Snow studies. Part I: A study of natural variability of snow terminal velocity

The variability and the uncertainties in snowfall velocity measurements are addressed in this study. The authors consider (i) the instrumental uncertainty in the fall velocity measurement, (ii) the effect of unstable falling motion on the accuracy of velocity measurement, and (iii) the natural variability of homogeneous snow terminal fall velocity. It is shown that, when periods of homogeneous characteristics of snow are selected to minimize the mixture of particles of different origin, the standard deviation of snowfall velocity within each period tends to stabilize at a value between 0.1 and 0.2 m s^-1. In addition, the variability of snow terminal fall velocity is examined with three control variables: surface temperature T_s, echo-top temperature T_t, and the depth of precipitation system H. The results show that the exponent b in the power-law relationship V = aD^b has little effect on the variability of snowfall velocity: the coefficient a correlates much better with the control variables (T_s, T_t,H) than the exponent b. Hence, snowfall velocity can be modeled with a varying coefficient a and a fixed exponent b = 0.18 (V = aD^0.18) with good accuracy.