Spectrum Evolution In The Raindrop Falling Process: Numerical Simulation

Basing on the data of raindrop spectra of air and ground at 9 July 1982 in Ningxia, a one-dimensional category model of raindrops was applied to simulate the process of falling raindrop after leaving cloud, which consider the process of evaporation, stochastic collection and stochastic breakup in the falling raindrop. The mechanisms which influenced the evolution of raindrop size distribution was discussed in detail.The temperature, dew point temperature and air pressure of each layers in the simulation as the initial background field was obtained by actual observation results, not to discuss these three parameters on the impact of the simulation; simulation did not consider the horizontal wind of each layer. Model using the quality of sub-file program, a total will be divided into 34 files.The parameters affecting the simulation results was discussed, including the forms of raindrop size distribution at first layer, cloud base height, air density, updraft, four important parameters. Simulation result was analyzed and the fallowing conclusions could be found:(1) According to the observations, using the M-P distribution and Gamma distribution, the height of 1500m, to fit the raindrop size distribution, respectively, the first layer of raindrop size distribution for model input values was obtained. The results show that using the Gamma distribution could simulate the small raindrop (D≤0.3mm) better, but that the results more stable when using the M-P distribution.(2) When the cloud base height was increased to 4000m from 1500m, the simulated time of stable precipitation, that the whole raindrop size spectra was appeared at ground, was added, the peak value of maximum number density in raindrop size distribution was smaller, the number of peak value at the small raindrop part become more, and the number of larger raindrops was fewer.(3) Using the standard atmospheric density (1.23 kg/m3), and the local air density of Ningxia (0.97 kg/m3), respectively, to simulate, the results show that due to the smaller air density, that the peak value of maximum number density in raindrop size distribution was decreases, and that for simulated time of stable precipitation was not significantly affected.(4) Updraft could delay the time which small raindrops fall on the ground, and the faster updraft velocity, the morn significantly delayed. But for the lager raindrop, there was not significant effect. Updraft could reduce the peak value of maximum number density in small raindrop size distribution.(5) In the simulation results, the peak value of raindrops number density are concentrated in the region of the less than lmm, particularly in the area of less than 0.3mm; there was not significant effect by various parameters for the evolution of more than lmm raindrop spectra at 60 minutes of integration time.Using the aircraft observations of raindrop size distribution of air and the boundary layer profiles of meteorological elements in Ningxia as the initial background field, a one-dimensional category model of raindrops was applied to simulate the process of falling raindrop after leaving cloud, compared ground and air, respectively, simulated and measured raindrop size distribution. And based on the sensitivity test, mechanisms of evaporation, coalescence and breakup for the effect of raindrop size distribution was discussed in detailed.The results show that the evaporation, coalescence and breakup in the process of raindrop falling could be simulated properly by the model, that the simulated raindrop spectra displayed a multiple peak distribution and that it is closer the measured raindrop size distribution than the traditional M-P distribution.The evaporation would cause the diameter of raindrop smaller, higher consumption of small size raindrops even more significantly than big size raindrops, increasing the number of concentrations of medium-sized raindrops, but that would not to change the overall spectral shape of raindrops. And the multi-peak raindrop size distribution was caused by the coalescence and breakup mechanism which were key factor to change the raindrop spectra.Analyzing the evolution of characteristic parameter of raindrop spectra of simulation results, the results show that the smaller average diameter of the raindrop size, narrower spectral width and decreasing number concentration, resulting in reduce rainfall intensity was causing by the evaporation; that the bigger average diameter of the raindrop size, broaded spectral width and increasing number concentration, resulting in raised rainfall intensity was causing by the coalescence and breakup.Overall, taking into account mechanisms of evaporation, coalescence and breakup, the process of raindrop falling below the cloud was simulated and the evolution of falling raindrop size distribution was reappeared by the model.