Abstract
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° N, 116.62° E, ~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 (Dm), and raindrop concentration (Nt) are stratified into six regimes to study their seasonal variation and relative rainfall contribution to the total seasonal rainfall. Heavy drizzle/light rain (R2: 0.2~2.5 mm h−1) has the maximum occurrence frequency throughout the year, while the total rainfall in summer is primarily from heavy rain (R4: 10~50 mm h−1). The rainfall for all seasons is contributed primarily from small raindrops (Dm2: 1.0~2.0 mm). The distribution of occurrence frequency of Nt and the relative rainfall contribution exhibit similar behavior during four seasons with Nt of 10~1000 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 log10Nw (Nw: mm−1m−3, the generalized intercept parameter) and Dm 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 (?) relationship (Z = ?R?) 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.
Highlights
As this study mainly focuses on rain, the 2-dimensional video distrometer (2DVD) measurements containing snow were removed according to the temperature data measured from automated surface stations at the SY site
The definition of a rain event was based on the method proposed by Tokay and Bashor [35], i.e., a rain event is defined on the basis of 1 h or a longer rain-free period between two consecutive rainy minutes
The R6 category only occurres in summer rain with the highest number concentration of small- to moderate-sized raindrops (D < 4.5 mm), showing a peak (104.7 mm−1 m−3 ) at the diameter size ~0.3 mm (Figure 4f). These results indicate that the shapes of Droplet Size Distribution (DSD) in MD (R1) and HD/LR (R2) rain categories present similar unimodal distribution in four seasons, and there are minor seasonal distinctions in the small- and large-drop ends
Summary
The microphysical process of cloud and precipitation plays an essential role in the formation and evolution of rainfall. Many studies concentrated on microscopic cloud physics have shown that considerable differences in the intensity and distribution of rain can reveal the different microphysical properties of clouds [1,2]. The space and time variation of raindrop size distribution (DSD) within an observation area is related to a series of microphysical processes, such as condensation, coalescence, freezing, attachment, melting, fragmentation, and evaporation of particles in a particular precipitation system [3]. The knowledge of DSD is helpful to understand the precipitation 4.0/).
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