Ice-phase Microphysical Processes Research Articles

Effects of anthropogenic aerosols and sea salt aerosols during a summer precipitation event in the Yangtze River Delta

Currently, we know little about the effects of anthropogenic aerosols (AAs) and sea salt aerosols (SSAs) on heavy precipitation over the densely populated coastal areas. In this view, we developed a local aerosol activation parametrization scheme which conforms to the Yangtze River Delta (YRD) characteristics using cloud condensation nucleus (CCN) with supersaturation at 0.4 (CCN0.4) and aerosol optical depth (AOD) data at 550 nm (AOD550) to improve the model performance. Meanwhile, we simulated a heavy precipitation over the YRD during early summer using the WRF-Chem model (version 4.1) with the Morrison microphysics scheme. Increased AAs and SSAs presented opposite responses in droplets concentrations and precipitation intensity. The increased AAs increased the aerosol concentrations, leading to more but smaller droplets; and the cloud droplets mixing ratio Qc was slightly increased by 17.7% due to the weakened auto-conversion from droplets to raindrops, while the rain mixing ratio Qr was decreased by about 5.7%. On the contrary, presence of SSAs increased the effective (EFC) radius of cloud droplets because of the larger particles. Along with increased AAs, due to the influences of decreased Qr and smaller droplets, probability density function (PDF) of the hourly surface rain rates shifted to the smaller values (< 0.5 mm/h), which was caused by lowered latent heat release resulting from the enhanced rain evaporative cooling and weakened condensation rate, weakening the convective intensity and updrafts. Meanwhile, decreased updrafts resulted in fewer cloud droplets being transformed to the upper air, less accretion of droplets by ice crystal and snow, and reduced ice mixing ratio Qi and snow mixing ratio Qs. The averaged surface precipitation accumulated for 24 h decreased by about 7.2%, and the intensity of radar reflectivity was decreased by about 7.0%. In contrast, presence of SSAs enhanced the auto-conversion droplets and weakened the evaporation of raindrops, thus increasing the Qr. In this case, the 24-h accumulated averaged surface rainfall was increased by about 30.1% with the larger radar reflectivity and increased area of large reflectivity values (> 20 dbz). The latent heat release and related microphysical processes were enhanced due to the changes of conversion rate, which, together with the increased Qr and larger droplets, strengthened the precipitation, increased the incidence of heavier precipitation (> 2 mm/h), and enhanced the ice-phase microphysical processes.

Investigation of aerosol effects on a deep convection case in Southeast China with budget analysis of hydrometeor mass and latent heat

It remains an open question which microphysical processes determine the sign of aerosol effects on deep convective rain. This lack of knowledge prevents us from understanding the uncertainties of the aerosol effects on deep convection. We try to answer this question through a cloud-resolving model (CRM) simulation on a summer storm that occurred in Southeast China, by analyzing both column hydrometeor mass and latent heat budget and quantifying the contribution of each microphysical process to the rain variations with aerosol. Our results show that warm-phase microphysical processes have stronger contributions (52% −99%) to the rain variations than ice-phase microphysical processes (1%−48%) when gradually increasing cloud condensation nuclei (CCN) concentration from 125 cm−3 (clean environment) to 16,000 cm−3 (highly polluted environment). Condensation and evaporation of cloud droplets are identified as two crucial microphysical processes, both of which are enhanced with increased aerosol loading. The competition between them (i.e., net condensation) essentially determines the sign of aerosol effects on surface rain. We also explore aerosol effects on updraft velocity from the perspective of buoyancy and latent heat budget, it reveals that the largely enhanced convective updrafts below 7 km are attributed to the increase of condensation heating. While the competition between enhanced condensation heating and suppressed deposition heating restricts the aerosol-induced convective invigoration above 7 km. This study highlights the dominant role of net condensation in the aerosol-imposed invigoration of deep convection under highly polluted environments, suggesting that an explicit treatment of vapor diffusion processes is necessary to simulate aerosol effects on deep convection more accurately.

Effects of anthropogenic and sea salt aerosols on a heavy rainfall event during the early-summer rainy season over coastal Southern China

Effects of anthropogenic and sea salt aerosols (AA and SSA, respectively) on heavy rainfall in the densely populated monsoon coastal areas remain poorly known. To address this issue, a representative heavy rainfall event over coastal Southern China is simulated using the Weather Research and Forecasting model coupled to Chemistry (WRF-Chem). Results show that the initial responses of clouds to AA and SSA are opposite due to their different size distributions: increasing the AA emission by tenfold leads to larger mixing ratio and number concentration of cloud droplets (Qc and Nc) with reduced size; on the contrary, presence of SSA results in less but larger cloud droplets as activation of smaller aerosols is inhibited on the order of 105 to 106 m−3 due to increased competition for the supersaturated water vapor between SSA and other smaller particles as cloud condensation nuclei. In the polluted environment, rain number concentration (Nr) is reduced to about a half of that in the clean environment due to weakened auto-conversion from cloud droplets to raindrops. The reduced Nr combined with enhanced accretion of cloud droplets by raindrops result in larger mean size and faster mean fall speed of raindrops, therefore probability distribution of surface hourly rain rate shifts toward values > about 2 mm h−1. The 24-h rainfall accumulation increases up to about 80 mm at the centers, although the region-averaged amount increases only 5.32%. Meanwhile, increasing the AA emission decreases total mixing ratio of ice-phase hydrometeors while the previously reported convective intensification is not evident. When the SSA emission is included in the polluted environment, the significantly inhibited activation of submicron aerosols leads to weakening of the following liquid-, mixed-, and ice-phase microphysical processes and the convective intensity to some extents. The probability distribution of hourly rain rates shifts toward values < about 5 mm h−1, while the 24-h rainfall accumulation hardly changes significantly with a decrease of 4.19% in the regionally averaged amount. The overall small impacts of aerosols, SSA in particular, are related to the strong synoptic forcing governing the heavy rainfall production in this case.

Evaluation of Microphysics Schemes in Tropical Cyclones Using Polarimetric Radar Observations: Convective Precipitation in an Outer Rainband

AbstractCloud microphysics significantly impact tropical cyclone precipitation. A prior polarimetric radar observational study by the authors revealed the ice-phase microphysical processes as the dominant microphysics mechanisms responsible for the heavy precipitation in the outer rainband of Typhoon Nida (2016). To assess the model performance regarding microphysics, three double-moment microphysics schemes (i.e., Thompson, Morrison, and WDM6) are evaluated by performing a set of simulations of the same case. While these simulations capture the outer rainband’s general structure, microphysics in the outer rainbands are strikingly different from the observations. This discrepancy is primarily attributed to different microphysics parameterizations in these schemes, rather than the differences in large-scale environments due to cloud–environment interactions. An interesting finding in this study is that the surface rain rate or liquid water content is inversely proportional to the simulated mean raindrop sizes. The mass-weighted raindrop diameters are overestimated in the Morrison and Thompson schemes and underestimated in the WDM6 scheme, while the former two schemes produce lower liquid water content than WDM6. Compared with the observed ice water content based on a new polarimetric radar retrieval method, the ice water content above the environmental 0°C level in all simulations is highly underestimated, especially at heights above 12 km MSL where large concentrations of small ice particles are typically prevalent. This finding suggests that the improper treatment of ice-phase processes is potentially an important error source in these microphysics schemes. Another error source identified in the WDM6 scheme is overactive warm-rain processes that produce excessive concentrations of smaller raindrops.

Numerical Simulation Study on the Microphysical Processes of Typhoon Wipha Using a New Moisture Conditioning Method

Yang, W.X.; Hu, Z.X.; Lei, H.C., and Li, B.D., 2020. Numerical simulation study on the microphysical processes of Typhoon Wipha using a new moisture conditioning method. In: Guido Aldana, P.A. and Kantamaneni, K. (eds.), Advances in Water Resources, Coastal Management, and Marine Science Technology. Journal of Coastal Research, Special Issue No. 104, pp. 156–159. Coconut Creek (Florida), ISSN 0749-0208.With the non-static mesoscale model ARPS (Advanced Regional Prediction System), by using the NCEP/ NCAR global final analysis data, and the global surface and sounding data under MICAPS (Meteorological Information Comprehensive Analysis and Process System) assimilated in 4 dimensions as the initial field and lateral boundary conditions, the initial moisture field is adjusted according to the surface accumulated rain of each 6 h, and the triple nested simulation is conducted to produce 3 dimensional high time -spatial (Wipha) are analysed. The results are significant for understanding the cloud structure and the precipitation forming mechanisms of the Typhoon storm. It is found that the ice phase microphysical processes are the key storm mechanisms in different periods. The melting graupel confirms that the rain is the most important mechanism. The intensity of the production of graupel varies in accordance with the Typhoon.

Effects of Anthropogenic Aerosols on a Heavy Rainstorm in Beijing

A heavy rainstorm occurred in Beijing on 19–20 July 2016. The Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) was used to investigate the effects of anthropogenic aerosols on precipitation and microphysical processes. Three conditions were simulated by altering the anthropogenic emissions. When the anthropogenic emissions were increased by 10 times, the area-average accumulated rainfall amount and maximum accumulated rainfall amount both decreased. The cloud water mixing ratio increased and the rain mixing ratio decreased. The radii of cloud droplets decreased, and the collision efficiency of cloud water by rain and the autoconversion rate of cloud water into rain were both low. When the anthropogenic emission was 10% of the original emission, the area of accumulated rainfall amounts greater than 25 mm in the Beijing area was 10% larger than those of the other two tests. The collision efficiency and autoconversion rate of cloud water into rain were high for large contact areas and large cloud droplets. The graupel mixing ratio was the largest. Thus, the process of melting of graupel into rain was the largest. In the WRF-Chem model, the aerosols did not participate as ice nuclei (IN) in the ice-phase microphysical processes, and therefore the aerosols could influence only the warm rain processes and mix-phased processes near the freezing level line. For no influence on ice-phase microphysical processes, the snow and ice mixing ratios did not show many differences among the different tests.

Differences between Convective and Stratiform Precipitation Budgets in a Torrential Rainfall Event

Differences in rainfall budgets between convective and stratiform regions of a torrential rainfall event were investigated using high-resolution simulation data produced by the Weather Research and Forecasting (WRF) model. The convective and stratiform regions were reasonably separated by the radar-based convective–stratiform partitioning method, and the three-dimensional WRF-based precipitation equation combining water vapor and hydrometeor budgets was further used to analyze the rainfall budgets. The results showed that the magnitude of precipitation budget processes in the convective region was one order larger than that in the stratiform region. In convective/stratiform updraft regions, precipitation was mainly from the contribution of moisture-related processes, with a small negative contribution from cloud-related processes. In convective/ stratiform downdraft regions, cloud-related processes played positive roles in precipitation, while moisture-related processes made a negative contribution. Moisture flux convergence played a dominant role in the moisture-related processes in convective or stratiform updraft regions, which was closely related to large-scale dynamics. Differences in cloud-related processes between convective and stratiform regions were more complex compared with those in moisture-related processes. Both liquid- and ice-phase microphysical processes were strong in convective/stratiform updraft regions, and ice-phase processes were dominant in convective/stratiform downdraft regions. There was strong net latent heating within almost the whole troposphere in updraft regions, especially in the convective updraft region, while the net latent heating (cooling) mainly existed above (below) the zero-layer in convective/stratiform downdraft regions.

Kinematics and Microphysics of Convection in the Outer Rainband of Typhoon Nida (2016) Revealed by Polarimetric Radar

Abstract This study analyzes the microphysics of convective cells in an outer rainband of Typhoon Nida (2016) using data collected by a newly upgraded operational polarimetric radar in China. The life cycle of these convective cells is divided into three stages: developing, mature, and decaying according to the intensity of the corresponding updraft. Composite analysis shows that deep columns of ZDR and KDP collocate well with the enhanced updraft as the cells develop to their mature stage. A layered microphysical structure is observed in the ice region with riming near the −5°C level within the updraft, aggregation around the −15°C level, and deposition anywhere above the 0°C level. These ice-phase microphysical processes are important pathways of particle growth in the outer rainbands. In particular, riming contributes significantly to surface heavy rainfall. These contrast to previously documented inner rainbands, where warm-rain processes are the predominant pathway of particle growth.

Investigation of the Effects of Anthropogenic Pollution on Typhoon Precipitation and Microphysical Processes Using WRF-Chem

Abstract Taking Typhoon Usagi (2013) as an example, this study used the Weather Research and Forecasting Model with Chemistry to investigate the influence of anthropogenic aerosols on typhoons. Three simulations (CTL, CLEAN, EXTREME) were designed according to the emission intensity of the anthropogenic pollution. The results showed that although anthropogenic pollution did not demonstrate clear influence on the track and strength of the typhoon, it clearly changed the precipitation, distribution of water hydrometeors, and microphysical processes. In the CLEAN experiment, the precipitation rate declined because cloud water collected by the rain decreased. Similarly, the precipitation rate decreased in the EXTREME experiment, because the autoconversion of cloud water to rain was restrained. Regarding precipitation type, the rate of stratiform precipitation in both the CLEAN and the EXTREME simulations was suppressed because the ice-phase microphysical processes weakened. Compared with the CTL run, the rate of stratiform precipitation at the periphery of the typhoon was reduced by about 28% in both the CLEAN and the EXTREME simulations. Moreover, the rate of convective precipitation within 140–160 km of the center of the typhoon in the EXTREME experiment was about 33% greater than in the CTL simulation. This increase was triggered by new convection at the periphery in the EXTREME simulation related to cloud water reevaporation. Finally, compared with the CTL experiment, the peaks of both convective and mixed precipitation in the CLEAN and EXTREME experiments shifted 10 km toward the typhoon periphery.

Mechanisms of the Negative Shortwave Cloud Feedback in Middle to High Latitudes

Abstract Increases in cloud optical depth and liquid water path (LWP) are robust features of global warming model simulations in high latitudes, yielding a negative shortwave cloud feedback, but the mechanisms are still uncertain. Here the importance of microphysical processes for the negative optical depth feedback is assessed by perturbing temperature in the microphysics schemes of two aquaplanet models, both of which have separate prognostic equations for liquid water and ice. It is found that most of the LWP increase with warming is caused by a suppression of ice microphysical processes in mixed-phase clouds, resulting in reduced conversion efficiencies of liquid water to ice and precipitation. Perturbing the temperature-dependent phase partitioning of convective condensate also yields a small LWP increase. Together, the perturbations in large-scale microphysics and convective condensate partitioning explain more than two-thirds of the LWP response relative to a reference case with increased SSTs, and capture all of the vertical structure of the liquid water response. In support of these findings, a very robust positive relationship between monthly mean LWP and temperature in CMIP5 models and observations is shown to exist in mixed-phase cloud regions only. In models, the historical LWP sensitivity to temperature is a good predictor of the forced global warming response poleward of about 45°, although models appear to overestimate the LWP response to warming compared to observations. The results indicate that in climate models, the suppression of ice-phase microphysical processes that deplete cloud liquid water is a key driver of the LWP increase with warming and of the associated negative shortwave cloud feedback.

Numerical Study of Precipitation Intensification and Ice-Phase Microphysical Processes in Typhoon Spiral Band

Numerical Study of Precipitation Intensification and Ice-Phase Microphysical Processes in Typhoon Spiral Band

A numerical investigation of a slow-moving convective line in a weakly sheared environment

A series of three-dimensional, cloud-resolving numerical simulations are performed to examine a slowpropagating, quasi-two-dimensional convective system in a weakly sheared environment during the Tropical Rainfall Measuring Mission Large-Scale Biosphere-Atmosphere (TRMM-LBA) field campaign. The focus is on the kinematics and thermodynamics, organization mechanisms, and dynamical effects of low-level shear, ice microphysics and tropospheric humidity. The control simulation, which is initialized with the observed sounding and includes full microphysics, successfully replicates many observed features of the convective system, such as the linear structure, spatial orientation, life cycle, and sluggish translation. The system at the mature stage displays a line-normal structure similar to that associated with squalltype convective systems, but the corresponding mesoscale circulation and thermodynamic modification are much weaker. Ice-phase microphysical processes are not necessary to the formation of the convective system, but they play a non-trivial role in the late evolution stage. In contrast, the low-level shear, albeit shallow and weak, is critical to the realistic realization of the convective line. The tropospheric moisture above the planetary boundary layer has an important impact on the behavior of convective organization. In particular, a dry layer in the lower troposphere significantly suppresses convective development and inhibits the generation of organized convection even though the convective available potential energy is substantial. The free-atmosphere humidity has received little attention in previous studies of organized convection and warrants further investigation.

Precipitation Parameterization in a Simulated Mei-Yu Front

Observational and numerical studies have consistently shown the importance of latent heat release associated with frontal precipitation in the development of a Mei-Yu front. However, a systematic evaluation of precipitation parameterization in the simulation of a Mei-Yu front has been rare in the literature. In order to enhance our understanding on precipitation parameterization of Mei-Yu fronts, this study conducts numerical experiments to evaluate the performance of subgrid-scale cumulus schemes and resolvable-scale microphysics schemes to simulate the Mei-Yu frontal system on 4-5 June 1998 at grid resolutions of 45 km and 15 km, using the Penn State/NCAR mesoscale model MM5. Principal findings are summarized here. The horizontal extent and intensity of precipitation, the partitioning off precipitation into grid-resolvable and subgrid-scale portions, the vertical thermodynamic profile in the precipitation region and the embedded mesoscale structure are extremely sensitive to the choice of cumulus parameterization schemes. This is true for both the 45- and 15-km grids. The partitioning of precipitation into subgrid scale and resolvable scale is sensitive to the particular cumulus parameterization that is used in the model, but it is nearly the same on both the 4S- and IS-km grids for a given cumulus parameterization. The detailed ice-phase microphysical processes do not have a significant impact on the rainfall pattern on either the 45- and 15-km grids. However, the inclusion of cloud ice-snow-graupel microphysical processes increases the total surface precipitation amount by 30% compared to the amount with only warm raid processes. Variations in the subgrid-scale cumulus parameterization have a much larger impact on the distribution and amount of Mei-Yu frontal precipitation than do variations in the resolvable-scale microphysics parameterization at mesoscale grid resolutions of 10-50 km.

A modeling study of the effects of ice-phase microphysical processes on trace chemical removal efficiencies

A simple cloud-chemistry model has been developed to study the effects of specific microphysical processes of precipitation formation and scavenging on the efficiency with which clouds remove trace chemicals form the atmosphere. The model is designed to let an isolated precipitation particle fall through the mixed-phase zone of a steady-state background cloud derived from the ascent of a Lagrangian parcel in which given mixing ratios of SO 2, CO 2 and NH 3 are in equilibrium with the aqueous phase. The precipitation particle is allowed to acquire water mass by both vapor deposition and accretion of supercooled cloud water and solute content by gaseous entrapment in rime ice, as parameterized from prior laboratory experiments. The concept of a relative removal efficiency has been employed to identify the important microphysical fractionation processes within the cloud. The model calculations show that the removal efficiencies of the chemical constituents in the cloud nuclei and soluble trace gases are controlled largely by the accretional growth process. Even with modest solubilities, a trace gas such as SO 2 is estimated to be removed much more efficiently than is water vapor via cold-cloud precipitation because of substantial entrapment of the gas during rime-ice formation.

Sulfate Production and Deposition in Midlatitude Continental Cumulus Clouds. Part II: Chemistry Model Formulation and Sensitivity Analysis

Abstract We examine characteristics of in-cloud sulfate chemistry using a 1.5-dimensional Eulerian cumulus cloud model. Specifically we present here a study of (i) the relative importance of aerosol scavenging to in-cloud production of sulfate through oxidation of sulfur dioxide by ozone and hydrogen peroxide, (ii) the importance of the entrainment of environmental air in the formation and subsequent deposition of sulfate, (iii) the role of ice phase microphysical processes in determining the chemical properties of precipitation, and (iv) the chemical effects on clouds due to a modification of the environment by previous clouds. The results for a continental background or moderately polluted atmosphere indicate that aerosol scavenging accounts for between 50% and 80% of the in-cloud sulfate ultimately deposited; that ozone oxidation of sulfur dioxide may be important in the upper regions of the cloud; that although entrainment of environmental air is important in the cloud dynamics and microphysics, it is...