Precipitation In Warm Clouds Research Articles

Combined impacts of aerosols and urbanization on a highly threatened extreme precipitation event in Beijing, China

On July 21, 2012, a catastrophic precipitation event occurred in Beijing, highlighting the serious threat of extreme precipitation on socio-economic development and human health under climate change. Nevertheless, whether, how and to what extent aerosols and urbanization, as the two main influencing factors of urban extreme precipitation, have affected this highly damaging extreme event remains largely unexplored. Here, we employed the weather research and forecasting model coupled with chemistry (WRF-Chem) and a single-layer urban canopy model to investigate the influences of urbanization, aerosols and their interactions on this extreme precipitation event. We found that the joint intensification effects of urbanization and aerosols on extreme precipitation events greatly enhance its negative influence on megacities. The results indicate that aerosols are enhanced by increasing cloud droplet numbers, thereby intensifying the feedback between precipitation and latent heating. Consequently, the total precipitation increased by 22.6%, raising the precipitation in the Beijing area increase by at least 50 mm. By stimulating atmospheric instability and strengthening vertical air motion (over 0.25 m s−1), the urban heat island effect considerably influences the temporal and spatial distributions of extreme precipitation events, resulting in an increase in warm cloud precipitation (80%) and a decrease (30%) in frontal precipitation. Consequently, joint intensification effects resulted in more concentrated precipitation in the southwest of Beijing, leading to a substantial increase (more than 40%, ∼80 mm). This condition may be an important reason for the most severe disasters in the southwest of Beijing.

An underestimated negative cloud feedback from cloud lifetime changes

As the atmosphere warms, part of the cloud population shifts from ice and mixed-phase (‘cold’) to liquid (‘warm’) clouds. Because warm clouds are more reflective and longer-lived, this phase change reduces the solar flux absorbed by the Earth and constitutes a negative radiative feedback. This cooling feedback is weaker in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) than in the fifth phase (CMIP5), contributing to greater greenhouse warming. Although this change is often attributed to improvements in the simulated cloud phase, another model bias persists: warm clouds precipitate too readily, potentially leading to underestimated negative lifetime feedbacks. In this study we modified a climate model to better simulate warm-rain probability and found that it exhibits a cloud lifetime feedback nearly three times larger than the default model. This suggests that model errors in cloud-precipitation processes may bias cloud feedbacks by as much as the CMIP5-to-CMIP6 climate sensitivity difference. Reliable climate model projections therefore require improved cloud process realism guided by process-oriented observations and observational constraints. CMIP6 models simulate higher and more accurate cloud liquid water fraction relative to CMIP5, but both ensembles overestimate warm cloud precipitation. Correcting these warm cloud processes in a model exposes compensating biases large enough to offset CMIP5–CMIP6 climate sensitivity differences.

Lightning activity and its associations with cloud structures in a rainstorm dominated by warm precipitation

Lightning activity and its associations with cloud structures during a rainstorm dominated by warm cloud precipitation were studied in Guangdong, China on May 7, 2017, using three-dimensional lightning location and polarimetric radar data. The overall convection and lightning activities of the rainstorm were weak. The rainstorm generally showed a typical tripolar charge structure with the main negative charge core located between the −15 and − 8 °C environmental isotherms in the first 4 h. The height of the charge regions clearly decreased after this period, with the main negative charge core being below the −8 °C isotherm. Lightning discharges were more concentrated in areas featuring relatively weak convection and relatively low precipitation intensity. Most of the locations with lightning discharges were dominated by dry aggregated snow and weak updrafts and downdrafts. This investigation demonstrated that the lightning discharges were spatially separated from the area of origin of charging in this rainstorm. It is proposed that, with weak convection in the rainstorm, the charging rate was lower than the speed of charge transfer from the area of origin, causing a relatively low charge density and a low frequency of lightning in the area of origin of charging. Meanwhile, the aggregation of small charged particles in regions away from the area of origin of charging might be conducive to the formation of a relatively high charge density and therefore relatively frequent lightning flashes. This situation is different from a typical thunderstorm with strong convection.

Developing and Testing of an Expansion Cloud Chamber for Cloud Physics Research

The expansion cloud chamber is an important equipment for the research on nucleation process and mechanism of aerosol and weather modification seeding agents. It provides experimental conditions, in which the vapor can transform to water or ice super saturated. But for a long time, expansion cloud chamber with advanced cloud particle spectrum and image measurement system is absent in China. A independently developed expansion cloud chamber is established recently to investigate cloud physical and chemical processes. It consists of cloud chamber equipment, environmental and cloud physics parameter measurement systems, data communication systems and control system.The cloud chamber system mainly includes an experimental reaction chamber and a pre-vacuum tank, and adopts the split design. They are connected through controllable valves. The inside capacity of the experimental reaction chamber and pre-vacuum tank are 1.5 m3 and 9 m3, respectively. The control target air pressure of the reaction chamber is 100 hPa, and that of the pre-vacuum tank is 30 hPa. The cloud chamber system is equipped with advanced measuring instruments, such as environmental parameter detectors, cloud particle spectrometer, the precipitation particle spectra instrument, cloud particle imager, visibility meter, aerosol particle spectrometer, high speed camera system, etc.It is the first time that the chamber system uses domestic cloud particle spectrometer and imager measurement system. Tests show that the cloud chamber system has good temperature and pressure controlling ability. The average cooling rate can reach 0.26℃·min-1 and temperature distribution in cloud chamber is uniform. In the chamber, the fog formed by expansion process lasts 4 minutes. The maximum concentration of fog droplets reaches 6.1 cm-3, the droplet spectrum range is 3-8 μm, and the average effective diameter is about 6.5 μm. At the same time, clear fog droplet images can be obtained by high speed camera. It can control the low-temperature environment from room temperature to -50℃, and achieve pressure expansion cloud simulation and microphysical parameter monitoring. The lack of indoor experimental equipment for aerosols and warm cloud seeding agents will be solved. It has important significance for verification of nuclear properties of warm cloud seeding agents and improving the technology of warm cloud precipitation enhancement.

Improving collisional growth in Lagrangian cloud models: development and verification of a new splitting algorithm

Abstract. Lagrangian cloud models (LCMs) are increasingly used in the cloud physics community. They not only enable a very detailed representation of cloud microphysics but also lack numerical errors typical for most other models. However, insufficient statistics, caused by an inadequate number of Lagrangian particles to represent cloud microphysical processes, can limit the applicability and validity of this approach. This study presents the first use of a splitting and merging algorithm designed to improve the warm cloud precipitation process by deliberately increasing or decreasing the number of Lagrangian particles under appropriate conditions. This new approach and the details of how splitting is executed are evaluated in box and single-cloud simulations, as well as a shallow cumulus test case. The results indicate that splitting is essential for a proper representation of the precipitation process. Moreover, the details of the splitting method (i.e., identifying the appropriate conditions) become insignificant for larger model domains as long as a sufficiently large number of Lagrangian particles is produced by the algorithm. The accompanying merging algorithm is essential to constrict the number of Lagrangian particles in order to maintain the computational performance of the model. Overall, splitting and merging do not affect the life cycle and domain-averaged macroscopic properties of the simulated clouds. This new approach is a useful addition to all LCMs since it is able to significantly increase the number of Lagrangian particles in appropriate regions of the clouds, while maintaining a computationally feasible total number of Lagrangian particles in the entire model domain.

A study of macrophysical and microphysical properties of warm clouds over the Northern Hemisphere using CloudSat/CALIPSO data

This study investigates the general macrophysical and microphysical properties of single-layer warm clouds over the Northern Hemisphere using standard CloudSat products during 2008. The yearly averaged occurrence frequency of single-layer warm clouds is 20.9% over oceans and 9.5% over land, respectively. The cloud top heights over land in different latitude zones exhibit a broader spectral width and more frequent occurrence of strong cloud systems in tropics than that over higher latitudes. The maximum values of liquid water content over land are approximately 10–30% smaller but the occurring altitudes are about 0.5 km higher than those over oceans with the same liquid water path (LWP). The reflectivity profiles clearly demonstrate the growth processes of cloud particles and how these processes vary with increasing LWP. The evolutions of reflectivity over land show a frequency shift toward weaker dBZ values relative to that over oceans with the same LWP, suggesting that high aerosol concentrations may induce suppressed drizzle and delayed precipitation in warm clouds. Additionally, there exists a significant difference between warm clouds formation at the initial stage over ocean and land areas. When LWP is 0.1–0.2 kg m−2 and optical depth exceeds 25, the dBZ values over oceans increase rapidly downward with increasing optical depth (decreasing height). The faster growth may be caused by the evaporation-condensation mechanism. Over land, however, large droplets are unlikely to appear in the early stage possibly because continental aerosols prevent the evaporation-condensation mechanism to occur, resulting in partially delayed drizzle formation.

Deconstructing the precipitation susceptibility construct: Improving methodology for aerosol‐cloud precipitation studies

It is generally thought that an increase in aerosol particles suppresses precipitation in warm clouds. The nature and magnitude of this effect are highly uncertain owing to numerous microphysical and macrophysical processes that influence clouds over a wide range of spatial and temporal scales. This work addresses the need to improve the evidence for and quantification of aerosol effects on precipitation by using observational data. Previous work introduced the concept of precipitation susceptibility as a metric for changes in precipitation that result from aerosol perturbations. Motivated by the difficulty in obtaining statistically significant aerosol measurements in the vicinity of clouds, this study explores breaking up the precipitation susceptibility construct into separate components: an aerosol‐cloud interaction component and a cloud‐precipitation component. These are used to quantify precipitation susceptibility, while also accounting for meteorological factors that could obfuscate the response of clouds to aerosol perturbations. The utility of this technique is demonstrated using a diverse set of tools, including data from NASA's A‐Train constellation of satellites, aircraft measurements, and models of various complexities. Employing this method results in increased confidence in causal relationships between aerosol perturbations and precipitation.

The validity of the kinetic collection equation revisited – Part 2: Simulations for the hydrodynamic kernel

Abstract. The kinetic collection equation (KCE) has been widely used to describe the evolution of the average droplet spectrum due to the collection process that leads to the development of precipitation in warm clouds. This deterministic, integro-differential equation only has analytic solution for very simple kernels. For more realistic kernels, the KCE needs to be integrated numerically. In this study, the validity time of the KCE for the hydrodynamic kernel is estimated by a direct comparison of Monte Carlo simulations with numerical solutions of the KCE. The simulation results show that when the largest droplet becomes separated from the smooth spectrum, the total mass calculated from the numerical solution of the KCE is not conserved and, thus, the KCE is no longer valid. This result confirms the fact that for kernels appropriate for precipitation development within warm clouds, the KCE can only be applied to the continuous portion of the mass distribution.

A new statistically based autoconversion rate parameterization for use in large‐scale models

The autoconversion rate is a key process for the formation of precipitation in warm clouds. In climate models, physical processes such as autoconversion rate, which are calculated from grid mean values, are biased, because they do not take subgrid variability into account. Recently, statistical cloud schemes have been introduced in large‐scale models to account for partially cloud‐covered grid boxes. However, these schemes do not include the in‐cloud variability in their parameterizations. In this paper, a new statistically based autoconversion rate considering the in‐cloud variability is introduced and tested in three cases using the Canadian Single Column Model (SCM) of the global climate model. The results show that the new autoconversion rate improves the model simulation, especially in terms of liquid water path in all three case studies.

Coalescence Efficiency Measurements for Minimally Charged Cloud Drops

Laboratory measurements were made of the collision cross section for water drops freely falling in air to evaluate coalescence efficiencies for cloud drops of 55‐105-mm radius in a radius ratio range of 0.5‐1.0. The charge on drops was minimized by control of the electric field at the location of drop formation and was further reduced using bipolar ions. The drop charge was below values expected in the developing stage of warm cloud precipitation and well below thresholds needed to promote coalescence. The horizontal distance between drop centers, measured from photographs, provided data on collision offsets that were used to determine coalescence efficiencies. Experiment uncertainty was calculated from numerical trials using random offsets with variance from measurement errors. The analysis demonstrated that the coalescence efficiency for collisions between cloud drops is certainly greater than 91% and likely greater than 95%. Because the present experiment shows that colliding cloud drops with minimal charge will coalesce, rather than bounce apart, the rate of growth of cloud drops by collision‐coalescence can be calculated using available theoretical collision efficiencies based on the interactions of uncharged spheres.

Precipitation and radiation modeling in a general circulation model: Introduction of cloud microphysical processes

Cloud microphysical processes are introduced in the precipitation parameterization of a general circulation model (GCM). Three microphysical processes are included in this representation of warm cloud precipitation: autoconversion of droplets, collection of droplets by falling raindrops, and evaporation of raindrops falling in clear sky. The mean droplet radius, r, is calculated from the cloud water mixing ratio, which is computed in the model, and the cloud droplet number concentration, N, which is prescribed. The autoconversion rate is set to zero for r < r0, a prescribed threshold mean droplet radius. We investigate the model sensitivity to r0 and to N, the cloud droplet concentration, which is linked to the concentration of cloud condensation nuclei and is likely to vary. We find that an increase in N leads to an increase in the amount of cloud water stored in the atmosphere. In further experiments the mean droplet radius used in the parameterization of cloud optical properties is calculated in the same way as in the precipitation parameterization in order to bring more consistency between the different schemes. We again investigate the model sensitivity to r0 and to N and we find that an increase in N significantly enhances cloud albedo.

Modification of Precipitation from Warm Clouds—A Review

Abstract This review is begun with a brief summary of the current status of our understanding of the physics of precipitation in warm clouds. The impact of warm-cloud precipitation processes on the evolution of the ice phase in supercooled clouds also is discussed. This is followed by a review of experimental attempts to modify the microstructure of warm clouds. Modeling studies of warm cloud modification and observational studies of inadvertent warm cloud modification also are drawn upon to further elucidate the physics of warm cloud modification. The hypotheses, and evidence, for dynamic modification of warm clouds are then discussed. A few brief comments an modeling of warm cloud processes also are given. These comments are intended to serve as a warning to the non-modeler to be very cautious in taking the results of the modeling studies at face value. Finally, the review is concluded with specific recommendations regarding the current status of warm cloud modification, and future directions for the sc...

Surface Measurements of the Electrical Properties of Warm Clouds over the Sea

THE electrical properties associated with warm cloud precipitation have been measured routinely at Hilo, on the windward coast of the Island of Hawaii, for the past three years. Warm clouds in this locality typically form over the ocean east of the island and move inland with the north-east trade wind. The present series of measurements is part of a continuing investigation of the mechanism of charge separation in warm clouds and of how such processes may be related to thunderstorm electricity. It is important to study the electrical properties of non-freezing clouds, which consist only of liquid cloud droplets, to clarify the role of the liquid phase in the complex liquid-ice continuum of thunderstorms, and changes in electrical properties also are intimately associated with the dynamic behaviour of clouds. We believe that atmospheric electrical measurements will provide useful information on the growth and development of warm clouds.