scholarly journals Are simulated aerosol-induced effects on deep convective clouds strongly dependent on saturation adjustment?

2012 ◽  
Vol 12 (4) ◽  
pp. 10059-10114 ◽  
Author(s):  
Z. J. Lebo ◽  
H. Morrison ◽  
J. H. Seinfeld
Keyword(s):  
Mass Flux ◽  
Cold Pool ◽  
Latent Heating ◽  
Time Step ◽  
Microphysics Scheme ◽  
Surface Precipitation ◽  
Convective Clouds ◽  
Aerosol Loading ◽  
Deep Convective Clouds ◽  
Bulk Model

Abstract. Three configurations of a bulk microphysics scheme in conjunction with a detailed bin scheme are implemented in the Weather Research and Forecasting (WRF) model to specifically address the role of the saturation adjustment assumption (i.e., condensing/evaporating the surplus/deficit water vapor relative to saturation in one time step) on aerosol-induced invigoration of deep convective clouds. The bulk model configurations are designed to treat cloud droplet condensation/evaporation using either saturation adjustment, as employed in most bulk models, or an explicit representation of supersaturation over a time step, as used in bin models. Results demonstrate that the use of saturation adjustment artificially enhances condensation and latent heating at low levels and limits the potential for an increase in aerosol concentration to increase buoyancy at mid to upper levels. This leads to a small weakening of the time- and domain-averaged convective mass flux (~ -3%) in polluted compared to clean conditions. In contrast, the bin model and bulk scheme with explicit prediction of supersaturation simulate an increase in latent heating aloft and the convective updraft mass flux is weakly invigorated (~5%). The bin model also produces a large increase in domain-mean cumulative surface precipitation in polluted conditions (~18%), while all of the bulk model configurations simulate little change in precipitation. Finally, it is shown that the cold pool weakens substantially with increased aerosol loading when saturation adjustment is applied, which acts to reduce the low-level convergence and weaken the convective dynamics. With an explicit treatment of supersaturation in the bulk and bin models there is little change in cold pool strength, so that the convective response to polluted conditions is influenced more by changes in latent heating aloft. It is concluded that the use of saturation adjustment can explain differences in the response of cold pool evolution and convective dynamics with aerosol loading simulated by the bulk and bin models, but cannot explain large differences in the response of surface precipitation between these models.

scholarly journals Are simulated aerosol-induced effects on deep convective clouds strongly dependent on saturation adjustment?

2012 ◽  
Vol 12 (20) ◽  
pp. 9941-9964 ◽  
Author(s):  
Z. J. Lebo ◽  
H. Morrison ◽  
J. H. Seinfeld
Keyword(s):  
Mass Flux ◽  
Cold Pool ◽  
Latent Heating ◽  
Time Step ◽  
Microphysics Scheme ◽  
Surface Precipitation ◽  
Convective Clouds ◽  
Aerosol Loading ◽  
Deep Convective Clouds ◽  
Bulk Model

Abstract. Three configurations of a bulk microphysics scheme in conjunction with a detailed bin scheme are implemented in the Weather Research and Forecasting (WRF) model to specifically address the role of the saturation adjustment assumption (i.e., condensing/evaporating the surplus/deficit water vapor relative to saturation in one time step) on aerosol-induced invigoration of deep convective clouds. The bulk model configurations are designed to treat cloud droplet condensation/evaporation using either saturation adjustment, as employed in most bulk models, or an explicit representation of supersaturation over a time step, as used in bin models. Results demonstrate that the use of saturation adjustment artificially enhances condensation and latent heating at low levels and limits the potential for an increase in aerosol concentration to increase buoyancy at mid to upper levels. This leads to a small weakening of the time- and domain-averaged convective mass flux (~-3%) in polluted compared to clean conditions. In contrast, the bin model and bulk scheme with explicit prediction of supersaturation simulate an increase in latent heating aloft and the convective updraft mass flux is weakly invigorated (~5%). The bin model also produces a large increase in domain-mean cumulative surface precipitation in polluted conditions (~18%), while all of the bulk model configurations simulate little change in precipitation. Finally, it is shown that the cold pool weakens substantially with increased aerosol loading when saturation adjustment is applied, which acts to reduce the low-level convergence and weaken the convective dynamics. With an explicit treatment of supersaturation in the bulk and bin models there is little change in cold pool strength, so that the convective response to polluted conditions is influenced more by changes in latent heating aloft. It is concluded that the use of saturation adjustment can explain differences in the response of cold pool evolution and convective dynamics with aerosol loading simulated by the bulk and bin models, but cannot explain large differences in the response of surface precipitation between these models.

scholarly journals Theoretical basis for convective invigoration due to increased aerosol concentration

2011 ◽  
Vol 11 (11) ◽  
pp. 5407-5429 ◽  
Author(s):  
Z. J. Lebo ◽  
J. H. Seinfeld
Keyword(s):  
Number Concentration ◽  
Latent Heating ◽  
Heating Rates ◽  
Microphysics Scheme ◽  
Convective Clouds ◽  
Aerosol Loading ◽  
Significant Difference ◽  
Deep Convective Clouds ◽  
Bin Microphysics ◽  
Cumulative Precipitation

Abstract. The potential effects of increased aerosol loading on the development of deep convective clouds and resulting precipitation amounts are studied by employing the Weather Research and Forecasting (WRF) model as a detailed high-resolution cloud resolving model (CRM) with both detailed bulk and bin microphysics schemes. Both models include a physically-based activation scheme that incorporates a size-resolved aerosol population. We demonstrate that the aerosol-induced effect is controlled by the balance between latent heating and the increase in condensed water aloft, each having opposing effects on buoyancy. It is also shown that under polluted conditions, increases in the CCN number concentration reduce the cumulative precipitation due to the competition between the sedimentation and evaporation/sublimation timescales. The effect of an increase in the IN number concentration on the dynamics of deep convective clouds is small and the resulting decrease in domain-averaged cumulative precipitation is shown not to be statistically significant, but may act to suppress precipitation. It is also shown that even in the presence of a decrease in the domain-averaged cumulative precipitation, an increase in the precipitation variance, or in other words, andincrease in rainfall intensity, may be expected in more polluted environments, especially in moist environments. A significant difference exists between the predictions based on the bin and bulk microphysics schemes of precipitation and the influence of aerosol perturbations on updraft velocity within the convective core. The bulk microphysics scheme shows little change in the latent heating rates due to an increase in the CCN number concentration, while the bin microphysics scheme demonstrates significant increases in the latent heating aloft with increasing CCN number concentration. This suggests that even a detailed two-bulk microphysics scheme, coupled to a detailed activation scheme, may not be sufficient to predict small changes that result from perturbations in aerosol loading.

scholarly journals Theoretical basis for convective invigoration due to increased aerosol concentration

2011 ◽  
Vol 11 (1) ◽  
pp. 2773-2842 ◽  
Author(s):  
Z. J. Lebo ◽  
J. H. Seinfeld
Keyword(s):  
Cloud Microphysics ◽  
Number Concentration ◽  
Latent Heating ◽  
Microphysics Scheme ◽  
Convective Clouds ◽  
Aerosol Loading ◽  
Significant Difference ◽  
Deep Convective Clouds ◽  
Bin Microphysics ◽  
Cumulative Precipitation

Abstract. The potential effects of increased aerosol loading on the development of deep convective clouds and resulting precipitation amounts are studied by employing the Weather Research and Forecasting (WRF) model as a detailed high-resolution cloud resolving model (CRM) with both detailed bulk and bin microphysics schemes. The bulk microphysics scheme incorporates a physically based parameterization of cloud droplet activation as well as homogeneous and heterogeneous freezing in order to explicitly resolve the possible aerosol-induced effects on the cloud microphysics. These parameterizations allow one to segregate the effects of an increase in the aerosol number concentration into enhanced cloud condensation nuclei (CCN) and/or ice nuclei (IN) concentrations using bulk microphysics. The bin microphysics scheme, with its explicit calculations of cloud particle collisions, is shown to better predict cumulative precipitation. Increases in the CCN number concentration may not have a monotonic influence on the cumulative precipitation resulting from deep convective clouds. We demonstrate that the aerosol-induced effect is controlled by the balance between latent heating and the increase in condensed water aloft, each having opposing effects on buoyancy. It is also shown that under polluted conditions and in relatively dry environments, increases in the CCN number concentration reduce the cumulative precipitation due to the competition between the sedimentation and evaporation/sublimation timescales. The effect of an increase in the IN number concentration on the dynamics of deep convective clouds is small, but may act to suppress precipitation. A comparison of the predictions using the bin and bulk microphysics schemes demonstrate a significant difference between the predicted precipitation and the influence of aerosol perturbations on updraft velocity within the convective core. The bulk microphysics scheme is shown to be unable to capture the changes in latent heating that occur as a result of changes in the CCN number concentration, while the bin microphysics scheme demonstrates significant increases in the latent heating aloft with increasing CCN number concentration. This suggests that a detailed two-bulk microphysics scheme, which is more computationally efficient than bin microphysics schemes, may not be sufficient, even when coupled to a detailed activation scheme, to predict small changes that result from perturbations in aerosol loading.

scholarly journals Sensitivity study of the aerosol effects on a supercell storm throughout its lifetime

2014 ◽  
Vol 14 (17) ◽  
pp. 24087-24118 ◽  
Author(s):  
A. Takeishi ◽  
T. Storelvmo
Keyword(s):  
Droplet Size ◽  
Cloud Condensation Nuclei ◽  
Wrf Model ◽  
Convective Cloud ◽  
Sensitivity Study ◽  
Cloud Microphysics ◽  
Microphysics Scheme ◽  
Convective Clouds ◽  
Aerosol Loading ◽  
Deep Convective Clouds

Abstract. An increase in atmospheric aerosol loading could alter the microphysics, dynamics, and radiative characteristics of deep convective clouds. Earlier modeling studies have shown that the effects of increased aerosols on the amount of precipitation from deep convective clouds are model-dependent. This study aims to understand the effects of increased aerosol loading on a deep convective cloud throughout its lifetime with the use of the Weather Research and Forecasting (WRF) model as a cloud-resolving model (CRM). It simulates an idealized supercell thunderstorm with 8 different aerosol loadings, for three different cloud microphysics schemes. Variation in aerosol concentration is mimicked by varying either cloud droplet number concentration or the number of activated cloud condensation nuclei. We show that the sensitivity to aerosol loading is dependent on the choice of microphysics scheme. For the schemes that are sensitive to aerosols loading, the production of graupel via riming of snow is the key factor determining the precipitation response. The formulation of snow riming depends on the microphysics scheme and is usually a function of two competing effects, the size effect and the number effect. In many simulations, a decrease in riming is seen with increased aerosol loading, due to the decreased droplet size that lowers the riming efficiency drastically. This decrease in droplet size also results in a delay in the onset of precipitation, as well as so-called warm rain suppression. Although these characteristics of convective invigoration (Rosenfeld et al., 2008) are seen in the first few hours of the simulations, variation in the accumulated precipitation mainly stems from graupel production rather than convective invigoration. These results emphasize the importance of accurate representations of graupel formation in microphysics schemes.

scholarly journals Non-Monotonic Dependencies of Cloud Microphysics and Precipitation on Aerosol Loading in Deep Convective Clouds: A Case Study Using the WRF Model with Bin Microphysics

Atmosphere ◽  
2018 ◽  
Vol 9 (11) ◽  
pp. 434 ◽  
Author(s):  
Ye-Lim Jeon ◽  
Sungju Moon ◽  
Hyunho Lee ◽  
Jong-Jin Baik ◽  
Jambajamts Lkhamjav
Keyword(s):  
Precipitation Amount ◽  
Cloud Microphysics ◽  
Ice Crystals ◽  
Precipitation Event ◽  
Cold Pool ◽  
Convective Clouds ◽  
Aerosol Loading ◽  
Optimal Value ◽  
Deep Convective Clouds ◽  
Bin Microphysics

Aerosol-cloud-precipitation interactions in deep convective clouds are investigated through numerical simulations of a heavy precipitation event over South Korea on 15–16 July 2017. The Weather Research and Forecasting model with a bin microphysics scheme is used, and various aerosol number concentrations in the range N0 = 50–12,800 cm−3 are considered. Precipitation amount changes non-monotonically with increasing aerosol loading, with a maximum near a moderate aerosol loading (N0 = 800 cm−3). Up to this optimal value, an increase in aerosol number concentration results in a greater quantity of small droplets formed by nucleation, increasing the number of ice crystals. Ice crystals grow into snow particles through deposition and riming, leading to enhanced melting and precipitation. Beyond the optimal value, a greater aerosol loading enhances generation of ice crystals while the overall growth of ice hydrometeors through deposition stagnates. Subsequently, the riming rate decreases because of the smaller size of snow particles and supercooled drops, leading to a decrease in ice melting and a slight suppression of precipitation. As aerosol loading increases, cold pool and low-level convergence strengthen monotonically, but cloud development is more strongly affected by latent heating and convection within the system that is non-monotonically reinforced.

scholarly journals Concurrent Sensitivities of an Idealized Deep Convective Storm to Parameterization of Microphysics, Horizontal Grid Resolution, and Environmental Static Stability

2015 ◽  
Vol 143 (6) ◽  
pp. 2082-2104 ◽  
Author(s):  
Hugh Morrison ◽  
Annareli Morales ◽  
Cecille Villanueva-Birriel
Keyword(s):  
Mass Flux ◽  
Convective Available Potential Energy ◽  
Static Stability ◽  
Cold Pool ◽  
Convective Storm ◽  
Microphysics Scheme ◽  
Surface Precipitation ◽  
Cold Pools ◽  
Spatial Coverage ◽  
Horizontal Grid

Abstract This study investigated the sensitivity of idealized deep convective storm simulations to microphysics parameterization, horizontal grid spacing (Δx), and environmental static stability. Three different bulk microphysics schemes in the Weather Research and Forecasting Model were tested for Δx between 0.125 and 2 km and three different environmental soundings, modified by altering static stability above 5 km. Horizontally and temporally averaged condensation and surface precipitation rates and convective updraft mass flux were sensitive to microphysics scheme and Δx for all environmental soundings. Microphysical sensitivities were similar for 0.125 < Δx < 1 km, but they varied for different soundings. Sensitivities of these quantities to Δx were less robust and varied with microphysics scheme. Other statistical convective characteristics, such as the mean updraft width and strength, exhibited similar sensitivities to Δx for all of the microphysics schemes. Microphysical sensitivities were primarily attributed to interactions between microphysics, cold pools, and dynamics that affected the spatial coverage of convective updrafts and hence the horizontally averaged convective mass flux, condensation rate, and surface precipitation. However, these linkages were less clear for the lowest convective available potential energy (CAPE) sounding, and in this case other mechanisms compensated to give a similar spatial coverage of convective updrafts even in simulations without a cold pool. For higher CAPE, there was considerable production of rimed ice from all of the microphysics schemes and its assumed characteristics, especially the fall speed, were important in explaining sensitivity via microphysical impacts on the cold pool. These results highlight the need for continued improvement in representing the production of rimed ice and its characteristics in microphysics schemes.

Dynamical Effects of Aerosol Perturbations on Simulated Idealized Squall Lines

2014 ◽  
Vol 142 (3) ◽  
pp. 991-1009 ◽  
Author(s):  
Zachary J. Lebo ◽  
Hugh Morrison
Keyword(s):  
Mass Flux ◽  
Wind Shear ◽  
Cold Pool ◽  
Strong Wind ◽  
Low Level ◽  
Aerosol Loading ◽  
Squall Lines ◽  
Wide Range ◽  
Wind Shears ◽  
Convective Mass Flux

Abstract The dynamical effects of increased aerosol loading on the strength and structure of numerically simulated squall lines are explored. Results are explained in the context of Rotunno–Klemp–Weisman (RKW) theory. Changes in aerosol loading lead to changes in raindrop size and number that ultimately affect the strength of the cold pool via changes in evaporation. Thus, the balance between cold pool and low-level wind shear–induced vorticities can be changed by an aerosol perturbation. Simulations covering a wide range of low-level wind shears are performed to study the sensitivity to aerosols in different environments and provide more general conclusions. Simulations with relatively weak low-level environmental wind shear (0.0024 s−1) have a relatively strong cold pool circulation compared to the environmental shear. An increase in aerosol loading leads to a weakening of the cold pool and, hence, a more optimal balance between the cold pool– and environmental shear–induced circulations according to RKW theory. Consequently, there is an increase in the convective mass flux of nearly 20% in polluted conditions relative to pristine. This strengthening coincides with more upright convective updrafts and a significant increase (nearly 20%) in cumulative precipitation. An increase in aerosol loading in a strong wind shear environment (0.0064 s−1) leads to less optimal storms and a suppression of the convective mass flux and precipitation. This occurs because the cold pool circulation is weak relative to the environmental shear when the shear is strong, and further weakening of the cold pool with high aerosol loading leads to an even less optimal storm structure (i.e., convective updrafts begin to tilt downshear).

scholarly journals Aerosol effects on deep convection: the propagation of aerosol perturbations through convective cloud microphysics

2019 ◽  
Vol 19 (4) ◽  
pp. 2601-2627 ◽  
Author(s):  
Max Heikenfeld ◽  
Bethan White ◽  
Laurent Labbouz ◽  
Philip Stier
Keyword(s):  
Latent Heat ◽  
Cloud Condensation Nuclei ◽  
Cloud Microphysics ◽  
Latent Heating ◽  
Microphysics Scheme ◽  
Convective Clouds ◽  
Wide Range ◽  
Microphysical Processes ◽  
Aerosol Effects ◽  
The Impact

Abstract. The impact of aerosols on ice- and mixed-phase processes in deep convective clouds remains highly uncertain, and the wide range of interacting microphysical processes is still poorly understood. To understand these processes, we analyse diagnostic output of all individual microphysical process rates for two bulk microphysics schemes in the Weather and Research Forecasting model (WRF). We investigate the response of individual processes to changes in aerosol conditions and the propagation of perturbations through the microphysics all the way to the macrophysical development of the convective clouds. We perform simulations for two different cases of idealised supercells using two double-moment bulk microphysics schemes and a bin microphysics scheme. The simulations cover a comprehensive range of values for cloud droplet number concentration (CDNC) and cloud condensation nuclei (CCN) concentration as a proxy for aerosol effects on convective clouds. We have developed a new cloud tracking algorithm to analyse the morphology and time evolution of individually tracked convective cells in the simulations and their response to the aerosol perturbations. This analysis confirms an expected decrease in warm rain formation processes due to autoconversion and accretion for more polluted conditions. There is no evidence of a significant increase in the total amount of latent heat, as changes to the individual components of the integrated latent heating in the cloud compensate each other. The latent heating from freezing and riming processes is shifted to a higher altitude in the cloud, but there is no significant change to the integrated latent heat from freezing. Different choices in the treatment of deposition and sublimation processes between the microphysics schemes lead to strong differences including feedbacks onto condensation and evaporation. These changes in the microphysical processes explain some of the response in cloud mass and the altitude of the cloud centre of gravity. However, there remain some contrasts in the development of the bulk cloud parameters between the microphysics schemes and the two simulated cases.

The role of aerosol spatial inhomogeneity in mixed-phase deep convective clouds and torrential rain in urban areas

2021 ◽  
Author(s):  
Seoung Soo Lee ◽  
Byung-Gon Kim ◽  
Zhanqing Li
Keyword(s):  
Spatial Distribution ◽  
Urban Areas ◽  
Mixed Phase ◽  
Spatial Inhomogeneity ◽  
Extreme Weather Events ◽  
Torrential Rain ◽  
Convective Clouds ◽  
Aerosol Loading ◽  
Deep Convective Clouds ◽  
Mixed Phase Clouds

<p>This study examines the role played by aerosol in mixed-phase deep convective clouds and torrential rain that occurred in the Seoul area, which is a conurbation area where urbanization has been rapid in the last few decades, using cloud-system resolving model (CSRM) simulations. The model results show that the spatial variability of aerosol concentrations causes the inhomogeneity of the spatial distribution of evaporative cooling and the intensity of associated outflow around the surface. This inhomogeneity generates a strong convergence field and the associated spatial inhomogeneity of condensation, deposition and associated cloud mass, leading to the formation of torrential rain.  With the increases in the variability of aerosol concentrations, the occurrence of torrential rain increases. This study finds that the effects of the increases in the variability play a much more important role in the increases in the intensity of mixed-phase clouds and torrential rain than the much-studied effects of the increases in aerosol loading. Results in this study demonstrate that for a better understanding of extreme weather events such as torrential rain in urban areas, not only changing aerosol loading but also changing aerosol spatial distribution since industrialization should be considered in aerosol-precipitation interactions. </p>

scholarly journals Evaluation of Orographic Cloud Seeding Using a Bin Microphysics Scheme: Two-Dimensional Approach

2017 ◽  
Vol 56 (5) ◽  
pp. 1443-1462 ◽  
Author(s):  
István Geresdi ◽  
Lulin Xue ◽  
Roy Rasmussen
Keyword(s):  
Wrf Model ◽  
Cloud Formation ◽  
Two Dimensional ◽  
Diffusional Growth ◽  
Microphysics Scheme ◽  
Surface Precipitation ◽  
Convective Clouds ◽  
Seeding Effect ◽  
Orographic Clouds ◽  
Bin Microphysics

AbstractA new version of a bin microphysical scheme implemented into the Weather Research and Forecasting (WRF) Model was used to study the effect of glaciogenic seeding on precipitation formation in orographic clouds. The tracking of silver iodide (AgI) particles inside of water drops allows the proper simulation of the immersion nucleation. The ice formations by deposition, condensational freezing, and contact nucleation of AgI particles are also simulated in the scheme. Cloud formation—both stably stratified and convective—and the spread of AgI particles were simulated by idealized flow over a two-dimensional (2D) bell-shaped mountain. The results of numerical experiments show the following: (i) Only the airborne seeding enhances precipitation in stably stratified layer clouds. Seeding can reduce or enhance precipitation in convective clouds. AgI seeding can significantly affect the spatial distribution of the surface precipitation in orographic clouds. (ii) The positive seeding effect is primarily due to additional diffusional growth of AgI-nucleated ice crystals in layer clouds. In convective clouds, seeding-induced changes of both diffusion and riming processes determine the seeding effect. (iii) The seeding effect is inversely related to the natural precipitation efficiency. (iv) Bulk seeding parameterization is adequate to simulate AgI seeding impacts on wintertime orographic clouds. More uncertainties of ground-seeding effects are found between bulk and bin simulations.

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