Publications by Guy Dagan


Atmospheric energy budget response to idealized aerosol perturbation in tropical cloud systems

Atmospheric Chemistry and Physics Copernicus GmbH 20 (2020) 4523-4544

G Dagan, P Stier, M Christensen, G Cioni, D Klocke, A Seifert

<jats:p>Abstract. The atmospheric energy budget is analysed in numerical simulations of tropical cloud systems to better understand the physical processes behind aerosol effects on the atmospheric energy budget. The simulations include both shallow convective clouds and deep convective tropical clouds over the Atlantic Ocean. Two different sets of simulations, at different dates (10–12 and 16–18 August 2016), are simulated with different dominant cloud modes (shallow or deep). For each case, the cloud droplet number concentration (CDNC) is varied as a proxy for changes in aerosol concentrations without considering the temporal evolution of the aerosol concentration (for example due to wet scavenging, which may be more important under deep convective conditions). It is shown that the total column atmospheric radiative cooling is substantially reduced with CDNC in the deep-cloud-dominated case (by ∼10.0 W m−2), while a much smaller reduction (∼1.6 W m−2) is shown in the shallow-cloud-dominated case. This trend is caused by an increase in the ice and water vapour content at the upper troposphere that leads to a reduced outgoing longwave radiation, an effect which is stronger under deep-cloud-dominated conditions. A decrease in sensible heat flux (driven by an increase in the near-surface air temperature) reduces the warming by ∼1.4 W m−2 in both cases. It is also shown that the cloud fraction response behaves in opposite ways to an increase in CDNC, showing an increase in the deep-cloud-dominated case and a decrease in the shallow-cloud-dominated case. This demonstrates that under different environmental conditions the response to aerosol perturbation could be different. </jats:p>


Effects of aerosol in simulations of realistic shallow cumulus cloud fields in a large domain

Atmospheric Chemistry and Physics European Geosciences Union (2019)

G Spill, P Stier, PR Field, G Dagan


Analysis of the atmospheric water budget for elucidating the spatial scale of precipitation changes under climate change

Geophysical Research Letters American Geophysical Union 46 (2019) 10504-10511

G Dagan, P Stier, D Watson-Parris

Global mean precipitation changes due to climate change were previously shown to be relatively small and well constrained by the energy budget. However, local precipitation changes can be much more significant. In this paper we propose that for large enough scales, for which the water budget is closed (precipitation [P] roughly equals evaporation [E]), changes in P approach the small global mean value. However, for smaller scales, for which P and E are not necessarily equal and convergence of water vapor still plays a role, changes in P could be much larger due to dynamical contributions. Using 40 years of two reanalysis data sets, 39 CMIP5 models and additional numerical simulations, we identify the scale of transition in the importance of the different terms in the water budget to precipitation to be ~3500-4000 km and demonstrate its relation to the spatial scale of precipitation changes under climate change.


Contrasting response of precipitation to aerosol perturbation in the tropics and extratropics explained by energy budget considerations

Geophysical Research Letters American Geophysical Union 46 (2019) 7828-7837

G Dagan, P Stier, D Watson-Parris

Precipitation plays a crucial role in the Earth's energy balance, the water cycle, and the global atmospheric circulation. Aerosols, by direct interaction with radiation and by serving as cloud condensation nuclei, may affect clouds and rain formation. This effect can be examined in terms of energetic constraints, that is, any aerosol‐driven diabatic heating/cooling of the atmosphere will have to be balanced by changes in precipitation, radiative fluxes, or divergence of dry static energy. Using an aqua‐planet general circulation model (GCM), we show that tropical and extratropical precipitation have contrasting responses to aerosol perturbations. This behavior can be explained by contrasting ability of the atmosphere to diverge excess dry static energy in the two different regions. It is shown that atmospheric heating in the tropics leads to large‐scale thermally driven circulation and a large increase in precipitation, while the excess energy from heating in the extratropics is constrained due to the effect of the Coriolis force, causing the precipitation to decrease.


Effects of aerosol in simulations of realistic shallow cumulus cloud fields in a large domain

Atmospheric Chemistry and Physics European Geosciences Union 19 (2019) 19
Part of a series from Atmospheric Chemistry and Physics

G Spill, P Stier, PR Field, G Dagan

Previous study of shallow convection has generally suffered from having to balance domain size with resolution, resulting in high-resolution studies which do not capture large-scale behaviour of the cloud fields. In this work we hope to go some way towards addressing this by carrying out cloud-resolving simulations on large domains. Simulations of trade wind cumulus are carried out using the Met Office Unified Model (UM), based on a case study from the Rain In Cumulus over the Ocean (RICO) field campaign. The UM is run with a nested domain of 500 km with 500 m resolution, in order to capture the large-scale behaviour of the cloud field, and with a double-moment interactive microphysics scheme. Simulations are run using baseline aerosol profiles based on observations from RICO, which are then perturbed. We find that the aerosol perturbations result in changes to the convective behaviour of the cloud field, with higher aerosol leading to an increase (decrease) in the number of deeper (shallower) clouds. However, despite this deepening, there is little increase in the frequency of higher rain rates. This is in contrast to the findings of previous work making use of idealised simulation setups. In further contrast, we find that increasing aerosol results in a persistent increase in domain mean liquid water path and decrease in precipitation, with little impact on cloud fraction.


Core and margin in warm convective clouds – Part 1: Core types and evolution during a cloud's lifetime

Atmospheric Chemistry and Physics Copernicus GmbH 19 (2019) 10717-10738

RH Heiblum, L Pinto, O Altaratz, G Dagan, I Koren

<jats:p>&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; The properties of a warm convective cloud are determined by the competition between the growth and dissipation processes occurring within it. One way to observe and follow this competition is by partitioning the cloud to core and margin regions. Here we look at three core definitions, namely positive vertical velocity (&lt;span class="inline-formula"&gt;&lt;i&gt;W&lt;/i&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt;), supersaturation (RH&lt;span class="inline-formula"&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt;), and positive buoyancy (&lt;span class="inline-formula"&gt;&lt;i&gt;B&lt;/i&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt;), and follow their evolution throughout the lifetime of warm convective clouds.&lt;/p&gt; &lt;p&gt;Using single cloud and cloud field simulations with bin-microphysics schemes, we show that the different core types tend to be subsets of one another in the following order: &lt;span class="inline-formula"&gt;&lt;math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;B&lt;/mi&gt;&lt;mi mathvariant="normal"&gt;core&lt;/mi&gt;&lt;/msub&gt;&lt;mo&gt;⊆&lt;/mo&gt;&lt;msub&gt;&lt;mi mathvariant="normal"&gt;RH&lt;/mi&gt;&lt;mi mathvariant="normal"&gt;core&lt;/mi&gt;&lt;/msub&gt;&lt;mo&gt;⊆&lt;/mo&gt;&lt;msub&gt;&lt;mi&gt;W&lt;/mi&gt;&lt;mi mathvariant="normal"&gt;core&lt;/mi&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;span&gt;&lt;svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="107pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="4b48f5ce235ae08f6aa376e6e7adc73c"&gt;&lt;svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-10717-2019-ie00001.svg" width="107pt" height="13pt" src="acp-19-10717-2019-ie00001.png"/&gt;&lt;/svg:svg&gt;&lt;/span&gt;&lt;/span&gt;. This property is seen for several different thermodynamic profile initializations and is generally maintained during the growing and mature stages of a cloud's lifetime. This finding is in line with previous works and theoretical predictions showing that cumulus clouds may be dominated by negative buoyancy at certain stages of their lifetime. The RH&lt;span class="inline-formula"&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt;–&lt;span class="inline-formula"&gt;&lt;i&gt;W&lt;/i&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt; pair is most interchangeable, especially during the growing stages of the cloud.&lt;/p&gt; &lt;p&gt;For all three definitions, the core–shell model of a core (positive values) at the center of the cloud surrounded by a shell (negative values) at the cloud periphery applies to over 80&amp;amp;thinsp;% of a typical cloud's lifetime. The core–shell model is less appropriate in larger clouds with multiple cores displaced from the cloud center. Larger clouds may also exhibit buoyancy cores centered near the cloud edge. During dissipation the cores show less overlap, reduce in size, and may migrate from the cloud center.&lt;/p&gt; </jats:p>


Core and margin in warm convective clouds – Part 2: Aerosol effects on core properties

Atmospheric Chemistry and Physics Copernicus GmbH 19 (2019) 10739-10755

RH Heiblum, L Pinto, O Altaratz, G Dagan, I Koren

<jats:p>&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; The effects of aerosol on warm convective cloud cores are evaluated using single cloud and cloud field simulations. Three core definitions are examined: positive vertical velocity (&lt;span class="inline-formula"&gt;&lt;i&gt;W&lt;/i&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt;), supersaturation (RH&lt;span class="inline-formula"&gt;&lt;sub&gt;core&lt;/sub&gt;)&lt;/span&gt;, and positive buoyancy (&lt;span class="inline-formula"&gt;&lt;i&gt;B&lt;/i&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt;). As presented in Part 1 (Heiblum et al., 2019), the property &lt;span class="inline-formula"&gt;&lt;math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;B&lt;/mi&gt;&lt;mi mathvariant="normal"&gt;core&lt;/mi&gt;&lt;/msub&gt;&lt;mo&gt;⊆&lt;/mo&gt;&lt;msub&gt;&lt;mi mathvariant="normal"&gt;RH&lt;/mi&gt;&lt;mi mathvariant="normal"&gt;core&lt;/mi&gt;&lt;/msub&gt;&lt;mo&gt;⊆&lt;/mo&gt;&lt;msub&gt;&lt;mi&gt;W&lt;/mi&gt;&lt;mi mathvariant="normal"&gt;core&lt;/mi&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;span&gt;&lt;svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="107pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="4b48f5ce235ae08f6aa376e6e7adc73c"&gt;&lt;svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-10739-2019-ie00001.svg" width="107pt" height="13pt" src="acp-19-10739-2019-ie00001.png"/&gt;&lt;/svg:svg&gt;&lt;/span&gt;&lt;/span&gt; is seen during growth of warm convective clouds. We show that this property is kept irrespective of aerosol concentration. During dissipation core fractions generally decrease with less overlap between cores. However, for clouds that develop in low aerosol concentrations capable of producing precipitation, &lt;span class="inline-formula"&gt;&lt;i&gt;B&lt;/i&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt; and subsequently &lt;span class="inline-formula"&gt;&lt;i&gt;W&lt;/i&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt; volume fractions may increase during dissipation (i.e., loss of cloud mass). The RH&lt;span class="inline-formula"&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt; volume fraction decreases during cloud lifetime and shows minor sensitivity to aerosol concentration.&lt;/p&gt; &lt;p&gt;It is shown that a &lt;span class="inline-formula"&gt;&lt;i&gt;B&lt;/i&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt; forms due to two processes: (i) convective updrafts – condensation within supersaturated updrafts and release of latent heat – and (ii) dissipative downdrafts – subsaturated cloudy downdrafts that warm during descent and “undershoot” the level of neutral buoyancy. The former process occurs during cloud growth for all aerosol concentrations. The latter process only occurs for low aerosol concentrations during dissipation and precipitation stages where large mean drop sizes permit slow evaporation rates and subsaturation during descent.&lt;/p&gt; &lt;p&gt;The aerosol effect on the diffusion efficiencies plays a crucial role in the development of the cloud and its partition to core and margin. Using the RH&lt;span class="inline-formula"&gt;&lt;sub&gt;core&lt;/sub&gt;&lt;/span&gt; definition, it is shown that the total cloud mass is mostly dictated by core processes, while the total cloud volume is mostly dictated by margin processes. Increase in aerosol concentration increases the core (mass and volume) due to enhanced condensation but also decreases the margin due to evaporation. In clean clouds larger droplets evaporate much slower, enabling preservation of cloud size, and even increase by detrainment and dilution (volume increases while losing mass). This explains how despite having smaller cores and less mass, cleaner clouds may live longer and grow to larger sizes.&lt;/p&gt; </jats:p>


Non-Monotonic Aerosol Effect on Precipitation in Convective Clouds over Tropical Oceans

Scientific Reports Springer Science and Business Media LLC 9 (2019) 7809

H Liu, J Guo, I Koren, O Altaratz, G Dagan, Y Wang, JH Jiang, P Zhai, YL Yung


Feedback mechanisms of shallow convective clouds in a warmer climate as demonstrated by changes in buoyancy

Environmental Research Letters 13 (2018) 054033-054033

G Dagan, I Koren, O Altaratz, G Feingold


Quantifying the effect of aerosol on vertical velocity and effective terminal velocity in warm convective clouds

Atmospheric Chemistry and Physics 18 (2018) 6761-6769

G Dagan, I Koren, O Altaratz


The effects of the spatial distribution of direct anthropogenic aerosols radiative forcing on atmospheric circulation

Journal of Climate (2018)

R Chemke, G Dagan


Organization and oscillations in simulated shallow convective clouds

Journal of Advances in Modeling Earth Systems Wiley 10 (2018) 2287-2299

G Dagan, I Koren, A Kostinski, O Altaratz

Physical insights into processes governing temporal organization and evolution of cloud fields are of great importance for climate research. Here using large eddy simulations with a bin microphysics scheme, we show that warm convective cloud fields exhibit oscillations with two distinct periods (~10 and ~90 min, for the case studied here). The shorter period dominates the nonprecipitating phase, and the longer period is related to the precipitating phase. We show that rain processes affect the domain’s thermodynamics, hence forcing the field into a low-frequency recharge-discharge cycle of developing cloudiness followed by precipitation-driven depletion. The end result of precipitation is stabilization of the lower atmosphere by warming of the cloudy layer (due to latent heat release) and cooling of the subcloud layer (by rain evaporation, creating cold pools). As the thermodynamic instability weakens, so does the cloudiness, and the rain ceases. During the nonprecipitating phase of the cycle, surface fluxes destabilize the boundary layer until the next precipitation cycle. Under conditions that do not allow development of precipitation (e.g., high aerosol loading), high-frequency oscillations dominate the cloud field. Clouds penetrating the stable inversion layer trigger gravity waves with a typical period of ~10 min. In return, the gravity waves modulate the clouds in the field by modifying the vertical velocity, temperature, and humidity fields. Subsequently, as the polluted nonprecipitating simulations evolve, the thermodynamic instability increases and the cloudy layer deepens until precipitation forms, shifting the oscillations from high to low frequency. The organization of cold pools and the spatial scale related to these oscillations are explored.


Shallow Convective Cloud Field Lifetime as a Key Factor for Evaluating Aerosol Effects

iScience Elsevier BV 10 (2018) 192-202

G Dagan, I Koren, O Altaratz, Y Lehahn


Time-dependent, non-monotonic response of warm convective cloud fields to changes in aerosol loading

Atmospheric Chemistry and Physics 17 (2017) 7435-7444

G Dagan, I Koren, O Altaratz, RH Heiblum


How do changes in warm-phase microphysics affect deep convective clouds?

Atmospheric Chemistry and Physics 17 (2017) 9585-9598

Q Chen, I Koren, O Altaratz, RH Heiblum, G Dagan, L Pinto


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

Journal of Geophysical Research: Atmospheres 121 (2016) 6356-6373

RH Heiblum, O Altaratz, I Koren, G Feingold, AB Kostinski, AP Khain, M Ovchinnikov, E Fredj, G Dagan, L Pinto, R Yaish, Q Chen


Characterization of cumulus cloud fields using trajectories in the center of gravity versus water mass phase space: 1. Cloud tracking and phase space description

Journal of Geophysical Research: Atmospheres 121 (2016) 6336-6355

RH Heiblum, O Altaratz, I Koren, G Feingold, AB Kostinski, AP Khain, M Ovchinnikov, E Fredj, G Dagan, L Pinto, R Yaish, Q Chen


The effect of subtropical aerosol loading on equatorial precipitation

Geophysical Research Letters 43 (2016) 11,048-11,056

G Dagan, R Chemke


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

Scientific Reports 6 (2016)

G Dagan, I Koren, O Altaratz, RH Heiblum


Aerosol effect on the mobility of cloud droplets

Environmental Research Letters IOP Publishing 10 (2015) 104011-104011

I Koren, O Altaratz, G Dagan

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