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Drizzle initiation through the collision and coalescence of cloud droplets plays a crucial role in warm cloud precipitation. Recent theoretical studies suggest that the influence of collisional growth on the droplet size distribution can be quantified by a non‐dimensional drizzle number (Dz). Here, large‐eddy simulations with Lagrangian microphysics are employed to evaluate the theory by simulating a tall convection‐cloud chamber under various conditions. Results show that the smaller the Dz, the larger the impact of collisions on the right tail of the droplet size distribution, consistent with the theory. The simulations confirm that the collision rate can be estimated from the droplet size distribution interacting only with cloud droplets of the same size at the mode radius. This suggests that the idealized theory can be a useful tool to design a cloud chamber for drizzle investigation, as well as to represent drizzle formation in models of real atmospheric clouds.

This study investigates the effects of aerosol vertical distribution on a deep convective cloud system. We intend to elucidate the mechanisms for aerosols entering the cloud from different heights, and how they affect cloud microphysics and precipitation. A thermal bubble is released at 1.5 km initially to run an idealized case using the Weather Research and Forecast (WRF) model. The aerosol layer with high concentration was initially put at different altitudes in the model to study the mechanisms and the number of aerosols entering the cloud. It was found that there are three mechanisms for aerosols from different heights to enter the cloud, depending on their relative height with the thermal bubble. Aerosols from lower altitudes (below 1 km) enter the cloud through pumping, while aerosols from higher altitudes (2–3 km, 3–5 km) enter the cloud through entrainment. Both mechanisms lead to low cloud condensation nuclei (CCN) concentration in the cloud. Only aerosols from intermediate altitudes (1–2 km), which is the same as the initial height of the thermal bubble, enter the cloud mainly by ascending with the bubble and lead to high CCN concentration in the cloud. The differences in activated CCN concentration affect the microphysical processes and precipitation remarkably. For the simulations with an initial aerosol layer at 1–2 km and 0–5 km, aerosols can enter the cloud more efficiently than the other four simulations. More activated CCNs in these two simulations lead to more graupels with smaller sizes at higher altitudes, which delays the precipitation but makes the precipitation last longer. However, the accumulated precipitation is similar in all six simulations, no matter what aerosol vertical distribution is like. The results in this study indicate that the altitude of aerosol layers determines the mechanisms for aerosols entering clouds, CCN concentration in the cloud, and to what extent the cloud microphysical processes and precipitation are affected.

Insoluble particles influence weather and climate by means of heterogeneous freezing process. Current weather and climate models face considerable uncertainties in freezing-process simulation due to limited information regarding species and number concentrations of heterogeneous ice-nucleating particles, particularly insoluble particles. Here, for the first time, the size distribution and species of insoluble particles are analyzed in 30 shells of 12 hailstones collected from China using scanning electron microscopy and energy-dispersive X-ray spectrometry. A total of 289 461 insoluble particles were detected and divided into three species – organics, dust, and bioprotein – utilizing machine learning methods. The size distribution of the insoluble particles of each species varies greatly among the different hailstones but little in their shells. Further, a classic size distribution of organics and dust followed logarithmic normal distributions, which could potentially be adapted in future weather and climate models despite the existence of uncertainties. Our findings highlight the need for atmospheric chemistry to be considered in the simulation of ice-freezing processes.

Recent aircraft observations of marine stratocumulus clouds consistently showed that cloud microphysical relationships vary with altitude, indicating inhomogeneous mixing characteristics near cloud top and homogeneous mixing characteristics in mid‐levels of clouds. Here, we conduct model intercomparison of an idealized, non‐precipitating stratocumulus cloud to evaluate model consistency and examine whether simulations can reproduce the observed mixing characteristics. The results show that eleven large‐eddy simulations with various dynamics and microphysics schemes show good agreement on the thermodynamical, microphysical, and dynamical properties of the stratocumulus‐topped boundary layer in a steady state. The inter‐model spread in steady‐state liquid water path is significantly reduced compared to previous model intercomparison studies. This improvement might be due to better models and more consistent initial conditions than those used decades ago. In addition, most simulations, including a low‐dimensional simulation, capture inhomogeneous mixing characteristics near the cloud top and homogeneous mixing characteristics inside the cloud. Moreover, simulations using Lagrangian microphysics schemes agree better with the observed mixing characteristics compared with those using the bin microphysics schemes. Since most simulations do not fully resolve the entrainment process, the apparent mixing characteristics arise from the variations in the resolved cloud properties. Our results support the vertical circulation mixing hypothesis, which suggests that homogeneous mixing characteristics in mid‐levels of clouds are due to the vertical circulation of entrainment‐affected and diluted parcels from the cloud top moved to lower levels. Plain Language Summary Cloud microphysical properties respond differently to entrainment, which can lead to inhomogeneous mixing, where a portion of droplets evaporate completely while the mean droplet size remains constant, or homogeneous mixing, where all droplets evaporate uniformly due to entrained air. Recent aircraft observations of marine stratocumulus clouds showed features of inhomogeneous mixing near the cloud top and homogeneous mixing inside the cloud. One hypothesis is that the cloud top inhomogeneous mixing is physical, but the in‐cloud homogeneous mixing is due to the vertical circulation of entrainment‐affected, diluted parcels from the cloud top. To evaluate this hypothesis, we employ eleven large‐eddy simulation models and one low‐dimensional model to simulate an idealized non‐precipitating stratocumulus cloud. Results show that most simulations capture the observed mixing characteristics, supporting the vertical circulation hypothesis. Moreover, simulations using the microphysics schemes that suffer less from numerical diffusion show better agreement with the observations. Another important finding is that the inter‐model spread is much smaller than in similar model intercomparison studies decades ago, likely reflecting improvements in model formulations and greater consistency in model configurations. Key Points Eleven large‐eddy simulations and one low‐dimensional simulation of an idealized non‐precipitating stratocumulus cloud are conducted The inter‐model spread is reduced compared to previous studies, likely due to improved models and more consistent model setups Most simulations capture the observed mixing characteristics in stratocumulus clouds, supporting the vertical circulation mixing hypothesis

In order to improve the accuracy of return flow snowfall forecast in Beijing area, the three return flow snowfall cases occurring in Beijing area in February 2019 are analyzed by using the ERA5/GDAS reanalysis data and the HYSPLIT back trajectory model for the meteorological analysis and characterization of the water vapor transport. The results show that, for all the three cases, the water vapor content in the easterly flow is limited regardless of the typical or atypical return flow, and significant snowfall can be produced when there is a cooperation of the southerly water vapor channel. In the typical return flow case (Feb. 14), the high backward return flow and the Loop Inversion Trough cooperate, and the water vapor transport channel is deeper, which is conducive to snow. In the two atypical return flow cases (Feb. 6 and 12), the water vapor transport is concentrated in the lower layers of the atmosphere (below 850 hPa). The case of Feb. 12 has a significant contribution from the southerly water vapor t

A mesoscale convective system (MCS) embedded in a cold frontal cloud system over Beijing on July 16, 2018 was investigated using X-band dual-polarization radar observations from Fangshan Station and sounding data from the Beijing Observatory. This study focused on hydrometeor identification and microphysical characterization, yielding the following key findings. 1) The ice-phase hydromorphic species above the 0°C layer differed in different stages and regions of this MCS. From the stage of development to maturity to dissipation, the main hydromorphic species in the region with combined reflectance larger than 40 dBZ were graupel and snow, and the proportion of ice crystals was relatively low. In the region with combined reflectance between 20 and 40 dBZ, graupel, snow, and ice crystals were the main hydromorphic species. In these two regions, the proportion of graupel gradually decreased with time, while the proportion of snow and ice crystals gradually increased with time. In the region with combined reflect

为提高北京地区回流降雪预报的准确率,利用ERA5/GDAS全球格点再分析资料和HYSPLIT轨迹追踪模式,对2019年2月发生在北京地区的3次回流降雪过程进行天气学分析和水汽输送特征分析.结果表明,在3次回流降雪个例中,偏东气流中的水汽含量有限,当出现偏南水汽通道的配合时,可产生明显的降雪.在典型回流形势(2月14日)下,高后回流与河套倒槽配合,回流水汽输送通道更为深厚,利于降雪;在非典型回流形势(2月6日和12日)下,回流水汽输送集中于大气的低层(850 hPa以下).2月12日南支气流水汽输送贡献较大,对南支气流水汽输送的忽视是导致2月12日降雪漏报的原因之一.3次回流降雪个例在偏东和偏南路径上水汽输送至北京地区的时间与白天显著降雪的时段基本上对应,可作为预报回流降雪的关键因子.

Immersion freezing is an important heterogeneous ice nucleation process in mixed-phase clouds. Many researches on immersion freezing focus on the parameterization using the classical nucleation theory. However, the ice nucleating abilities of the atmospheric aerosol particles are not well understood, and therefore cause difficulties of the parameterization of immersion freezing. In this study, we investigate the ice nucleating ability of Marigold pollen from the Beijing area by doing drop immersion freezing experiments in the laboratory. Marigold pollen are first mixed with pure water. A population of drops with a volume of 10 mL from the mixture are then put on a PE film in a chamber with a cooling rate of about 0.5 °C min-1. We find that the median freezing temperature of drops with pollen as ice nuclei is -17 °C. The contact angle parameterization of immersion freezing using the classical nucleation theory is also given in this study. We use the single-θ and the θ-pdf parameterization schemes to explain the data. The results show that the θ-pdf parameterization scheme is better for describing the ice nucleating behavior of the pollen in Beijing.