Vertical Profiles of Raindrop Size Distribution Observed by Micro Rain Radar

Raindrop size distribution (DSD) is of great importance for understanding the microphysical process of precipitation, as well as improving the microphysical parameterization scheme in numerical model. Most studies of DSD focus on precipitation characteristics on the surface. However, vertical profiles of DSD and rain parameters are important for quantitatively accurate precipitation estimation from weather radars. Based on data observed by the ground-based PARSIVEL disdrometer and a vertical pointing micro rain radar (MRR) at Xingtai, Hebei Province located in North China from June to September in 2016, the vertical evolution of precipitation microphysical parameters and DSD of different rain rate classed for stratiform precipitation are analyzed. Measurements from MRR, rain gauge and ground PARSIVEL disdrometer are compared. Results show that measurements from MRR, rain gauge and PARSIVEL disdrometer have good agreement in rain rate. MRR and PARSIVEL disdrometer show good consistency in medium sized (1-2.5 mm) range of DSD but have slight differences for small and large raindrops. MRR observes much more small particles than PARSIVEL disdrometer. When the rain rate is low, with low relative humidity around the ground, both large and small drops decrease with the altitude decreasing, so as to the liquid water content and rain rate, which is explained by the evaporation. When the rain rate is high, the concentration of particles for precipitation is much larger, and the vertical variation of DSD is more obvious. The profiles of radar reflectivity show a positive slope(dZ/dH>0). The concentration of medium-sized and large raindrops increases obviously with decreasing altitude at the cost of reducing small raindrops for precipitation with rain rate between 2-20 mm·h^-1, indicating that the coalescence is the dominant process for 2-20 mm·h^-1. The largest contribution to the total number concentration is small drops (0-0.5 mm) with diameters between 0-1 mm and can reach up to 50% above altitude of 2 km. Small particles contribute less to the precipitation intensity as the altitude decreasing. These small raindrops account only 15% to the surface precipitation, while the medium-sized raindrops can contribute 60% with rain rate between 2-20 mm·h^-1. Large raindrops (>2 mm) is about 50% of the surface rainfall for the largest rain rate class. These results provide useful information for better understanding rain processes and quantitative estimation of precipitation in the future.