Microphysical Characteristics of Rainfall Observed by a 2DVD Disdrometer during Different Seasons in Beijing, China

The seasonal variations of raindrop size distribution (DSD) and rainfall are investigated using three-year (2016-2018) observations from a two-dimensional video disdrometer (2DVD) located at a suburban station (40.13 degrees N, 116.62 degrees E, similar to 30 m AMSL) in Beijing, China. The annual distribution of rainfall presents a unimodal distribution with a peak in summer with total rainfall of 966.6 mm, followed by fall. Rain rate (R), mass-weighted mean diameter (D-m), and raindrop concentration (N-t) are stratified into six regimes to study their seasonal variation and relative rainfall contribution to the total seasonal rainfall. Heavy drizzle/light rain (R2: 0.2 similar to 2.5 mm h(-1)) has the maximum occurrence frequency throughout the year, while the total rainfall in summer is primarily from heavy rain (R4: 10 similar to 50 mm h(-1)). The rainfall for all seasons is contributed primarily from small raindrops (D(m)2: 1.0 similar to 2.0 mm). The distribution of occurrence frequency of N-t and the relative rainfall contribution exhibit similar behavior during four seasons with N-t of 10 similar to 1000 m(-3) registering the maximum occurrence and rainfall contributions. Rainfall in Beijing is dominated by stratiform rain (SR) throughout the year. There is no convective rainfall (CR) in winter, i.e., it occurs most often during summer. DSD of SR has minor seasonal differences, but varies significantly in CR. The mean values of log(10)N(w) (N-w: mm(-1)m(-3), the generalized intercept parameter) and D-m of CR indicate that the CR during spring and fall in Beijing is neither continental nor maritime, at the same time, the CR in summer is close to the maritime-like cluster. The radar reflectivity (Z) and rain rate (R) relationship (Z = aR(b)) showed seasonal differences, but were close to the standard NEXRAD Z-R relationship in summer. The shape of raindrops observed from 2DVD was more spherical than the shape obtained from previous experiments, and the effect of different axis ratio relations on polarimetric radar measurements was investigated through T-matrix-based scattering simulations.

Automated summary

The study reveals that rainfall in Beijing is mainly influenced by small raindrops, with summer rainfall predominantly consisting of heavy rain, and annual rainfall being dominated by stratiform rain.