Guy Dagan

From aerosol-limited to invigoration of warm convective clouds

Invigorating convection in warm clouds Atmospheric aerosols—tiny airborne particles—affect the way clouds form and how they affect climate. Koren et al. investigated how the formation of warm clouds, such as those that form over the oceans, depends on pollution levels (see the Perspective by Remer). Aerosols affect cloud formation in cleaner air disproportionately more than in more polluted air. Before the widespread air pollution of the industrial era, it seems, warm convective clouds may have covered much less of the oceans than they do today. Science, this issue p. 1143; see also p. 1089 Low concentrations of atmospheric aerosols have a dramatic effect on the invigoration of convection in warm clouds. [Also see Perspective by Remer] Among all cloud-aerosol interactions, the invigoration effect is the most elusive. Most of the studies that do suggest this effect link it to deep convective clouds with a warm base and cold top. Here, we provide evidence from observations and numerical modeling of a dramatic aerosol effect on warm clouds. We propose that convective-cloud invigoration by aerosols can be viewed as an extension of the concept of aerosol-limited clouds, where cloud development is limited by the availability of cloud-condensation nuclei. A transition from pristine to slightly polluted atmosphere yields estimated negative forcing of ~15 watts per square meter (cooling), suggesting that a substantial part of this anthropogenic forcing over the oceans occurred at the beginning of the industrial era, when the marine atmosphere experienced such transformation.

Aerosol Effect on the Mobility of Cloud Droplets

Cloud droplet mobility is referred to here as a measure of the droplets ability to move with ambient air. We claim that an important part of the aerosol effect on convective clouds is driven by changes in droplet mobility. We show that the mass-weighted average droplet terminal velocity, defined here as the effective terminal velocity (eta) and its spread (sigma_eta) serve as direct measures of this effect. Moreover, we develop analytical estimations for eta and sigma_eta to show that changes in the relative dispersion of eta (epsilon_eta = sigma_eta/eta) can serve as a sensitive predictor of the onset of droplet-collection processes.

Aerosol effect on the evolution of the thermodynamic properties of warm convective cloud fields

Convective cloud formation and evolution strongly depend on environmental temperature and humidity profiles. The forming clouds change the profiles that created them by redistributing heat and moisture. Here we show that the evolution of the field’s thermodynamic properties depends heavily on the concentration of aerosol, liquid or solid particles suspended in the atmosphere. Under polluted conditions, rain formation is suppressed and the non-precipitating clouds act to warm the lower part of the cloudy layer (where there is net condensation) and cool and moisten the upper part of the cloudy layer (where there is net evaporation), thereby destabilizing the layer. Under clean conditions, precipitation causes net warming of the cloudy layer and net cooling of the sub-cloud layer (driven by rain evaporation), which together act to stabilize the atmosphere with time. Previous studies have examined different aspects of the effects of clouds on their environment. Here, we offer a complete analysis of the cloudy atmosphere, spanning the aerosol effect from instability-consumption to enhancement, below, inside and above warm clouds, showing the temporal evolution of the effects. We propose a direct measure for the magnitude and sign of the aerosol effect on thermodynamic instability.

The effect of subtropical aerosol loading on equatorial precipitation

Cloud-aerosol interactions are considered as one of the largest sources of uncertainties in the study of climate change. Here, another possible cloud-aerosol effect on climate is proposed. A series of Large Eddy Simulations (LES) with bin microphysics reveal a sensitivity of the total atmospheric water vapor amount to aerosol concentration. Under polluted conditions the rain is suppressed and the total amount of water vapor in the atmosphere increases with time compared to clean precipitating conditions. Theoretical examination of this aerosol effect on water vapor transport from the subtropics to the tropics, and hence on the equatorial rain and Hadley circulation, is conducted using an idealized General Circulation Model (GCM). It is shown that a reduction in the subtropical rain amount results in increased water vapor advection to the tropics and enhanced equatorial rain and Hadley circulation. This joins previously proposed mechanisms on the radiative aerosol effect on the general circulation.

The Effects of the Spatial Distribution of Direct Anthropogenic Aerosols Radiative Forcing on Atmospheric Circulation

AbstractThe large uncertainty in estimating the global aerosol radiative forcing (ARF) is one of the major challenges the climate community faces for climate projection. While the global-mean ARF m...

Core and margin in warm convective clouds. Part II: aerosol effects on core properties

This paper follows from Part I which sought to examine the various methods of defining the cores and margins of convective clouds by using buoyancy, RH, and vertical velocity to define the core. They showed that these core diagnostics can be subsets of one another, but that this varies in space and time. This follow-on study examines the impacts of varying the aerosol concentration on the core definitions. Given that aerosols can change the cloud DSD, the condensation/evaporation rates can change,

Characterization of cumulus cloud fields using trajectories in the center of gravity versus water mass phase space: 2. Aerosol effects on warm convective clouds: Center of Gravity Versus Water Mass 2

In Part I of this work a 3-D cloud tracking algorithm and phase space of center of gravity altitude versus cloud liquid water mass (CvM space) were introduced and described in detail. We showed how new physical insight can be gained by following cloud trajectories in the CvM space. Here this approach is used to investigate aerosol effects on cloud fields of warm cumuli. We show a clear effect of the aerosol loading on the shape and size of CvM clusters. We also find fundamental differences in the CvM space between simulations using bin versus bulk microphysical schemes, with the bin scheme precipitation expressingmuch higher sensitivity to changes in aerosol concentrations. Using the bin microphysical scheme, we find that the increase in cloud center of gravity altitude with increase in aerosol concentrations occurs for a wide range of cloud sizes. This is attributed to reduced sedimentation, increased buoyancy and vertical velocities, and increased environmental instability, all of which are tightly coupled to inhibition of precipitation processes and subsequent feedbacks of clouds on their environment. Many of the physical processes shown here are consistent with processes typically associated with cloud invigoration.