A lack of reliable estimates of cloud condensation nuclei (CCN) aerosols over oceans has severely limited our ability to quantify their effects on cloud properties and extent of cooling by reflecting ...solar radiation-a key uncertainty in anthropogenic climate forcing. We introduce a methodology for ascribing cloud properties to CCN and isolating the aerosol effects from meteorological effects. Its application showed that for a given meteorology, CCN explains three-fourths of the variability in the radiative cooling effect of clouds, mainly through affecting shallow cloud cover and water path. This reveals a much greater sensitivity of cloud radiative forcing to CCN than previously reported, which means too much cooling if incorporated into present climate models. This suggests the existence of compensating aerosol warming effects yet to be discovered, possibly through deep clouds.
The estimation of cloud radiative forcing due to aerosol‐cloud interactions, RFaci (also known as the first indirect effect), relies on approximating the cloud albedo susceptibility to changes in ...droplet concentration, β. β depends on the cloud albedo and droplet concentration, both of which can be observed by satellites. Satellite observations are often spatially aggregated to coarser resolutions, typically 1 × 1° scenes. However, on such spatial scales, the cloud albedo tends to be heterogeneous, whereas the β approximation assumes homogeneity. Here, we demonstrate that the common practice of aggregating satellite data and neglecting cloud albedo heterogeneity results in an average overestimation of 10% in previous estimates of the RFaci. Additionally, we establish a relationship between the magnitude of the bias in β and Stratocumulus morphologies, providing a physical context for cloud heterogeneity and the associated bias. Lastly, we propose a correction method that can be applied to cloud albedo gridded data.
Plain Language Summary
This paper explores the effect of cloud albedo morphology, which is a reflection of cloud heterogeneity, on radiative forcing due to aerosol‐cloud interactions (RFaci). The RFaci is estimated from satellite observations based on the assumption that clouds are homogeneous within a given scene. However, when satellite data is spatially aggregated to reduce the amount of data to a user‐friendly gridded format—a common practice—this assumption is no longer valid. Consequently, an overestimation of the RFaci occurs, particularly in heterogeneous scenes, where the overestimation can reach up to 50%. This means that the RFaci is lower than previously estimated. Our results also suggest that cloud albedo enhancement due to an increase in droplet concentrations would be most effective in homogeneous scenes. Therefore, marine cloud brightening strategies should take cloud albedo homogeneity into account to achieve the most effective albedo enhancement.
Key Points
The common practice of spatial aggregation of satellite data into 1 × 1° scenes leads to an average 10% overestimation of the RFaci
The overestimation is due to neglecting cloud albedo heterogeneity, and is associated to different types of Stratocumulus morphologies
A correction is proposed, which calls for the incorporation of cloud reflectance statistics in Level 3 data
The impact of aerosols on cloud properties is one of the largest
uncertainties in the anthropogenic radiative forcing of the climate.
Significant progress has been made in constraining this forcing ...using
observations, but uncertainty remains, particularly in the magnitude
of cloud rapid adjustments to aerosol perturbations. Cloud liquid
water path (LWP) is the leading control on liquid-cloud albedo, making
it important to observationally constrain the aerosol impact on LWP. Previous modelling and observational studies have shown that multiple
processes play a role in determining the LWP response to aerosol
perturbations, but that the aerosol effect can be difficult to isolate.
Following previous studies using mediating variables, this work investigates
use of the relationship between cloud droplet number concentration
(Nd) and LWP for constraining the role of aerosols. Using
joint-probability histograms to account for the non-linear relationship, this
work finds a relationship that is broadly consistent with previous studies.
There is significant geographical variation in the relationship, partly due
to role of meteorological factors (particularly relative humidity).
The Nd–LWP
relationship is negative in the majority of regions, suggesting that
aerosol-induced LWP reductions could offset a significant fraction of the
instantaneous radiative forcing from aerosol–cloud interactions (RFaci). However, variations in the Nd–LWP relationship in response to
volcanic and shipping aerosol perturbations indicate that the
Nd–LWP relationship overestimates the causal Nd impact
on LWP due to the role of confounding factors. The weaker LWP reduction
implied by these “natural experiments” means that this work provides an
upper bound to the radiative forcing from aerosol-induced changes in the LWP.
The number concentration of cloud particles is a
key quantity for understanding aerosol–cloud interactions and describing
clouds in climate and numerical weather prediction models. In contrast with
...recent advances for liquid clouds, few observational constraints exist
regarding the ice crystal number concentration (Ni). This study
investigates how combined lidar–radar measurements can be used to provide
satellite estimates of Ni, using a methodology that constrains
moments of a parameterized particle size distribution (PSD). The operational
liDAR–raDAR (DARDAR) product serves as an existing base for this method,
which focuses on ice clouds with temperatures Tc<-30 ∘C. Theoretical considerations demonstrate the capability for accurate retrievals
of Ni, apart from a possible bias in the concentration in small
crystals when Tc≳−50 ∘C, due to the assumption of
a monomodal PSD shape in the current method. This is verified via a
comparison of satellite estimates to coincident in situ measurements, which additionally
demonstrates the sufficient sensitivity of lidar–radar observations to
Ni. Following these results, satellite estimates of
Ni are evaluated in the context of a case study and a preliminary
climatological analysis based on 10 years of global data. Despite a lack
of other large-scale references, this evaluation shows a reasonable physical
consistency in Ni spatial distribution patterns. Notably,
increases in Ni are found towards cold temperatures and, more
significantly, in the presence of strong updrafts, such as those related to
convective or orographic uplifts. Further evaluation and improvement of this
method are necessary, although these results already constitute a first encouraging
step towards large-scale observational constraints for Ni. Part 2
of this series uses this new dataset to examine the controls on
Ni.
Quantifying the aerosol/cloud-mediated radiative effect at a global scale requires simultaneous satellite retrievals of cloud condensation nuclei (CCN) concentrations and cloud base updraft ...velocities (Wb
). Hitherto, the inability to do so has been a major cause of high uncertainty regarding anthropogenic aerosol/cloud-mediated radiative forcing. This can be addressed by the emerging capability of estimating CCN and Wb
of boundary layer convective clouds from an operational polar orbiting weather satellite. Our methodology uses such clouds as an effective analog for CCN chambers. The cloud base supersaturation (S) is determined by Wb
and the satellite-retrieved cloud base drop concentrations (Ndb
), which is the same as CCN(S). Validation against ground-based CCN instruments at Oklahoma, at Manaus, and onboard a ship in the northeast Pacific showed a retrieval accuracy of ±25% to ±30% for individual satellite overpasses. The methodology is presently limited to boundary layer not raining convective clouds of at least 1 km depth that are not obscured by upper layer clouds, including semitransparent cirrus. The limitation for small solar backscattering angles of <25° restricts the satellite coverage to ∼25% of the world area in a single day.
Important aspects of the adjustments to aerosol-cloud interactions can be examined using the relationship between cloud droplet number concentration (Nd) and liquid water path (LWP). Specifically, ...this relation can constrain the role of aerosols in leading to thicker or thinner clouds in response to adjustment mechanisms. This study investigates the satellite retrieved relationship between Nd and LWP for a selected case of mid-latitude continental clouds using high-resolution Large-eddy simulations (LES) over a large domain in weather prediction mode. Since the satellite retrieval uses the adiabatic assumption to derive the Nd, we have also considered adiabatic Nd (NAd) from the LES model for comparison. The joint histogram analysis shows that the NAd-LWP relationship in the LES model and the satellite is in approximate agreement. In both cases, the peak conditional probability (CP) is confined to lower NAd and LWP; the corresponding mean LWP (LWP) shows a weak relation with NAd. The CP shows a larger spread at higher NAd (>50 cm), and the LWP increases non-monotonically with increasing NAd in both cases. Nevertheless, both lack the negative NAd-LWP relationship at higher NAd, the entrainment effect on cloud droplets. In contrast, the model simulated Nd-LWP clearly illustrates a much more nonlinear (an increase in LWP with increasing Nd and a decrease in LWP at higher Nd) relationship, which clearly depicts the cloud lifetime and the entrainment effect. Additionally, our analysis demonstrates a regime dependency (marine and continental) in the NAd-LWP relation from the satellite retrievals. Comparing local vs large-scale statistics from satellite data shows that continental clouds exhibit only a weak nonlinear NAd-LWP relationship. Hence a regime-based Nd-LWP analysis is even more relevant when it comes to warm continental clouds and their comparison to satellite retrievals.
Marine stratocumulus clouds (MSC) cover large areas over the oceans and possess super sensitivity of their cloud radiative effect to changes in aerosol concentrations. Aerosols can cause transitions ...between regimes of fully cloudy closed cells and open cells. The possible role of aerosols in cloud cover has a big impact on the amount of reflected solar radiation from the clouds, thus potentially constitutes very large aerosol indirect radiative effect, which can exceed 100 Wm−2. It is hypothesized that continentally polluted clouds remain in closed cells regime for longer time from leaving continent and hence for longer distance away from land, thus occupying larger ocean areas with full cloud cover. Attributing this to anthropogenic aerosols would imply a very large negative radiative forcing with a significant climate impact. This possibility is confirmed by analyzing a detailed case study based on geostationary and polar‐orbiting satellite observations of the microphysical and dynamical evolution of MSC. We show that large area of closed cells was formed over the northeast Atlantic Ocean downwind of Europe in a continentally polluted air mass. The closed cells undergo cleansing process that was tracked for 3.5 days that resulted with a rapid transition from closed to open cells once the clouds started drizzling heavily. The mechanism leading to the eventual breakup of the clouds due to both meteorological and aerosol considerations is elucidated. We termed this cleansing and cloud breakup process maritimization. Further study is needed to assess the climatological significance of such situations.
Key Points
Polluted MSC remain as closed cells for longer time
Continents can act as huge aerosol sources and form “continent tracks”
Potentially hitherto unrecognized large negative RF over large area and time
A method for separating the three components of the marine stratocumulus (MSC) aerosol cloud interactions radiative effects, i.e., the cloud cover, liquid water path (LWP) and cloud drop radius ...(Twomey), was developed and tested. It is based on the assumption that changes in MSC cloud regimes that occur at short distance in homogeneous meteorological conditions are related to respective changes in the concentration of cloud condensation nuclei (CCN). The method was applied to 50 cases of well defined transitions from closed to open cells. It was found that the negative cloud radiative effect (CRE) over the closed cells is on average higher by 109±18Wm−2 than that over the adjacent open cells. This large negative CRE is composed of the cloud cover (42±8%), LWP (32±8%) and Twomey (26±6%) effects. This shows that the Twomey effect, which is caused by change in droplet concentration for a given LWP, contributes only a quarter of the difference in CRE, whereas the rest is contributed by added cloud water to the open cells both in the horizontal (cloud cover effect) and in the vertical (LWP effect) dimensions. The results suggest the possibility that anthropogenic aerosols that affect MSC-regime-changes might incur large negative radiative forcing on the global scale, mainly due to the cloud cover effect.
•Cloud mediated aerosol radiative forcing is decomposed into three components.•The components are cloud cover, liquid water path and cloud drop radius effect.•The drop radius effect (Twomey) contributes only a quarter of the total forcing.•Most of the forcing in marine stratocumulus is due to LWP and cover effects.
Documentation of the evolution of ship tracks during 42 h demonstrated that ship emissions are able to convert a marine stratocumulus regime of open cells into closed cells, along with significant ...negative radiative forcing. This was possible by examining continuous day and night geostationary satellite data that allowed for an uninterrupted documentation of the full life cycle of ship tracks. After nearly one day the ship tracks lost their linear appearance and expanded to cover large areas. These areas, when viewed out of sequence and context, would not be attributable to aerosol perturbations. A rejuvenation of previously dissipated ship tracks in the form of extensive closed cells was also observed. It is suggested that ship emissions may undergo photochemical reactions which nucleate new aerosols that, along with remaining ultrafine particles, grow to CCN that are activated hours later and close the open cells. The added radiative forcing from the closed cells that can be related to the ship tracks, which is mainly coming from the cloud cover effect, may exceed −100 Wm−2, depending on the season and latitude. This implies that anthropogenic aerosols that can be transported from continents through the boundary layer, or travel in the free troposphere and mix with the boundary layer from above, may explain the formation of large closed cells areas that are presently not recognized as originated by aerosol perturbations. The observations reported here pose a demanding test of the ability of cloud resolving models to replicate cloud‐aerosol interactions.
Key Points
Aerosols can close large and small open cells
Ship tracks expanded to large areas of closed cells
Large sources of aerosols may incur large climate forcing
In situ and spaceborne studies reveal the prevalence of thin clouds in the major Stratocumulus‐to‐Cumulus Transition (SCT) regions. Using instantaneous satellite and reanalysis data, this study ...investigates the properties of thin clouds in the Southeast Pacific Ocean and their impact on the cloud radiative effect (CRE). Our findings demonstrate that thin clouds are intrinsic to the SCT. The overcast stratocumulus‐dominated regime exhibits a minimal presence of thin clouds, which become notably prominent after the clouds breakup into the cumulus‐dominated regime. The regime dependence of the occurrence of thin clouds is also observed in terms of the marine cold‐air outbreak parameter and the sea surface temperature. Thin clouds at a given cloud cover significantly modulate the shortwave (SW) and longwave (LW) components of CRE. SW CRE decreases by 46 %–65 % with increasing thin cloud cover. They account for a larger variance in cloud albedo than the combined influence of the liquid water path and effective radius. Furthermore, LW CRE decreases by about 12 %–52 % with thin cloud cover. An increase in the fraction of thin clouds also leads to a larger fraction of negative SW CRE offset by positive LW CRE at a given cloud cover. This LW compensation ranges from approximately 8 % at overcast cloud cover to as much as 19 % at about 50 % cloud cover. These findings elucidate the crucial role of thin clouds, and thus cloud morphology, in modulating CRE and underscore the necessity of their accurate representation in climate models.
Plain Language Summary
The eastern regions of major global oceans are primarily covered by low‐altitude clouds. These bright clouds overlay the dark ocean surface, reflecting incoming solar radiation and cooling the Earth. Concurrently, they act as a barrier to the energy emitted by the ocean surface, preventing it from escaping the Earth and thereby contributing to the Earth's warming. Satellite observations reveal the prevalence of thin clouds in these oceanic regions. In contrast to their thicker counterparts, thin clouds are less effective in reflecting solar radiation and blocking surface emissions. This study investigates the impact of thin clouds on radiation using data from various sources. We show that the presence of thin clouds significantly reduces cloud brightness by approximately 46 %–65 %, even when the cloud cover remains constant. Thin clouds have a more substantial effect on a cloud's ability to reflect solar radiation compared to the combined influence of cloud water and cloud droplet size. Furthermore, since thin clouds allow some surface emissions to pass through, their presence at a constant cloud cover reduces their warming effect by up to 52 %. These findings emphasize the crucial role of thin clouds in shaping the contribution of clouds to Earth's energy balance.
Key Points
Thin clouds dim cloud brightness by up to 65 percent at a fixed cloud cover
Thin clouds exert a greater influence on cloud albedo than the combined effects of cloud water and droplet size under constant cloud cover
Thin clouds reduce longwave warming by up to 52 percent and modulate the balance between longwave warming and shortwave cooling
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