多普勒雷达的速度图像特征及其在一次降雪过程中的应用

The Doppler Radar Velocity Image Features and Its Application in a Snowfall Process

该文首先介绍了大面积降水的多普勒天气雷达径向速度PPI图像识别技术,包括:①冷暖平流与大尺度辐合辐散运动叠加的图像特征;②由零线的朝向和正负速度面积、径向速度值的大小判断风向风速辐合辐散的图像特征,然后介绍了利用EVAD技术由雷达基数据定量计算大气平均散度和平均垂直速度的方法,最后应用图像识别技术和EVAD方法对石家庄2004年冬季的一次大面积降雪过程进行了详细分析,发现:此次大面积降雪过程有明显的辐合辐散图像特征,径向速度值小,辐合辐散弱,始终存在暖平流。另外,降雪强度和散度、垂直速度关系密切:辐合层厚度加大,辐散抬高,则降雪加强;辐合、辐散层高度降低,则降雪减弱。同时,由EVAD技术定量计算的散度和相同时刻由径向速度图像产品定性分析的辐合、辐散基本一致。

A Doppler weather radar radial velocity PPI image identifying technique in large-scale rainfall is introduced： ① The image characters of the cold （warm） advection combined with large-scale divergence （convergence） are that the bending degree increases on a side while decreases on the other, which forms the zero-speed line differing in bending on both sides of the display center. ②The wind direction-related convergence or divergence features display an bow-like zero-velocity line, thereby the convergence or divergence can be determined. If the bow＇s ends bending towards the positive （negative） speed area, the negative （positive） area is larger than the positive one （negative）, which means pure convergence （divergence）. ③ The wind speed-related convergence （divergence） character can be estimated by means of the positive and negative speed areas and measures in a same distance ring. It is convergence if negative velocities are bigger than positive ones, otherwise it is divergence. The bigger the different value between positive and negative, the stronger the convergence or divergence. Furthermore, a method is introduced to quantitively calculate the atmospheric average divergence and vertical speed using EVAD technique： ① Extract average divergences in each height level from radar base data utilizing Thomas weighted coefficient method. ② In order to diminish the error added up while integrating along height when calculating air vertical speed by continuous equation, divergences are adjusted according to variation method. ③ Adjusted divergences are considered in continuous equation to calculate air vertical speeds. Applying the image identifying and EVAD techniques, a large-area snowfall process is analyzed in detail in Shijiazhuang, Hebei Province, in the winter of 2004, which suggests that there are obvious convergence （divergence） features during the process, radial velocities are small, convergences and divergences are weak and there is always the warm convection. In addition, the snowfall intensity is closely related to the divergence and vertical speeds, snowfall strengthens as soon as the thickness of convergence levels increases and divergence ascends up; otherwise snowfall weakens when the height of convergence and divergence levels descends down. The quantitive divergence calculated by EVAD is corresponded with those quality analyzed by radial speed picture produced in the same time.