Factors Determining the Impact of Aerosols on Surface Precipitation from Clouds: An Attempt at Classification

2008 ◽  
Vol 65 (6) ◽  
pp. 1721-1748 ◽  
Author(s):  
A. P. Khain ◽  
N. BenMoshe ◽  
A. Pokrovsky
Keyword(s):  
Forest Fires ◽  
Large Scale ◽  
Cloud Model ◽  
Aerosol Concentration ◽  
Cloud Base ◽  
Atmospheric Conditions ◽  
Diffusional Growth ◽  
Surface Precipitation ◽  
Freezing Level ◽  
The Impact

Abstract The simulation of the dynamics and the microphysics of clouds observed during the Large-Scale Biosphere–Atmosphere Experiment in Amazonia—Smoke, Aerosols, Clouds, Rainfall, and Climate (LBA–SMOCC) campaign, as well as extremely continental and extremely maritime clouds, is performed using an updated version of the Hebrew University spectral microphysics cloud model (HUCM). A new scheme of diffusional growth allows the reproduction of in situ–measured droplet size distributions including those formed in extremely polluted air. It was shown that pyroclouds forming over the forest fires can precipitate. Several mechanisms leading to formation of precipitation from pyroclouds are considered. The mechanisms by which aerosols affect the microphysics and precipitation of warm cloud-base clouds have been investigated by analyzing the mass, heat, and moisture budgets. The increase in aerosol concentration increases both the generation and the loss of the condensate mass. In the clouds developing in dry air, the increase in the loss is dominant, which suggests a decrease in the accumulated precipitation with the aerosol concentration increase. On the contrary, an increase in aerosol concentration in deep maritime clouds leads to an increase in precipitation. The precipitation efficiency of clouds in polluted air is found to be several times lower than that of clouds forming in clean air. A classification of the results of aerosol effects on precipitation from clouds of different types developing in the atmosphere with high freezing level (about 4 km) is proposed. The role of air humidity and other factors in precipitation’s response to aerosols is discussed. The analysis shows that many discrepancies between the results reported in different observational and numerical studies can be attributed to the different atmospheric conditions and cloud types analyzed.

scholarly journals Untangling Microphysical Impacts on Deep Convection Applying a Novel Modeling Methodology. Part II: Double-Moment Microphysics

2016 ◽  
Vol 73 (9) ◽  
pp. 3749-3770 ◽  
Author(s):  
Wojciech W. Grabowski ◽  
Hugh Morrison
Keyword(s):  
Large Scale ◽  
Cloud Condensation Nuclei ◽  
Deep Convection ◽  
Cloud Droplet ◽  
Cloud Base ◽  
Microphysics Scheme ◽  
Freezing Level ◽  
Modeling Methodology ◽  
Double Moment ◽  
The Impact

Abstract The suggested impact of pollution on deep convection dynamics, referred to as the convective invigoration, is investigated in simulations applying microphysical piggybacking and a comprehensive double-moment bulk microphysics scheme. The setup follows the case of daytime convective development over land based on observations during the Large-Scale Biosphere–Atmosphere (LBA) experiment in Amazonia. In contrast to previous simulations with single-moment microphysics schemes and in agreement with results from bin microphysics simulations by others, the impact of pollution simulated by the double-moment scheme is large for the upper-tropospheric convective anvils that feature higher cloud fractions in polluted conditions. The increase comes from purely microphysical considerations: namely, the increased cloud droplet concentrations in polluted conditions leading to the increased ice crystal concentrations and, consequently, smaller fall velocities and longer residence times. There is no impact on convective dynamics above the freezing level and thus no convective invigoration. Polluted deep convective clouds precipitate about 10% more than their pristine counterparts. The small enhancement comes from smaller supersaturations below the freezing level and higher buoyancies inside polluted convective updrafts with velocities between 5 and 10 m s−1. The simulated supersaturations are large, up to several percent in both pristine and polluted conditions, and they call into question results from deep convection simulations applying microphysical schemes with saturation adjustment. Sensitivity simulations show that the maximum supersaturations and the upper-tropospheric anvil cloud fractions strongly depend on the details of small cloud condensation nuclei (CCN) that can be activated in strong updrafts above the cloud base.

scholarly journals Ten Years of Aerosol Effects on Single-Layer Overcast Clouds over the US Southern Great Plains and the China Loess Plateau

2020 ◽  
Vol 2020 ◽  
pp. 1-15 ◽  
Author(s):  
Hongru Yan ◽  
Tianhe Wang
Keyword(s):  
Great Plains ◽  
Indirect Effect ◽  
Negative Relationship ◽  
Single Layer ◽  
Cloud Base ◽  
Atmospheric Conditions ◽  
Southern Great Plains ◽  
Aerosol Loading ◽  
The Us ◽  
The Impact

Using almost 10 years of observations of clouds and aerosols from the US Southern Great Plains (SGP) atmospheric observatory and the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) in China, the impact of aerosols on single-layer overcast clouds over continental land for different regimes were investigated. Atmospheric conditions at the two sites were first compared in an attempt to isolate the influence of aerosols on cloud properties from dynamic and thermodynamic influences. Cloud types and amounts are similar at the two sites. The dominant aerosol types at the SGP and SACOL sites are sulphate and dust, respectively, with greater aerosol optical depths (AODs) and absorption at the SACOL site. Aerosol first indirect effect (FIE) ranges from 0.021 to 0.152 and from −0.078 to 0.047 at the SGP and SACOL sites, respectively, when using the AOD below cloud base as CCN proxy. Although differences exist, the influence of meteorological conditions on the FIE at the two sites is consistent. FIEs are easily detected under descending motion and dry condition. The FIE at the SGP site is larger than that at the SACOL site, which suggests that the cloud albedo effect is more sensitive under relatively cleaner atmospheric conditions and the dominating aerosol at the SACOL site has less hygroscopicity. The radiative forcing of the FIE over the SGP site is −3.2 W m−2 for each 0.05 increment in FIE. Cloud durations generally prolong as aerosol loading increases, which is consistent with the hypothesis of the aerosol second indirect effect. The negative relationship between cloud duration time and aerosol loading when aerosol loading reaches a large value further might suggest a semidirect effect.

scholarly journals Risk of large-scale fires in boreal forests of Finland under changing climate

2016 ◽  
Vol 16 (1) ◽  
pp. 239-253 ◽  
Author(s):  
I. Lehtonen ◽  
A. Venäläinen ◽  
M. Kämäräinen ◽  
H. Peltola ◽  
H. Gregow
Keyword(s):  
Climate Change ◽  
Forest Fire ◽  
Forest Fires ◽  
Large Scale ◽  
Fire Danger ◽  
Short Period ◽  
Projected Change ◽  
Fire Activity ◽  
Model Variability ◽  
The Impact

Abstract. The target of this work was to assess the impact of projected climate change on forest-fire activity in Finland with special emphasis on large-scale fires. In addition, we were particularly interested to examine the inter-model variability of the projected change of fire danger. For this purpose, we utilized fire statistics covering the period 1996–2014 and consisting of almost 20 000 forest fires, as well as daily meteorological data from five global climate models under representative concentration pathway RCP4.5 and RCP8.5 scenarios. The model data were statistically downscaled onto a high-resolution grid using the quantile-mapping method before performing the analysis. In examining the relationship between weather and fire danger, we applied the Canadian fire weather index (FWI) system. Our results suggest that the number of large forest fires may double or even triple during the present century. This would increase the risk that some of the fires could develop into real conflagrations which have become almost extinct in Finland due to active and efficient fire suppression. However, the results reveal substantial inter-model variability in the rate of the projected increase of forest-fire danger, emphasizing the large uncertainty related to the climate change signal in fire activity. We moreover showed that the majority of large fires in Finland occur within a relatively short period in May and June due to human activities and that FWI correlates poorer with the fire activity during this time of year than later in summer when lightning is a more important cause of fires.

scholarly journals The effect of secondary ice production parameterization on the simulation of a cold frontal rainband

2018 ◽  
Vol 18 (22) ◽  
pp. 16461-16480 ◽  
Author(s):  
Sylvia C. Sullivan ◽  
Christian Barthlott ◽  
Jonathan Crosier ◽  
Ilya Zhukov ◽  
Athanasios Nenes ◽  
...  
Keyword(s):  
Secondary Production ◽  
Large Scale ◽  
Mesoscale Model ◽  
Ice Crystal ◽  
Surface Precipitation ◽  
Earth’S Climate ◽  
Ice Production ◽  
Cloud Ice ◽  
Droplet Shattering ◽  
The Impact

Abstract. Secondary ice production via processes like rime splintering, frozen droplet shattering, and breakup upon ice hydrometeor collision have been proposed to explain discrepancies between in-cloud ice crystal and ice-nucleating particle numbers. To understand the impact of this additional ice crystal generation on surface precipitation, we present one of the first studies to implement frozen droplet shattering and ice–ice collisional breakup parameterizations in a mesoscale model. We simulate a cold frontal rainband from the Aerosol Properties, PRocesses, And InfluenceS on the Earth's Climate campaign and investigate the impact of the new parameterizations on the simulated ice crystal number concentrations (ICNC) and precipitation. Near the convective regions of the rainband, contributions to ICNC can be as large from secondary production as from primary nucleation, but ICNCs greater than 50 L−1 remain underestimated by the model. The addition of the secondary production parameterizations also clearly intensifies the differences in both accumulated precipitation and precipitation rate between the convective towers and non-convective gap regions. We suggest, then, that secondary ice production parameterizations be included in large-scale models on the basis of large hydrometeor concentration and convective activity criteria.

The impact of large-scale winds on thermally driven flows and exchange processes over mountainous terrain

2020 ◽  
Author(s):  
Jan Weinkaemmerer ◽  
Ivan Bašták Ďurán ◽  
Jürg Schmidli
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Volume Effect ◽  
Turbulent Heat ◽  
Atmospheric Conditions ◽  
Free Atmosphere ◽  
Convective Boundary ◽  
Mean Values ◽  
Mountainous Regions ◽  
The Impact

<p>In the convective boundary layer over mountainous regions, the mean values and the fluxes of quantities like heat, mass, and momentum are strongly influenced by thermally induced flows. Several studies have pointed out that the enhanced warming of the air inside a valley can be explained by the valley-volume effect whereas the cross-valley circulation leads to a net export of heat to the free atmosphere. We are interested in the influence of an upper-level wind on the local circulations and the boundary-layer properties, both locally and in terms of the horizontal mean, as this aspect has not yet received much attention. LES are carried out over idealized, two-dimensional topographies using the CM1 numerical model. For the analysis, turbulent, mean-circulation, and large-scale contributions are systematically distinguished. Also, budget analyses are performed for the turbulence kinetic energy and the turbulent heat and mass flux. Based on the first results for periodic topographies, no crucial influence on the horizontally averaged heat-flux and temperature profile can be observed, even though the flow pattern of the thermal wind is qualitatively changed. In addition to that, the impact on moisture transport will be evaluated and simulations over different topographies as well as for different atmospheric conditions and surface properties will be presented.</p>

The Strong Impact of Weak Horizontal Convergence on Continental Shallow Convection

2020 ◽  
Vol 77 (9) ◽  
pp. 3119-3137
Author(s):  
Marcin J. Kurowski ◽  
Wojciech W. Grabowski ◽  
Kay Suselj ◽  
João Teixeira
Keyword(s):  
Large Scale ◽  
Cloud Microphysics ◽  
Cloud Base ◽  
Small Scale ◽  
Strong Impact ◽  
Shallow Convection ◽  
Liquid Water Path ◽  
Strong Response ◽  
The Impact ◽  
Benchmark Solutions

Abstract Idealized large-eddy simulation (LES) is a basic tool for studying three-dimensional turbulence in the planetary boundary layer. LES is capable of providing benchmark solutions for parameterization development efforts. However, real small-scale atmospheric flows develop in heterogeneous and transient environments with locally varying vertical motions inherent to open multiscale interactive dynamical systems. These variations are often too subtle to detect them by state-of-the-art remote and in situ measurements, and are typically excluded from idealized simulations. The present study addresses the impact of weak [i.e., O(10−6) s−1] short-lived low-level large-scale convergence/divergence perturbations on continental shallow convection. The results show a strong response of shallow nonprecipitating convection to the applied weak large-scale dynamical forcing. Evolutions of CAPE, mean liquid water path, and cloud-top heights are significantly affected by the imposed convergence/divergence. In contrast, evolving cloud-base properties, such as the area coverage and mass flux, are only weakly affected. To contrast those impacts with microphysical sensitivity, the baseline simulations are perturbed assuming different observationally based cloud droplet number concentrations and thus different rainfall. For the tested range of microphysical perturbations, the imposed convergence/divergence provides significantly larger impact than changes in the cloud microphysics. Simulation results presented here provide a stringent test for convection parameterizations, especially important for large-scale models progressing toward resolving some nonhydrostatic effects.

scholarly journals A six year satellite-based assessment of the regional variations in aerosol indirect effects

2009 ◽  
Vol 9 (12) ◽  
pp. 4091-4114 ◽  
Author(s):  
T. A. Jones ◽  
S. A. Christopher ◽  
J. Quaas
Keyword(s):  
Large Scale ◽  
Vertical Motion ◽  
Aerosol Concentration ◽  
Anthropogenic Sources ◽  
Aerosol Layer ◽  
Relative Importance ◽  
Atmospheric Conditions ◽  
Anthropogenic Aerosol ◽  
Western Atlantic ◽  
Cloud Properties

Abstract. Aerosols act as cloud condensation nuclei (CCN) for cloud water droplets, and changes in aerosol concentrations have significant microphysical impacts on the corresponding cloud properties. Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol and cloud properties are combined with NCEP Reanalysis data for six different regions around the globe between March 2000 and December 2005 to study the effects of different aerosol, cloud, and atmospheric conditions on the aerosol indirect effect (AIE). Emphasis is placed in examining the relative importance of aerosol concentration, type, and atmospheric conditions (mainly vertical motion) to AIE from region to region. Results show that in most regions, AIE has a distinct seasonal cycle, though the cycle varies in significance and period from region to region. In the Arabian Sea (AS), the six-year mean anthropogenic + dust AIE is −0.27 Wm−2 and is greatest during the summer months (<−2.0 Wm−2) during which aerosol concentrations (from both dust and anthropogenic sources) are greatest. Comparing AIE as a function of thin (LWP<20 gm−2) vs. thick (LWP≥20 gm−2) clouds under conditions of large scale ascent or decent at 850 hPa showed that AIE is greatest for thick clouds during periods of upward vertical motion. In the Bay of Bengal, AIE is negligible owing to less favorable atmospheric conditions, a lower concentration of aerosols, and a non-alignment of aerosol and cloud layers. In the eastern North Atlantic, AIE is weakly positive (+0.1 Wm−2) with dust aerosol concentration being much greater than the anthropogenic or sea salt components. However, elevated dust in this region exists above the maritime cloud layers and does not have a hygroscopic coating, which occurs in AS, preventing the dust from acting as CCN and limiting AIE. The Western Atlantic has a large anthropogenic aerosol concentration transported from the eastern United States producing a modest anthropogenic AIE (−0.46 Wm−2). Anthropogenic AIE is also present off the West African coast corresponding to aerosols produced from seasonal biomass burning (both natural and man-made). Interestingly, atmospheric conditions are not particularly favorable for cloud formation compared to the other regions during the times where AIE is observed; however, clouds are generally thin (LWP<20 gm−2) and concentrated very near the surface. Overall, we conclude that vertical motion, aerosol type, and aerosol layer heights do make a significant contribution to AIE and that these factors are often more important than total aerosol concentration alone and that the relative importance of each differs significantly from region to region.

scholarly journals On the limits of Köhler activation theory: how do collision and coalescence affect the activation of aerosols?

2017 ◽  
Vol 17 (13) ◽  
pp. 8343-8356 ◽  
Author(s):  
Fabian Hoffmann
Keyword(s):  
Critical Radius ◽  
Cloud Model ◽  
Cloud Droplet ◽  
Aerosol Concentration ◽  
Cloud Base ◽  
Cloud Droplets ◽  
Activation Theory ◽  
Large Eddy ◽  
Kohler Theory ◽  
Additional Process

Abstract. Activation is necessary to form a cloud droplet from an aerosol, and it is widely accepted that it occurs as soon as a wetted aerosol grows beyond its critical radius. Traditional Köhler theory assumes that this growth is driven by the diffusion of water vapor. However, if the wetted aerosols are large enough, the coalescence of two or more particles is an additional process for accumulating sufficient water for activation. This transition from diffusional to collectional growth marks the limit of traditional Köhler theory and it is studied using a Lagrangian cloud model in which aerosols and cloud droplets are represented by individually simulated particles within large-eddy simulations of shallow cumuli. It is shown that the activation of aerosols larger than 0. 1 µm in dry radius can be affected by collision and coalescence, and its contribution increases with a power-law relation toward larger radii and becomes the only process for the activation of aerosols larger than 0. 4–0. 8 µm depending on aerosol concentration. Due to the natural scarcity of the affected aerosols, the amount of aerosols that are activated by collection is small, with a maximum of 1 in 10 000 activations. The fraction increases as the aerosol concentration increases, but decreases again as the number of aerosols becomes too high and the particles too small to cause collections. Moreover, activation by collection is found to affect primarily aerosols that have been entrained above the cloud base.

scholarly journals Evaluating the effects of microphysical complexity in idealised simulations of trade wind cumulus using the Factorial Method

2010 ◽  
Vol 10 (10) ◽  
pp. 23497-23537 ◽  
Author(s):  
C. Dearden ◽  
P. J. Connolly ◽  
T. W. Choularton ◽  
P. R. Field
Keyword(s):  
State Of The Art ◽  
Model Development ◽  
Cloud Base ◽  
Trade Wind ◽  
One Dimensional ◽  
Surface Precipitation ◽  
Factorial Method ◽  
Modelling Framework ◽  
Development Tool ◽  
The Impact

Abstract. The effect of microphysical and environmental factors on the development of precipitation in warm idealised clouds are explored using an idealised process modelling framework. A simple one-dimensional column model is used to drive a suite of microphysics schemes including a flexible multi-moment bulk scheme (including both single and dual moment liquid water) and a state-of-the-art bin-resolved scheme with explicit treatments of liquid and aerosol. The Factorial Method is employed to quantify and compare the sensitivities of each scheme under a set of controlled conditions, in order to isolate the effect of additional microphysical complexity in terms of the impact on surface precipitation. For the schemes considered, and in the absence of entrainment, surface precipitation totals were found to depend increasingly on the meteorological conditions as the level of microphysical complexity is increased. The dual-moment liquid bulk scheme was shown to provide the best agreement with the bin scheme when the cloud base updraught speeds are relatively weak. At higher updraughts, all schemes show that the sensitivity to the magnitude of vertical velocity reduces dramatically, and any subsequent change in precipitation is governed almost entirely by the change in aerosol concentration. However the effect of changes in temperature were found to be underestimated in the bulk schemes compared to the bin scheme; this can be accounted for through differences in the depletion of rain below cloud base by evaporation. Collectively, these results demonstrate the usefulness of the Factorial Method as a model development tool for quantitatively comparing and contrasting the behaviour of microphysics schemes of differing levels of complexity within a specified parameter space.

scholarly journals Evaluation of Orographic Cloud Seeding Using a Bin Microphysics Scheme: Three-Dimensional Simulation of Real Cases

2020 ◽  
Vol 59 (9) ◽  
pp. 1537-1555
Author(s):  
István Geresdi ◽  
Lulin Xue ◽  
Noémi Sarkadi ◽  
Roy Rasmussen
Keyword(s):  
Water Content ◽  
Liquid Water ◽  
Cloud Condensation Nuclei ◽  
Three Dimensional ◽  
Liquid Water Content ◽  
Weather Modification ◽  
Diffusional Growth ◽  
Surface Precipitation ◽  
Orographic Clouds ◽  
The Impact

AbstractThe University of Pécs and NCAR Bin (UPNB) microphysical scheme was implemented into the mesoscale Weather Research and Forecast (WRF) Model that was used to study the impact of silver iodide (AgI) seeding on precipitation formation in winter orographic clouds. Four different experimental units were chosen from the Wyoming Weather Modification Pilot Project to simulate the seeding effect. The results of the numerical experiments show the following: (i) Comparisons with the soundings, snow gauges, and microwave radiometer data indicate that the three-dimensional simulations with detailed microphysics reasonably represent both the dynamics and the microphysics of real clouds. (ii) The dispersion of the AgI particles from the simulated ground-based seeding was effective because of turbulent mixing. (iii) In the investigated cases (surface temperature is less than 0°C), surface precipitation and precipitation efficiency show low susceptibility to the concentrations of cloud condensation nuclei and natural ice nucleating particles. (iv) If the available liquid water content promotes the enhancement of the number of snowflakes by diffusional growth, the surface precipitation can be increased by more than 5%. A novel parameter relevant to orographic clouds, horizontally integrated liquid water path (LWP), was evaluated to find the relation between seeding efficiency and liquid water content. The impact of seeding is negligible if the horizontal LWP is less than 0.1 mm and is apparent if the horizontal LWP is larger than 1 mm, as based on the cases investigated in this study.

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