Development for measurement of physical characteristics of falling snowflake and its application to radio wave attenuation

人们认为，雪花是通过雪花晶体在坠落过程中的碰撞和粘合而生长的。这一生长过程受雪花形状和下落运动的影响。它不仅在与云物理有关的考虑方面很重要，而且对于各种工程应用，如遥感和电信，以及最近在全球气候监测领域的应用也是重要的。降水现象在与无线电波衰减和传播相关的领域中发挥着重要作用。在研究降水时，使用的主要仪器之一是雷达。雷达测量降水是基于雷达反射率因子Z和降雨率R之间的关系。为了确定降雪的Z-R关系，必须在短时间间隔内高精度地独立测量Z和R。然而，目前还没有开发出高精度和时间分辨率的降雪率测量系统，为了测量雪花的物理参数，用电视摄像机拍摄了自然落下的雪花。从这些图像中计算出每个雪粒的直径、速度和密度。根据图像数据计算的降雪率是使用电子天平直接获得的值来计算的。在这次观测中，用一台小型X波段多普勒雷达测量了接收功率。研究了降雪率与X波段波衰减的关系，并与雪粒的物理特性进行了比较。结果表明，功率谱与雪粒密度有关。

It is thought that snowflakes grow by the collision and adhesion of snowflake crystals during their fall. This growing process is effected by shape and falling motion of snowflake. It is not only important in consideration relating to the cloud physics, but also for various engineering applications such as remote sensing and telecommunications and lately in the field of global climate monitoring. Precipitation phenomena play an important part in fields related to radio wave attenuation and propagation. In the study of precipitation, one of the main instrument used is the radar. Radar measurements of precipitation are based on the relation between the radar reflectivity factor Z and precipitation rate R. In order to determine the Z-R relationship for snowfall, Z and R have to be measured independently with high accuracy at short time intervals. However, few systems have been developed for measurement of snowfall rate with high degree of accuracy and temporal resolution.In order to measure the physical parameter of snowflakes, natural falling snowflakes were photographed by TV cameras. The diameter, velocity and density of each snow particle were computed from these images. Snowfall rates calculated from image data are computed with values directly obtained using an electronic balance. During this observation, the received power was measured using a small X-band Doppler radar. The relationship between X-band wave attenuation and snowfall rate was investigated and compared to the physical characteristics of snow particles. It was suggested that the power spectrum was related with the density of snow particles.