Shallow marine mixed-phase clouds are important for the Earth's radiative balance, but modelling their formation and dynamics is challenging. These clouds depend on boundary layer turbulence and ...cloud top radiative cooling, which is related to the cloud phase. The fraction of frozen droplets depends on the availability of suitable ice-nucleating particles (INPs), which initiate droplet freezing. While mineral dust is the dominating INP type in most regions, high-latitude boundary layer clouds can be dependent on local marine INP emissions, which are often related to biogenic sources including phytoplankton. Here we use high resolution large eddy simulations to examine the potential effects of marine emissions on boundary layer INP concentrations and their effects on clouds. Surface emissions have a direct effect on INP concentration in a typical well-mixed boundary layer whereas a steep inversion can block the import of background INPs from the free troposphere. The importance of the marine source depends on the background INP concentration, so that marine INP emissions become more important with lower background INP concentrations. For the INP budget it is also important to account for INP recycling. Finally, with the high-resolution model we show how ice nucleation hotspots and high INP concentrations are focused on updraught regions. Our results show that marine INP emissions contribute directly to the boundary layer INP budget and therefore have an influence on mixed-phase clouds.
In this paper, we consider the cloud drop activation of aerosol particles consisting of water soluble material and an insoluble core. Based on the Köhler theory, we derive analytical equations for ...the critical diameters and supersaturations of such particles. We demonstrate the use of the equations by comparing the critical supersaturations of particles composed of ammonium sulfate and insoluble substances with those of model organic particles with varying molecular sizes.
The large-eddy model UCLALES–SALSA, with an exceptionally detailed aerosol description for both aerosol number and chemical composition, has been extended for ice and mixed-phase clouds.
Comparison ...to a previous mixed-phase cloud model intercomparison study confirmed the accuracy of newly implemented ice microphysics.
A further simulation with a heterogeneous ice nucleation scheme, in which ice-nucleating particles (INPs) are also a prognostic variable, captured the typical layered structure of Arctic mid-altitude mixed-phase cloud: a liquid layer near cloud top and ice within and below the liquid layer.
In addition, the simulation showed a realistic freezing rate of droplets within the vertical cloud structure.
The represented detailed sectional ice microphysics with prognostic aerosols is crucially important in reproducing mixed-phase clouds.
The goal of this study is to investigate the role of
organic aerosols emitted with sea spray or formed from marine gas phase
emissions of volatile organic compounds (VOCs) in influencing the ...stability
of stratiform marine clouds. We aim to point out the processes and
drivers that could be relevant for global climate and should thus be
considered in large-scale models. We employ a large eddy simulator coupled with an
aerosol–cloud microphysical model together with different parameterizations
for emission of sea salt, primary organic aerosol, and VOCs from sea surface
and formation of secondary organic aerosol (SOA), to simulate the conditions
of the second Dynamics and Chemistry of Marine Stratocumulus observational campaign characterized by low-level
stratocumulus clouds transitioning from closed cells to drizzling open cell
structure. We find that the inclusion of sea spray emissions can both extend and
shorten the transitioning timescale between closed and open cells based on
the parameterization employed. Fine sea spray provides extra cloud
condensation nuclei (CCN) and delays the onset of drizzle as the
collision–coalescence process is slowed down due to smaller cloud droplet
mean size. The coarse mode has an opposite effect due to giant CCN (GCCN)
speeding up the drizzle formation through the enhanced collision–coalescence
processes. The balance between two processes depends on the model
parameterization employed. Compared to differences between different sea
spray parameterizations, the sensitivity of the clouds to the variations in
organic fraction of sea spray and hygroscopicity of the emitted particles is
relatively limited. However, our results show that it is important to
account for the size dependence of the sea spray organic fraction as
attributing organic emissions to coarse mode noticeably reduces the GCCN
effect. In addition, including the secondary organic aerosol formation
from VOCs can potentially have a noticeable impact, but only when emitting
the highest observed fluxes of monoterpenes. This impact is also highly
sensitive on the size distribution of the background aerosol population. SOA
production from isoprene is visible only if aqueous phase SOA production
pathways are included, and even then, the effect is lower than from
monoterpenes.
The global aerosol–climate model ECHAM6.3–HAM2.3 (E63H23)
as well as the previous model versions ECHAM5.5–HAM2.0 (E55H20) and ECHAM6.1–HAM2.2
(E61H22) are evaluated using global observational ...datasets for clouds and
precipitation. In E63H23, the amount of low clouds, the liquid and ice water path, and
cloud radiative effects are more realistic than in previous model versions.
E63H23 has a more physically based aerosol activation scheme, improvements
in the cloud cover scheme, changes in the detrainment of convective clouds,
changes in the sticking efficiency for the accretion of ice crystals by snow,
consistent ice crystal shapes throughout the model, and changes in mixed-phase
freezing; an inconsistency in ice crystal number concentration (ICNC) in
cirrus clouds was also removed. Common biases in ECHAM and in E63H23 (and in
previous ECHAM–HAM versions) are a cloud amount in stratocumulus
regions that is too low and deep convective clouds over the Atlantic and Pacific oceans
that form too close to the continents (while tropical land precipitation is
underestimated). There are indications that ICNCs are overestimated in
E63H23. Since clouds are important for effective radiative forcing due to
aerosol–radiation and aerosol–cloud interactions (ERFari+aci) and
equilibrium climate sensitivity (ECS), differences in ERFari+aci
and ECS between the model versions were also analyzed. ERFari+aci is weaker
in E63H23 (−1.0 W m−2) than in E61H22 (−1.2 W m−2) (or E55H20;
−1.1 W m−2). This is caused by the weaker shortwave ERFari+aci
(a new aerosol activation scheme and sea salt emission parameterization in
E63H23, more realistic simulation of cloud water) overcompensating for the
weaker longwave ERFari+aci (removal of an inconsistency in ICNC in
cirrus clouds in E61H22). The decrease in ECS in E63H23 (2.5 K) compared to E61H22 (2.8 K) is due to
changes in the entrainment rate for shallow convection (affecting the cloud
amount feedback) and a stronger cloud phase feedback. Experiments with minimum cloud droplet number concentrations (CDNCmin) of
40 cm−3 or 10 cm−3 show that a higher value of CDNCmin reduces
ERFari+aci as well as ECS in E63H23.
In this study we introduce an in-cloud wet deposition scheme for liquid and ice phase clouds for global aerosol–climate models which use a size-segregated aerosol description. For in-cloud nucleation ...scavenging, the scheme uses cloud droplet activation and ice nucleation rates obtained from the host model. For in-cloud impaction scavenging, we used a method where the removal rate depends on the wet aerosol size and cloud droplet radii. We used the latest release version of ECHAM-HAMMOZ (ECHAM6.3-HAM2.3-MOZ1.0) with the Sectional Aerosol module for Large Scale Applications version 2.0 (SALSA) microphysics package to test and compare our scheme. The scheme was compared to a scheme that uses fixed scavenging coefficients. The comparison included vertical profiles and mass and number distributions of wet deposition fluxes of different aerosol compounds and for different latitude bands. Using the scheme presented here, mass concentrations for black carbon, organic carbon, sulfate, and the number concentration of particles with diameters larger than 100 nm are higher than using fixed scavenging coefficients, with the largest differences in the vertical profiles in the Arctic. On the other hand, the number concentrations of particles smaller than 100 nm in diameter show a decrease, especially in the Arctic region. These results could indicate that, compared to fixed scavenging coefficients, nucleation scavenging is less efficient, resulting in an increase in the number concentration of particles larger than 100 nm. In addition, changes in rates of impaction scavenging and new particle formation (NPF) can be the main cause of reduction in the number concentrations of particles smaller than 100 nm. Without further adjustments in the host model, our wet deposition scheme produced unrealistically high aerosol concentrations, especially at high altitudes. This also leads to a spuriously long lifetime of black carbon aerosol. To find a better setup for simulating aerosol vertical profiles and transport, sensitivity simulations were conducted where aerosol emission distribution and hygroscopicity were altered. Vertical profiles of aerosol species simulated with the scheme which uses fixed scavenging rates and the abovementioned sensitivity simulations were evaluated against vertical profiles from aircraft observations. The lifetimes of different aerosol compounds were also evaluated against the ensemble mean of models involved in the Aerosol Comparisons between Observations and Models (AEROCOM) project. The best comparison between the observations and the model was achieved with our wet deposition scheme when black carbon was emitted internally mixed with soluble compounds instead of keeping it externally mixed. This also produced atmospheric lifetimes for the other species which were comparable to the AEROCOM model means.
This is the second of two papers in which we study the dependency of the impacts of stratospheric sulfur injections on the model and injection strategy used. Here, aerosol optical properties from ...simulated stratospheric aerosol injections using two aerosol models (modal scheme M7 and sectional scheme SALSA), as described in Part 1 (Laakso et al., 2022), are implemented consistently into the EC-Earth, MPI-ESM and CESM Earth system models (ESMs) to simulate the climate impacts of different injection rates ranging from 2 to 100 Tg(S) yr−1. Two sets of simulations were run with the three ESMs: (1) regression simulations, in which an abrupt change in CO2 concentration or stratospheric aerosols over pre-industrial conditions was applied to quantify global mean fast temperature-independent climate responses and quasi-linear dependence on temperature, and (2) equilibrium simulations, in which radiative forcing of aerosol injections with various magnitudes compensated for the corresponding radiative forcing of CO2 enhancement to study the dependence of precipitation on the injection magnitude. The latter also allow one to explore the regional climatic responses. Large differences in SALSA- and M7-simulated radiative forcing in Part 1 translated into large differences in the estimated surface temperature and precipitation changes in ESM simulations; for example, an injection rate of 20 Tg(S) yr−1 in CESM using M7-simulated aerosols led to only 2.2 K global mean cooling, while EC-Earth–SALSA combination produced a 5.2 K change. In equilibrium simulations, where aerosol injections were utilized to offset the radiative forcing caused by an atmospheric CO2 concentration of 500 ppm, the decrease in global mean precipitation varied among models, ranging from −0.7 % to −2.4 % compared with the pre-industrial climate. These precipitation changes can be explained by the fast precipitation response due to radiation changes caused by the stratospheric aerosols and CO2, as the global mean fast precipitation response is shown to be negatively correlated with global mean atmospheric absorption. Our study shows that estimating the impact of stratospheric aerosol injection on climate is not straightforward. This is because the simulated capability of the sulfate layer to reflect solar radiation and absorb long-wave radiation is sensitive to the injection rate as well as the aerosol model used to simulate the aerosol field. These findings emphasize the necessity for precise simulation of aerosol microphysics to accurately estimate the climate impacts of stratospheric sulfur intervention. This study also reveals gaps in our understanding and uncertainties that still exist related to these controversial techniques.
We consider the equilibrium behavior of a polydisperse aqueous droplet population as a function of relative humidity (RH) when a soluble trace gas, such as nitric acid, is present in the system. The ...droplet population experiences a splitting when the RH is increased sufficiently. This splitting is not related to the traditional Köhler activation of cloud droplets, as it may occur at relative humidities below 100%. Remarkably, the splitting always takes place in such a way that the largest size class of the (discretized) droplet population starts taking up the soluble trace gas efficiently, growing steeply as a function of RH, and forcing the smaller droplets to shrink. We consider this behavior in terms of open and closed system Köhler curves (open system referring to one in which the trace gas concentration remains constant and closed system to one in which the gas concentration decreases as a result of uptake of the trace gas). We show how the open and closed system Köhler curves are related, and that the splitting of the population can be explained in terms of closed system curves crossing the Köhler maxima of the open system curves. We then go on to consider time-dependent situations, and show that due to gas-phase mass transfer limitations, the splitting of the size distributions moves toward smaller sizes as the rate of RH increase becomes more rapid. Finally, we consider stratospheric supercooled ternary solution droplet populations, and show that the splitting described using the new theory may lead to formation of bimodal size distributions in the stratosphere.
Atmospheric aerosol particle concentrations are strongly affected by various wet processes, including below and in-cloud wet scavenging and in-cloud aqueous-phase oxidation. We studied how wet ...scavenging and cloud processes affect particle concentrations and composition during transport to a rural boreal forest site in northern Europe. For this investigation, we employed air mass history analysis and observational data. Long-term particle number size distribution (â¼15 years) and composition measurements (â¼8 years) were combined with air mass trajectories with relevant variables from reanalysis data. Some such variables were rainfall rate, relative humidity, and mixing layer height. Additional observational datasets, such as temperature and trace gases, helped further evaluate wet processes along trajectories with mixed effects models.
The uptake of water by atmospheric aerosols has a pronounced effect on particle light scattering properties, which in turn are strongly dependent on the ambient relative humidity (RH). Earth system ...models need to account for the aerosol water uptake and its influence on light scattering in order to properly capture the overall radiative effects of aerosols. Here we present a comprehensive model–measurement evaluation of the particle light scattering enhancement factor f(RH), defined as the particle light scattering coefficient at elevated RH (here set to 85 %) divided by its dry value. The comparison uses simulations from 10 Earth system models and a global dataset of surface-based in situ measurements. In general, we find a large diversity in the magnitude of predicted f(RH) amongst the different models, which can not be explained by the site types. Based on our evaluation of sea salt scattering enhancement and simulated organic mass fraction, there is a strong indication that differences in the model parameterizations of hygroscopicity and model chemistry are driving at least some of the observed diversity in simulated f(RH). Additionally, a key point is that defining dry conditions is difficult from an observational point of view and, depending on the aerosol, may influence the measured f(RH). The definition of dry also impacts our model evaluation, because several models exhibit significant water uptake between RH = 0 % and 40 %. The multisite average ratio between model outputs and measurements is 1.64 when RH = 0 % is assumed as the model dry RH and 1.16 when RH = 40 % is the model dry RH value. The overestimation by the models is believed to originate from the hygroscopicity parameterizations at the lower RH range which may not implement all phenomena taking place (i.e., not fully dried particles and hysteresis effects). This will be particularly relevant when a location is dominated by a deliquescent aerosol such as sea salt. Our results emphasize the need to consider the measurement conditions in such comparisons and recognize that measurements referred to as dry may not be dry in model terms. Recommendations for future model–measurement evaluation and model improvements are provided.
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