Huige Di

Hydrometeor growth and sedimentation in mixed-phase clouds are key processes linking cloud evolution to surface precipitation, yet continuous observations of when fast-sedimenting hydrometeors establish effective downward transport remain limited. We combine observations from a millimeter-wave cloud radar, multiwavelength polarization lidar, microwave radiometer, and radiosondes to investigate the evolution of a cloud system from a high-level ice cloud to a stratiform mixed-phase cloud and ultimately to short-lived light precipitation. High-level ice hydrometeors developed and began to sediment under ice-saturated or ice-supersaturated conditions. As they descended into warmer cloud layers containing liquid water, radar reflectivity and hydrometeor terminal fall velocity increased concurrently with Doppler spectral broadening, consistent with further hydrometeor growth and enhanced sedimentation. The mixed-phase cloud subsequently persisted for nearly 8 h, while liquid water and fast-sedimenting spectral components were sustained without continuous surface precipitation. We define the high-velocity spectral energy fraction (Ffast) and combine it with the linear radar reflectivity factor to construct a Hydrometeor Sedimentation Index (HSI). Ffast was present well before surface precipitation, whereas HSI increased markedly later and extended toward lower altitudes, showing a clearer temporal correspondence with short-lived light precipitation. The concurrent enhancement of fast-sedimenting spectral contributions and strong radar echo signal, together with their downward extension, was a key observational feature of the transition to precipitation. These results provide a process-oriented framework for using continuous Doppler spectral observations to characterize hydrometeor sedimentation evolution and the cloud-to-precipitation transition in mixed-phase clouds.

During severe haze events, the boundary layer exhibits a complex vertical structure, while high aerosol loadings hinder high-resolution temperature and humidity measurements. To address this, a Raman-Mie lidar and retrieval algorithms for temperature, humidity, and aerosol optical properties were developed at Xi’an University of Technology, enabling high-resolution profiling of haze vertical structures. A 12-day haze episode was continuously monitored from formation to dissipation, providing detailed spatiotemporal variations of temperature, relative humidity, and aerosols. The boundaries of temperature inversion (TI) and aerosol layers were identified using a threshold method. The results revealed a strong coupling between aerosols and temperature during pollution evolution. Dome and stove effects were observed, with possible coexistence and interaction. Three dome-shaped TIs were identified. The top of a decreasing-type aerosol layer formed a stratified dome structure that constrained vertical diffusion, with the temperature gradient of the elevated TI varying inversely with its depth. Both TI strength and humidity were strongly correlated with surface PM_2.5 concentrations. Surface-based TI exhibited a clear diurnal variation, with TI peaks preceding aerosol peaks. The results indicated that strong elevated TI and weak turbulence in the lower layer favored aerosol accumulation. Clouds and virga not only suppressed radiative heating but also enhanced humidity, further driving the rapid increase in surface PM_2.5 concentrations. During the dissipation stage, the rapid breakdown of TI and enhanced solar heating were critical for pollutant removal, while efficient horizontal transport facilitated the complete clearance of aerosols within the boundary layer.

During severe haze events, the boundary layer exhibits a complex vertical structure, while high aerosol loadings hinder high-resolution temperature and humidity measurements. To address this, a Raman-Mie lidar and retrieval algorithms for temperature, humidity, and aerosol optical properties were developed at Xi’an University of Technology, enabling high-resolution profiling of haze vertical structures. A 12-day haze episode was continuously monitored from formation to dissipation, providing detailed spatiotemporal variations of temperature, relative humidity, and aerosols. The boundaries of temperature inversion (TI) and aerosol layers were identified using a threshold method. The results revealed a strong coupling between aerosols and temperature during pollution evolution. Dome and stove effects were observed, with possible coexistence and interaction. Three dome-shaped TIs were identified. The top of a decreasing-type aerosol layer formed a stratified dome structure that constrained vertical diffusion, with the temperature gradient of the elevated TI varying inversely with its depth. Both TI strength and humidity were strongly correlated with surface PM_2.5 concentrations. Surface-based TI exhibited a clear diurnal variation, with TI peaks preceding aerosol peaks. The results indicated that strong elevated TI and weak turbulence in the lower layer favored aerosol accumulation. Clouds and virga not only suppressed radiative heating but also enhanced humidity, further driving the rapid increase in surface PM_2.5 concentrations. During the dissipation stage, the rapid breakdown of TI and enhanced solar heating were critical for pollutant removal, while efficient horizontal transport facilitated the complete clearance of aerosols within the boundary layer.

During severe haze events, the boundary layer exhibits a complex vertical structure, while high aerosol loadings hinder high-resolution temperature and humidity measurements. To address this, a Raman-Mie lidar and retrieval algorithms for temperature, humidity, and aerosol optical properties were developed at Xi’an University of Technology, enabling high-resolution profiling of haze vertical structures. A 12-day haze episode was continuously monitored from formation to dissipation, providing detailed spatiotemporal variations of temperature, relative humidity, and aerosols. The boundaries of temperature inversion (TI) and aerosol layers were identified using a threshold method. The results revealed a strong coupling between aerosols and temperature during pollution evolution. Dome and stove effects were observed, with possible coexistence and interaction. Three dome-shaped TIs were identified. The top of a decreasing-type aerosol layer formed a stratified dome structure that constrained vertical diffusion, with the temperature gradient of the elevated TI varying inversely with its depth. Both TI strength and humidity were strongly correlated with surface PM_2.5 concentrations. Surface-based TI exhibited a clear diurnal variation, with TI peaks preceding aerosol peaks. The results indicated that strong elevated TI and weak turbulence in the lower layer favored aerosol accumulation. Clouds and virga not only suppressed radiative heating but also enhanced humidity, further driving the rapid increase in surface PM_2.5 concentrations. During the dissipation stage, the rapid breakdown of TI and enhanced solar heating were critical for pollutant removal, while efficient horizontal transport facilitated the complete clearance of aerosols within the boundary layer.

This study utilizes Ka-band millimeter-wave cloud radar (MMCR), assisted by a precipitation phenomenon instrument, to conduct case studies and analyses of convective precipitation, cumulus precipitation, and stratus precipitation in the Xi’an region. Using the Doppler spectral data of the MMCR, dynamic parameters such as vertical air motion velocity (updraft and downdraft) and particle terminal fall velocity within these three types of cloud precipitation were retrieved. The results show that above the melting layer, the maximum updraft velocity in convective clouds reaches 15 m·s⁻¹, and the strong updraft drives cloud droplets to move upward at an average velocity of about 5 m·s⁻¹. The average updraft velocity in cumulus clouds is greater than that in stratus clouds, with updrafts in cumulus and stratus mainly distributed within 1.5–3 m·s⁻¹ and 1–2 m·s⁻¹, respectively. The reflectivity factor of precipitation particles (Ze) is used to correct the equivalent reflectivity factor (Ka-Ze) after attenuation correction below the MMCR melting layer. The accuracy of calculating the raindrop concentration using the Ka-Ze of MMCR was improved below the melting layer. Based on the relationship between terminal fall velocity and particle diameter and using the conversion between the MMCR power spectrum and raindrop spectrum, the concentration, fall velocity, and particle diameter of raindrops are calculated below the melting layer. The results show that the average reflectivity factor, average concentration, and average particle diameter of raindrops follow the order of convective precipitation > cumulus precipitation > stratiform precipitation. However, the average terminal fall velocity distribution of raindrop particles follows a different order: convective precipitation > stratiform precipitation > cumulus precipitation.

The Dual-frequency Precipitation Radar (DPR) aboard the Global Precipitation Measurement (GPM) mission provides valuable three-dimensional precipitation structure data on a global scale and has been widely used in hydrometeorological research. However, due to its spatial resolution limitations and inherent algorithmic assumptions, the accuracy of GPM precipitation estimates can exhibit systematic biases, especially under complex terrain conditions or in the presence of variable precipitation structures, such as light stratiform rain or intense convective storms. In this study, we evaluated the near-surface precipitation rate estimates from the GPM-DPR Level 2A product using over 1440 min of disdrometer observations collected across China from 2021 to 2023. Based on three years of stable stratiform precipitation data from the Jinghe station, we developed a least squares linear correction model for radar reflectivity. Independent validation using national disdrometer data from 2023 demonstrated that the corrected reflectivity significantly improved rainfall estimates under light precipitation conditions, although improvements were limited for convective events or in complex terrain. To further enhance retrieval accuracy, we introduced a regionally adaptive R–Z relationship scheme stratified by precipitation type and terrain category. Applying these localized relationships to the corrected reflectivity yielded more consistent rainfall estimates across diverse conditions, highlighting the importance of incorporating regional microphysical characteristics into satellite retrieval algorithms. The results indicate that the accuracy of GPM precipitation retrievals is more significantly influenced by precipitation type than by terrain complexity. Under stratiform precipitation conditions, the GPM-estimated precipitation data demonstrate the highest reliability. The correction framework proposed in this study is grounded on ground-based observations and integrates regional precipitation types with terrain characteristics. It effectively enhances the applicability of GPM-DPR products across diverse environmental conditions in China and offers a methodological reference for correcting satellite precipitation biases in other regions.

An autocollimator is a goniometer established according to the principle of autocollimation, but it is ineffective for measuring the roll angle. This paper proposes an improved autocollimator available for large-range roll angle measurement, which maintains the optical structure of the classic one while incorporating a wedge prism (WP) working in transmissive mode as a roll angle sensing element to dissociate the collimated beam into two beams. According to the moving paths of the two light spots focused on the photodetector, the roll angle of the WP can be solved. The measuring method is expounded, and the calibration results reveal that the improved autocollimator has an accuracy of ±13.55 arcsec over a range of 360°, confirming its feasibility for roll angle measurement where a large measuring range is required.

An autocollimator is a popular angle measuring apparatus which lacks the capability to measure the roll angle. This paper proposes a novel roll angle sensor with a large measuring range that is based on the autocollimation principle. A modified right-angle prism (MRP) functions as a reflector to admit a collimated beam and return two outgoing beams to the sensor head. The roll angle of the MRP can be attained by analyzing the moving tracks of the two light spots focused on a photodetector. The mathematical model is derived in detail, and the experimental results show that the measuring accuracy of the proposed sensor is ±13.85 arcsec over a range of 360°. These results verify the feasibility of the proposed sensor for roll angle measurements that require a large measuring range.

The seeding effect of upper clouds on lower clouds affects the evolution of clouds, especially the seeding from upper ice clouds on lower stratiform clouds or convective clouds, which can stimulate the precipitation of lower clouds and even produce extreme precipitation. When seeders of the seeding cloud enter the feeding cloud, the interaction between cloud particles results in the change in macro- and micro-parameters of the feeding cloud. Based on the observation data of a ground-based Ka-band millimeter-wave cloud radar (MMCR) and microwave radiometer (MWR) in spring and autumn from 2021 to 2022, the seeder–feeder phenomenon among double-layer clouds in Xi'an, China, is studied. The study on 11 cases of seeder–feeder processes shows that the processes can be divided into three types by defining the height difference (HD) between the seeding cloud base and the feeding cloud top and the effective seeding depth (ESD). Through analysis of the reflectivity factor (Z) and the radial velocity (V_r) of cloud particles detected by the MMCR and on the retrieved cloud dynamics parameters (vertical velocity of airflow, V_a, and terminal velocity of cloud particles, V_f), it is shown that the reflectivity factor and particle terminal velocity in the cloud are significantly enhanced during the seeder–feeder period for the three types of processes. But the enhancement magnitudes of the three seeder–feeder processes are different. The results also show that the impact of seeding on the feeding cloud is limited. The lower the height and thinner the thickness of the HD, the lower the height and thicker the thickness of the ESD. On the contrary, the higher the height and the thicker the thickness of the HD, the higher the height and the thinner the thickness of the ESD.

The seeding effect of upper clouds on lower clouds affects the evolution of clouds, especially the seeding from upper ice clouds on lower stratiform clouds or convective clouds, which can stimulate the precipitation of lower clouds and even produce extreme precipitation. Because when seeders of seeding cloud enter the feeding cloud, the interaction between cloud particles results in the change of macro and micro parameters of the feeding cloud. Based on the observation data of the ground-based Ka-band millimeter-wave cloud radar (MMCR) and microwave radiometer (MWR) in spring and autumn from 2020 to 2022, the seeder-feeder phenomenon among double-layer clouds in China Xi'an was studied. The study on 11cases of seeder-feeder processes shows that the processes can be divided into three categories by defining the height difference (HD) between the seeding cloud base and the feeding cloud top, and the effective seeding depth (ESD). Through the analysis on the reflectivity factor and the radial velocity of cloud particles detected by MMCR and on the retrieved cloud dynamics parameters (vertical velocity of airflow and falling velocity of cloud particles), it is shown that the reflectivity factor in the cloud are significantly enhanced during the seeder-feeder period for the three types of processes. But there are different enhancements among the reflectivity factor profiles for the three seeder-feeder processes. The results also showed the limited depth as seeders entering the top of the feeding cloud. The lower and thinner the HD height was, the lower and thicker the ESD height was. On the contrary, the higher the HD height, the higher and thinner the ESD height.