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

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. ② The 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 diver-gence. 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.