Review: Cloud invigoration by aerosols—Coupling between microphysics and dynamics
Introduction
Processes that shape cloud and cloud field properties can be divided into three classes: microphysical (including chemistry), dynamical and radiative. Microphysical processes are related to the formation, evolution and properties of cloud and precipitation particles (Pruppacher and Klett, 1978). Dynamical processes describe the movement of air in clouds and their immediate environments (Houze, 1993). Dynamics span a large range of scales of motion, from turbulence to synoptic scales. Atmospheric radiation processes describe the interactions between atmospheric compounds (gases, clouds and aerosol) and electromagnetic radiation (Petty, 2006). Processes discussed in this review are related to the microphysical and dynamical domains.
The main source of complexity in understanding clouds is the tight coupling between scales and processes. One of the main questions leading today's cloud-climate research is: “How tightly are microphysical and dynamic processes coupled?” This is a two-way question. In one direction, the question of how do dynamics affect microphysics? is well-appreciated, emerging from the basic droplet diffusional growth equations (Pruppacher and Klett, 1978). The major uncertainty of the coupling resides in the reverse question: “How do microphysics affect dynamical processes” (Khain et al., 2005, van den Heever et al., 2006, Levin and Cotton, 2009). Embedded in this question is the need to define the role, if any, of aerosol effects on clouds of all types, including aerosol invigoration of these clouds and cloud systems.
Aerosol properties serve as the initial conditions for droplet activation in the cloud. Therefore, changes in aerosol properties inherently produce changes in the cloud microphysical processes. There is a growing body of work, to be discussed below, that shows how changes in aerosol size-distribution affect the timing of the onset of rain and its vertical location within the cloud. This by itself suggests that microphysics affect the water distribution within the cloud. Moreover, rain is the end result of a long chain of linear and non-linear processes, starting from activation and droplet growth by diffusion and continuing to collision-coalescence related processes. Therefore, aerosol effects on the initiation of rain reflect earlier influences on condensation and evaporation, drag-forces, latent heat fluxes, droplet terminal velocities, drop collection and entrainment. It is through these processes that microphysical changes affect the dynamics.
Studies that address cloud invigoration by aerosols link changes in aerosol properties to changes in the cloud dimensions, condensate spatial distribution and rain patterns. Such associations suggest significant coupling between cloud microphysics and cloud or cloud system dynamics. A cloud system is a group of organized clouds. It is this coupling that leads to invigoration, the enhanced development of the clouds. This paper reviews attempts to answer where, when and in what way, convective clouds are affected by the microphysical impact on the dynamics branch of the coupling, covering observation-based and cloud resolving modeling studies.
Clouds are key players in the climate system, significantly affecting Earth's energy balance and providing a fundamental link in the water cycle. At the same time, clouds are the least understood climate process (Forster et al., 2007). The radiation budget, water cycle and energy balance of the Earth's system depend on cloud microphysical and macrophysical properties, such as, coverage, vertical extent and internal properties. Therefore, a better understanding of the role of clouds in the climate system requires a good understanding of the processes that affect cloud properties.
Specifically, clouds serve as atmospheric radiation modulators in both the short wave and long wave portions of the spectrum (Baker and Peter, 2008, Trenberth et al., 2009). Clouds cool the atmosphere by reflecting solar radiation to space, and account for approximately 2/3's of the Earth's albedo (Trenberth et al., 2009). In the long-wave infrared (LWIR) range, clouds act like a greenhouse gas and warm the atmosphere by absorbing the Earth's outgoing LWIR radiation. The net radiative effect of a single cloud or a cloud system (organized structures of dynamically related clouds) depends on the cloud type, size and location within the atmospheric column. For example, low-altitude stratiform clouds (like Marine-Stratocumulus, MSc) cool the Earth–atmosphere system by reflecting sunlight back to space and do not significantly disturb the outgoing LWIR radiation that continues to radiate at a similar blackbody temperature as the ocean (Hartmann and Doelling, 1991, Wood, 2012). On the other hand the net radiative effect of high-altitude cirrus clouds is warming, driven by their cold temperature compared to the earth's surface and relative low reflectance in the visible (Stephens and Webster, 1981, Fusina et al., 2007).
Equally important is the effect of cloud processes and properties on precipitation. Rain rate patterns and the total amount of precipitation affect the amount of water available for human use. Also, cloud processes that affect precipitation likewise contribute to the energy balance of the atmosphere. Cloud internal thermodynamic processes, such as condensation, evaporation, freezing, deposition, sublimation, melting and precipitation fallout provide a significant component of energy input and output from the atmosphere, through latent heat related processes (Trenberth et al., 2009). Furthermore, large-scale atmospheric tropical and sub-tropical circulations are driven by latent heat release in deep convective cloud systems in the tropical belts (Riehl and Malkus, 1958, Riehl and Simpson, 1979). Therefore, energy distribution and balance can be significantly affected by changes in cloud processes (Grabowski and Petch, 2009).
Thus, to understand the Earth's interconnected energy and water budgets and how those budgets will be modified in a changing climate, we need to understand and predict changes to cloud processes and properties. Such understanding and prediction is extremely difficult. Numerous dynamic and thermodynamic, chemical and radiative processes act and interact on spatial scales ranging from the size of an aerosol particle (< 10− 7 m) to the size of a cloud field (∼ 105 m). As in many dynamical systems, a cloud system's response to changes in its environmental conditions will not be linear. Some of the systems response may be controlled by negative feedbacks, resulting in a relatively small overall result: a buffered system (Stevens and Feingold, 2009). Whereas others, might result in shifting the cloud system into a different state, characterized by different cloud lifetimes, coverage, morphology and precipitation patterns. The transition from open to closed cells of shallow marine stratiform clouds is an example of a clear shifting from one state to the other (Rosenfeld et al., 2006, Koren and Feingold, 2011, Goren and Rosenfeld, 2012). Therefore, the nonlinear nature of cloud fields and the many spatial and temporal scales involved, make it extremely complicated to capture the net effect of cloud responses to changing conditions with in-situ or remote sensing measurements, or to represent the responses accurately in numerical model simulations.
Environmental conditions such as the temperature, humidity and wind profiles, heat and moisture surface fluxes (and their evolution in time), determine the potential of clouds to form and to produce liquid, mixed-phase, or ice hydrometeors. However, aerosol particles, liquid or solid matter suspended in the atmosphere, are a necessary component in a complete description of cloud processes.
Even today it is common to name aerosol effects on clouds as “indirect effects”. In this review we choose to use the terminology introduced above of microphysical, dynamical and radiative aerosol effects instead of indirect effects. Traditionally, cloud responses to changes in aerosol concentration and properties were discussed in the context of microphysical impact alone (Twomey, 1977, Albrecht, 1989) or solely on the radiative impact of aerosol on the clouds surrounding environment, by changing thermal and humidity profiles (Hansen et al., 1997). As detailed hereafter, in recent years, we have come to understand that this decoupling of responses is not realistic.
Droplet and ice crystal formation depend on the availability and properties of aerosols. Here we use the term “aerosol” as a generic term that serves as a measure for both subsets of cloud condensation nuclei (CCN) and ice nuclei (IN). CCNs reduce dramatically the energy barrier required for droplet nucleation, hence reducing the required supersaturation from several hundred percentage points (in the case of homogenous nucleation) to less than one percent (in heterogeneous nucleation) (Pruppacher and Klett, 1978). INs freeze supercooled water at higher temperatures (− 7 °C to − 15 °C) than in cases of homogenous freezing (below − 32 °C). The ability of an aerosol to serve as an efficient CCN or IN, depends on its size, mass, shape and chemical composition (Seinfeld and Pandis, 2012). The relationship between the amount of aerosol and initial number of droplets is complex, but usually more aerosols (smaller than 1 μm in radius) will result in more but smaller droplets, near the cloud base (Ramanathan et al., 2001). Large aerosol particles, acting as giant cloud condensation nuclei (GCCN, radius > 1 μm), may elongate the upper end of the size distribution, creating a tail of large droplets that will affect the evolution of the cloud differently than would small aerosols (Feingold et al., 1999, Yin et al., 2000). However, in this review, unless specifically indicated, when we discuss aerosols we are referring to small particles (< 1 μm in radius). Cloud processes influence aerosol properties in many aspects as well, including transfer of material from the gas phase into particles and changes in the particles' composition and sizes (Hoppel et al., 1986, Hoppel et al., 1990). In addition the rain washes out aerosol particles and reduces aerosol concentrations (Garrett et al., 2006). These processes are major controllers of aerosol properties, but are not a topic of this review.
A cloud developing in a high aerosol loading environment will have more but smaller droplets and a narrower size distribution, (Squires, 1958, Rosenfeld and Lensky, 1998, Andreae et al., 2004, Koren et al., 2005). Since clouds are non-linear systems, this single change may trigger a series of feedbacks that affect cloud processes and properties. In the warm part of the cloud (that contains liquid water only), the changes will reduce the efficiency of the collision-coalescence process (Squires, 1958, Warner, 1968, Albrecht, 1989) and delay raindrop formation (usually considered as drops with a radius > 40 μm). The altered size distribution also affects condensation and evaporation rates and the location and amount of latent heat release within and below the cloud (Khain et al., 2005, Rosenfeld et al., 2008). The above changes, initiated by additional aerosol available to the cloud, as well as changes in terminal velocities, drag forces and mixing, provide the pathways leading to modifications to dynamical processes on the cloud scale that later on may evolve to alterations of the entire cloud system (Khain et al., 2005, Tao et al., 2007, Khain et al., 2008, Lee et al., 2010).
Regarding mixed phase (containing water and ice) and cold (ice) processes, the web of feedbacks is much more complex, involving many types of processes and particles (homogenous and heterogeneous freezing, aggregation, riming, shedding, sublimation, melting and condensation). The warm part of the cloud serves as both an initial and boundary condition for the mixed part of the cloud, since it determines the timing and location of the freezing processes (Fletcher, 1962, Mason, 1971, Pruppacher and Klett, 1978, Rosenfeld and Woodley, 2000, Williams et al., 2002, Andreae et al., 2004, Koren et al., 2005). The warm part dictates the properties of the drops reaching the freezing level and by that determines the freezing processes that depend on the size distribution and chemical properties of the drops (condensation–freezing nucleation). In addition the change in the availability of INs forces additional changes in the freezing processes (immersion and contact freezing) and in various other ice processes. These processes and feedbacks and the induced changes in cloud properties and rain patterns are described in detail in Section 3.
Aerosols independently can change the atmospheric environment through the radiative effect. The radiative effect of aerosols refers to aerosol particle perturbations to the solar radiation budget by absorbing and scattering radiation. When particles absorb (and scatter) radiation aloft they cause heating in the atmospheric layer in which they reside (Hansen et al., 1997, Koren et al., 2004, Koren et al., 2008, Davidi et al., 2009) and reduction of the relative humidity in that layer, while cooling the surface. This process stabilizes the temperature profile below the aerosol layer and reduces the surface heat and moisture fluxes. Studies showed association between increased loading of absorbing aerosols, increased stability in low-level layers, and inhibition of cloud development (Yu et al., 2002, Feingold et al., 2005). This change in atmospheric stability can impact atmospheric circulations at scales that can even reach the size of the Intertropical Convergence Zone (ITCZ) and Indian monsoon. A recently published review by Wang, 2013, summarizes this subject.
Radiative effects change the temperature and humidity profiles that can modify microphysical processes (such as cloud particle condensation and evaporation rates) and impact dynamical processes as well. Thus, the radiative effect of absorbing aerosols, if aerosol loading is sufficient, can act oppositely from microphysical effects to suppress cloud development, which may conceal expected invigoration from the microphysical–dynamical coupling (Koren et al., 2008). Further discussion about this subject will appear in Section 2.1.3.
The first type of aerosol effects on cloud microphysical and radiative processes was determined based on observational studies of marine stratocumulus (MSc) clouds. Twomey (1977) showed that more but smaller drops, for the same liquid water content, produce a more reflective cloud, due to a larger cross-sectional area of the cloud droplets. Following that, Albrecht (1989) suggested that an increase in aerosol loading decreases the efficiency of warm rain processes by narrowing the size spectrum of smaller drops, and causing an increase in the liquid water path (LWP) and cloud life time. For a complete review of the literature that follows Twomey and Albrecht's pioneering work, see (Feingold and Siebert, 2009). In contrast to stratiform clouds, convective clouds offer ample opportunity to find additional important insight into aerosol–cloud interaction processes. This is because convective clouds are not usually confined beneath a strong inversion layer, allowing aerosol perturbations (to the microphysics, dynamics and environmental conditions) to produce significant changes in cloud microphysical, macrophysical and precipitation characteristics.
Invigoration refers here to the process in which aerosols act to deepen clouds. It can also manifest itself as optically thicker clouds with a larger area and more condensate mass that is located higher in the atmosphere. Those clouds can penetrate more frequently into the tropopause, and have larger anvils. We also expect changes in rain patterns and stronger electrical activity. This review begins in Section 2 with presentation of our current state of understanding of the invigoration effect from a literature survey of both observational and numerical modelling studies. In Section 3 we discuss the chain of processes responsible for the invigoration effect. In Section 4 there is a discussion of broader impact, and in Section 5 we discuss main unresolved issues and suggested future work.
We note two reviews recently published on aerosol–cloud interactions that deal with the aspect of cloud invigoration from different perspectives (Tao et al., 2012, Rosenfeld et al., 2013). In this paper we offer a review, not on the broad subject of aerosol–cloud interaction, but instead focused on the essence of aerosol invigoration of convective clouds. The level of complexity involved with cloud invigoration is high posing a true challenge in describing it in a clear way. Therefore here we offer an “ideal invigoration scenario” describing a theoretical sequence of effects and feedbacks starting from small scale changes in the droplets' size distribution and ending with possible effects on the cloud and cloud-field scales. In parallel we discuss possible deviations from the ideal scenario linked to the cloud type and environmental conditions. In addition we discuss the complexity and challenges involved, pointing to the advantages and disadvantages of the different approaches and suggesting directions for future work.
To obtain a shorter perspective on the topic we offer in Section 3, a concise physical model that explains the generalizations gathered from the observational and numerical studies, and also the studies that deviate from those generalizations. The model is still qualitative, but it provides a basis from which to unify much of the confusing literature.
Section snippets
Observational studies
In this section we discuss the methods and types of data used to observe the invigoration effect in clouds and their limitations, and then provide evidence of an aerosol effect on a wide range of cloud parameters. We also present studies where the expected invigoration signal was not seen, some of which can be explained by environmental factors and some of which, especially precipitation characteristics, remain unresolved.
Looking for evidence of aerosol invigoration of the convection through
Physical mechanisms behind the invigoration effect
The observational and numerical results presented above show that the coupled microphysical and dynamical processes can manifest themselves in a variety of cloud responses to the enhanced aerosol loading. These varied responses depend on the environmental conditions and cloud characteristics. Among the influencing environmental parameters are the instability of the atmosphere (measured by CAPE for example), the relative humidity profile, the wind shear along the atmosphere and the height of the
Discussion of broader impact
The aerosol-induced changes in cloud properties will drive impacts on scales larger than individual clouds or cloud fields.
Convective clouds play a key role in the Earth's radiation balance, in the water cycle and for atmospheric circulations (Section 1.1). The invigoration effect provides a pathway to bring about changes in the size distribution of water drops and ice particles and in the size of clouds, creating deeper and bigger clouds. Once the cloud's microphysical and macrophysical
Unresolved issues and future work
There is ample evidence from observations and results from numerical modeling to support the significance of the invigoration effect, despite the inherent complexity of the system, the uncertainty in the observational data and the difficulties with numerical models. Still many of the unresolved issues remain. A few of the key unresolved issues related to invigoration are listed below with suggested directions for future work.
Acknowledgment
We want to thank the two anonymous reviewers for their contribution that significantly improved the paper. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007–2013)/ERC Grant agreement no. 306965. L. A. Remer acknowledges funding for this project from NASA grant NNX12AK81G.
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