对此类杂波的特点,设计了考虑回波水平、垂直平滑度,基于动态阈值区分非气象回波的滤除方法。试验评估显示,该方法对于晴空、对流、层云和暖雨4类天气背景下的此类非气象回波都能进行较好的识别并正确地滤除。

With the construction of expressways along the Hangzhou Bay Bridge,Jiaxing coast and Shanghai Yangshan Harbor Bridge over the past years,the Ningbo,Zhoushan and Shanghai Doppler radar detected a kind of non-meteorological echo,with its location corresponding to the expressways.Analysis shows that the radar clutter is mainly caused by motor vehicles and bridges,with the characteristics similar but not always to ground echo clutter.The similarity lies in the fact that the echo appears at low elevation angle with relatively fixed positions and decreases rapidly with angle rising.But the echo intensity changes both spatially and temporally,and the radial velocity and spectrum width are different from the normal ground clutter.In addition,the clutter area varies with wind direction and atmospheric stability at low levels,implying the meteorological condition effect on the atmospheric refractive index.The main reason for the spatial and temporal change of clutter is the vehicle moving and the change of the low atmospheric refractive index on the path of the radar beam propagation.Statistics on the Ningbo radar diurnal variations from Jan.to Dec.of 2014 show that the clutters are stronger at daytime than at nighttime,reflecting that the traffic volume may have impact on the clutter.Based on traditional radar data quality control algorithms,a specific technique is designed to remove this kind of clutter,which mainly includes the horizontal smoothness of echo,the vertical smoothness of echo,dynamical threshold for non-meteorological clutter removal technique,non-precipitation echo processing method.Comparisons show the technique displays well under 4 different weather patterns including clear weather,convective rainfall,stratiform rainfall and warm-cloud rainfall.