Vertical structures and microphysical mechanisms of winter precipitation over the southwestern mountainous area of China

Abstract

The vertical structures and microphysical processes of winter precipitation were investigated using comprehensive observations of three PARSIVELs and one MRR during January–March 2022 over the southwestern mountainous area of China. Twelve precipitation events were classified into four precipitation categories, including solid-phase precipitation, mixed-phase precipitation, light liquid-phase precipitation and heavy liquid-phase precipitation. Statistics reveal that, the majority of winter precipitation originates from snowflakes, with significantly higher precipitation top altitudes in liquid-phase precipitation compared to solid-phase and mixed-phase precipitation. Among the four different precipitation categories, there are distinct layers in which different microphysical processes dominate. For solid-phase precipitation, there is single solid-phase layer with few phase-type transitions, increased reflectivity (Ze) and precipitation rate (R) above 2.1 km altitude are likely affected by deposition and aggregation growth of snowflakes, increased fall velocity (Vf) and dispersed spectral width (Vsw) below 2.1 km altitude might suggest that most snowflakes rime into graupel and a few snowflakes melt. For mixed-phase precipitation, there are two distinct phase-type layers with weak phase-type transitions; in solid-phase layer (above 2.1 km altitude), accompanied by decreased Ze and R, decreased (increased) proportion of liquid-phase/mixed-phase hydrometeors above (below) 4.5 km altitude is likely attributed to refreezing/sublimation with fragmentation (sublimation); in mixed-phase layer (below 2.1 km altitude), significantly increased Vf and more dispersed Vsw suggest that snowflakes rime/melt into graupel, mixed-phase and liquid-phase hydrometeors. Although both light and heavy liquid-phase precipitation are divided into three distinct phase-type layers with remarkable phase-type transitions, significant differences exist in their vertical structures: first, heavy liquid-phase precipitation exhibits substantially higher precipitation top altitudes and a greater abundance of supercooled hydrometeors, resulting more active microphysical processes and noticeable increases of Ze and R at all altitudes; second, in melting layer, Ze in light liquid-phase precipitation initially increases and then decreases, known as bright band signature, however, Ze in heavy liquid-phase precipitation continuously increases due to enhanced melting and collision processes; third, in liquid-phase layer, both Ze and R in light liquid-phase precipitation show decreasing trends due to evaporation or break-up process, while Ze and R in heavy liquid-phase precipitation show remarkable increasing trends attributed to collision coalescence processes. Moreover, there are significant differences in ground particle size distributions, with small-size high-concentration hydrometeors in solid-phase precipitation, medium-size medium-concentration hydrometeors in mixed-phase precipitation, small-size low-concentration hydrometeors in light liquid-phase precipitation, and large-size high-concentration hydrometeors in heavy liquid-phase precipitation, resulting different Z-R relationships among the four precipitation categories.