Dominant role by vertical wind shear in regulating aerosol effects on deep convective clouds

Fan, Jiwen; Yuan, Tianle; Comstock, J. M.; Ghan, Steven J.; Хаин, А.; Leung, L. Ruby; Li, Zhanqing; Martins, Vanderlei; Ovchinnikov, Mikhail

doi:10.1029/2009jd012352

Cited by 310 publications

(363 citation statements)

References 60 publications

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“…Increased IN are thereby depicted to reduce the net cooling impact of such clouds on the planet and vice versa (8). Nevertheless, we must note that the sign and magnitude of the impact remains in question, because cloud modeling studies find that IN increases may lead as well to precipitation decreases, and their impact cannot necessarily be isolated from impacts of increases in all types of aerosols on the warm phase (>0°C) regions of clouds (9), nor the dynamical context in which clouds form (10). The sign and magnitude of the effects of increased IN on high (cirrus) clouds is similarly uncertain, but the impact is likely to be a decrease in the net planetary warming of this cloud type.…”

mentioning

confidence: 99%

Predicting global atmospheric ice nuclei distributions and their impacts on climate

DeMott

Prenni²,

Liu³

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

1,105

1,218

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Knowledge of cloud and precipitation formation processes remains incomplete, yet global precipitation is predominantly produced by clouds containing the ice phase. Ice first forms in clouds warmer than −36°C on particles termed ice nuclei. We combine observations from field studies over a 14-year period, from a variety of locations around the globe, to show that the concentrations of ice nuclei active in mixed-phase cloud conditions can be related to temperature and the number concentrations of particles larger than 0.5 μm in diameter. This new relationship reduces unexplained variability in ice nuclei concentrations at a given temperature from ∼10 3 to less than a factor of 10, with the remaining variability apparently due to variations in aerosol chemical composition or other factors. When implemented in a global climate model, the new parameterization strongly alters cloud liquid and ice water distributions compared to the simple, temperature-only parameterizations currently widely used. The revised treatment indicates a global net cloud radiative forcing increase of ∼1 W m −2 for each order of magnitude increase in ice nuclei concentrations, demonstrating the strong sensitivity of climate simulations to assumptions regarding the initiation of cloud glaciation.aerosol indirect effects | climate forcing | ice nucleation T he formation of ice in clouds is of vital importance to life on Earth, as ice formation is one of the key processes initiating precipitation. In addition, since ice nucleation is tied to the action of specific aerosol particles, natural and human impacts on ice nucleation in the atmosphere can lead to alteration of the energy and hydrological cycles (1). Ice nucleation in clouds occurs via two primary pathways: homogeneous freezing of liquid particles below about −36°C and heterogeneous ice nucleation, triggered by "ice nuclei" that possess surface properties favorable to lowering the energy barrier to crystallization. Once ice is formed, some circumstances may favor generation of ice from preexisting ice particles, or secondary ice formation (2). Heterogeneous ice nucleation remains an enigmatic topic involving multiple mechanistic processes (3) that sometimes defy ready investigation or description. Despite the lack of a complete understanding of heterogeneous ice formation processes, a variety of techniques have been developed and used to detect the presence of and quantify the number concentrations of atmospheric ice nuclei as a function of temperature (4). These measurements show that, although generally representing only 1 in 10 5 of ambient particles in the free troposphere (5), ice nuclei (IN) can nevertheless exert an influence on cold cloud microphysical processes that is disproportionate to their low number concentrations. For example, the concentrations of IN needed to explain observed precipitation rates range from as small as 10 −3 per standard liter at −10°C (6) to more typical estimates of a few IN per standard liter at −20°C (7).The sensitivity of precipitation initiation fro...

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mentioning

confidence: 99%

Predicting global atmospheric ice nuclei distributions and their impacts on climate

DeMott

Prenni²,

Liu³

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

1,105

1,218

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“…Many environmental parameters, such as the CAPE (18-21), RH (13,20,22,23), and VWS (12,22,26,28,34), have been known to influence MCSs' lifetime. However, to our knowledge, this is the first satellite-based global tropical continental-scale assessment of the relative roles of meteorological conditions versus aerosols in determining the variations of cloud ice and lifetime of the MCSs using collocated geostationary satellites, closely synchronized A-train polar orbital satellite, and MERRA reanalysis datasets on MCSs' lifetime.…”

Section: Resultsmentioning

confidence: 99%

“…For example, model simulation has shown that an increase in aerosol concentrations up to an optimal level can invigorate the MCSs under weak vertical wind shear (VWS) and higher RH but suppress the MCSs under strong VWS in a dry environment (12,13). They found that, due to a significant enhancement in the convective available potential energy (CAPE), corresponding to an increase in RH from 50% to 70%, aerosol impact on ice crystal mass becomes pronounced, with a dramatic increase in the size of the anvils and the mass of ice crystals of the deep convection.…”

mentioning

confidence: 99%

Relative influence of meteorological conditions and aerosols on the lifetime of mesoscale convective systems

Chakraborty

Massie

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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Using collocated measurements from geostationary and polar-orbital satellites over tropical continents, we provide a large-scale statistical assessment of the relative influence of aerosols and meteorological conditions on the lifetime of mesoscale convective systems (MCSs). Our results show that MCSs' lifetime increases by 3-24 h when vertical wind shear (VWS) and convective available potential energy (CAPE) are moderate to high and ambient aerosol optical depth (AOD) increases by 1 SD (1σ). However, this influence is not as strong as that of CAPE, relative humidity, and VWS, which increase MCSs' lifetime by 3-30 h, 3-27 h, and 3-30 h per 1σ of these variables and explain up to 36%, 45%, and 34%, respectively, of the variance of the MCSs' lifetime. AOD explains up to 24% of the total variance of MCSs' lifetime during the decay phase. This result is physically consistent with that of the variation of the MCSs' ice water content (IWC) with aerosols, which accounts for 35% and 27% of the total variance of the IWC in convective cores and anvil, respectively, during the decay phase. The effect of aerosols on MCSs' lifetime varies between different continents. AOD appears to explain up to 20-22% of the total variance of MCSs' lifetime over equatorial South America compared with 8% over equatorial Africa. Aerosols over the Indian Ocean can explain 20% of total variance of MCSs' lifetime over South Asia because such MCSs form and develop over the ocean. These regional differences of aerosol impacts may be linked to different meteorological conditions. mesoscale convective systems | aerosols | meteorological parameters T he hypothesis that aerosols may delay precipitation and increase cloud lifetime of shallow marine clouds (1) has motivated many researchers to study the aerosol indirect effect on convective clouds; however, the influence of aerosols on enhancing cloud lifetime has remained under debate. Mesoscale convective systems (MCSs) are deep convective clouds that cover several hundred kilometers. Previous studies have shown that aerosols affect deep convection, in particular that aerosols increase the number of smaller size cloud condensation nuclei (CCN) (2), which weaken coagulation and coalescence that form rain droplets, and consequently delay warm rainfall (3, 4). These processes allow more cloud droplets to rise above the freezing level and increase latent heat released due to glaciation (5), resulting in stronger updraft speed, enhanced cloud ice content (6), larger anvil size (5), and higher cloud top height (7). The top of the troposphere warms owing to the aerosol-induced changes in convective anvils (8). Although these aerosol effects have been seen in observations from field campaigns (9, 10), they have been undetectable on large spatial and multiyear scales. Rosenfeld et al. (11) have attributed this lack of detectability on the large scale of aerosol invigoration of convection to its variation with meteorological conditions and to the lack of knowledge of the relative humidity (RH) outside the clo...

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“…In deep convective clouds rain forms eventually, but the aerosol-induced delay in its formation to greater heights was shown to cause in some conditions cloud invigoration (Andreae et al, 2004;Rosenfeld et al, 2008a;Fan et al, 2012) and additional electrification (Yuan et al, 2011). The invigoration was shown to occur mainly for situations with weak wind shear and for clouds with warm base, in which there is large vertical distance between cloud base and the freezing level (Fan et al, 2009;Li et al, 2011). The aerosolinduced vertical growth and the consequent expansion of the anvils into cirrus was observed (Koren et al, 2010) and simulated (Fan et al, 2012) to inflict large positive radiative forcing, in contrast to the strong negative forcing that is caused by the aerosol effect on shallow clouds, at least in heavily drizzling marine stratocumulus (e.g., Albrecht, 1989).…”

Section: Ways By Which Aerosols Affect Cloud Microphysical Precipitamentioning

confidence: 99%

The scientific basis for a satellite mission to retrieve CCN concentrations and their impacts on convective clouds

et al. 2012

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Abstract. The cloud-mediated aerosol radiative forcing is widely recognized as the main source of uncertainty in our knowledge of the anthropogenic forcing on climate. The current challenges for improving our understanding are (1) global measurements of cloud condensation nuclei (CCN) in the cloudy boundary layer from space, and (2) disentangling the effects of aerosols from the thermodynamic and meteorological effects on the clouds. Here, we present a new conceptual framework to help us overcome these two challenges, using relatively simple passive satellite measurements in the visible and infared (IR). The idea is to use the clouds themselves as natural CCN chambers by retrieving simultaneously the number of activated aerosols at cloud base, N a , and the cloud base updraft speed. The N a is obtained by analyzing the distribution of cloud drop effective radius in convective elements as a function of distance above cloud base. The cloud base updraft velocities are estimated by double stereoscopic viewing and tracking of the evolution of cloud surface features just above cloud base. In order to resolve the vertical dimension of the clouds, the field of view will be 100 m for the microphysical retrievals, and 50 m for the stereoscopic measurements. The viewing geometry will be eastward and 30 degrees off nadir, with the Sun in the back at 30 degrees off zenith westward, requiring a Sun-synchronous orbit at 14 LST. Measuring simultaneously the thermodynamic environment, the vertical motions of the clouds, their microstructure and the CCN concentration will allow separating the dynamics from the CCN effects. This concept is being applied in the proposed satellite mission named Clouds, Hazards and Aerosols Survey for Earth Researchers (CHASER).

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Dominant role by vertical wind shear in regulating aerosol effects on deep convective clouds

Cited by 310 publications

References 60 publications

Predicting global atmospheric ice nuclei distributions and their impacts on climate

Predicting global atmospheric ice nuclei distributions and their impacts on climate

Relative influence of meteorological conditions and aerosols on the lifetime of mesoscale convective systems

The scientific basis for a satellite mission to retrieve CCN concentrations and their impacts on convective clouds

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