Marine low clouds are one of the greatest sources of uncertainty for climate projection. We present an observed climatology of cloud albedo susceptibility to cloud droplet number concentration ...perturbations (S0) with changing sea surface temperature (SST) and estimated inversion strength for single‐layer warm clouds over the North Atlantic Ocean, using eight years of satellite and reanalysis data. The key findings are that SST has a dominant control on S0 in the presence of co‐varying synoptic conditions and aerosol perturbations. Regions conducive to aerosol‐induced darkening (brightening) clouds occur with high (low) local SST. Higher SST significantly hastens cloud‐top evaporation with increasing aerosol loading, by accelerating entrainment and facilitating entrainment drying. In a global‐warming‐like scenario where aerosol loading is reduced, less cloud darkening is expected, mainly as a result of reduced entrainment drying. Our results imply a less positive low‐cloud liquid water path feedback in a warmer climate with decreasing aerosol loading.
Plain Language Summary
Low clouds over the ocean are a poorly quantified component of the climate system. Here we use eight years of space‐based measurements and meteorological data to quantify how the brightness of single‐layer low clouds over the North Atlantic Ocean might respond to cloud droplet number concentration perturbations in a warmer world. We find that under higher sea surface temperatures, increases in drop number tend to reduce cloud brightness by accelerating evaporation of cloud water. Thus in a warmer world, low clouds will reflect less energy to space in response to an increase in aerosol loading. If aerosol sources decrease then we expect more robust clouds and more offsetting of greenhouse gas warming.
Key Points
Sea surface temperature (SST) has a strong influence on the relative occurrence of aerosol‐induced brightness of clouds over the North Atlantic Ocean
Aerosol perturbations are locally confined and have less influence on the brightness of clouds compared to SST
In a warmer climate where aerosol loading is reduced, we expect a less positive liquid water path feedback
Using 15‐year observations obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board both NASA Terra and Aqua from March 2003 to February 2018, this study investigated the ...spatio‐temporal variations of both macro‐ and micro‐physical cloud properties over China, including cloud fraction (CF), cloud top pressure (CTP), cloud top temperature (CTT), cloud optical thickness (COT), and effective radius (r
e) of both liquid water and ice clouds. Multi‐year averaged CF is around 61% over whole China region. However, CF varies with both regions and seasons. The CFs are about 6–8% larger in summer and winter (~64–65%) than in spring and autumn (~58%). By classifying China into five regimes, which are northwestern China (NW), northeastern China (NE), Tibetan Plateau (TP), northern China (N), and southern China (S), there is a clear CF regional distribution pattern. In general, there are large amount of clouds in S and southeast of TP, and small amount in NE, N, NW and most TP. Moreover, there are generally more clouds over ocean than over land, and much more clouds over S than over N. The CFs are larger (smaller) in the afternoon than in the morning over most land (ocean) regions. Furthermore, the largest CF differences between afternoon and morning occur over the TP region in China. COT demonstrates almost the same regional distribution pattern as the CF for all four seasons. Specifically, COT is higher in S than in N, which is most likely associated with the type of clouds and the availability of water vapour. Cloud r
e shows larger values in NW and TP than in eastern China regions in all seasons except for summer, which could be related to the heavy aerosol pollution in eastern China regions. Accompanying with the cold cloud tops over TP, a low CTP centre is often located there.
Using 15‐year MODIS observations, this study statistically analyses the cloud characteristics over China along with five regions of N, S, NW, NE, and TP. It is found that multi‐year averaged CF is ~61% over China, with ~64–65% in summer and winter, and ~58% in spring and autumn; and for most seasons, CF and COT are larger in S than N, CTT and CTP are lowest over TP, cloud r
e is larger in NW and TP than other regions.
Clouds strongly affect the absorption and reflection of shortwave and longwave radiation in the atmosphere. A key bias in climate models is related to excess absorbed shortwave radiation in the ...high-latitude Southern Ocean. Model evaluation studies attribute these biases in part to midtopped clouds, and observations confirm significant midtopped clouds in the zone of interest. However, it is not yet clear what cloud properties can be attributed to the deficit in modeled clouds. Present approaches using observed cloud regimes do not sufficiently differentiate between potentially distinct types of midtopped clouds and their meteorological contexts.
This study presents a refined set of midtopped cloud subregimes for the high-latitude Southern Ocean, which are distinct in their dynamical and thermodynamic background states. Active satellite observations fromCloudSatandCloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations(CALIPSO) are used to study the macrophysical structure and microphysical properties of the new cloud regimes. The subgrid-scale variability of cloud structure and microphysics is quantified within the cloud regimes by identifying representative physical cloud profiles at high resolution from the radar–lidar (DARDAR) cloud classification mask.
The midtopped cloud subregimes distinguish between stratiform clouds under a high inversion and moderate subsidence; an optically thin cold-air advection cloud regime occurring under weak subsidence and including altostratus over low clouds; optically thick clouds with frequent deep structures under weak ascent and warm midlevel anomalies; and a midlevel convective cloud regime associated with strong ascent and warm advection. The new midtopped cloud regimes for the high-latitude Southern Ocean will provide a refined tool for model evaluation and the attribution of shortwave radiation biases to distinct cloud processes and properties.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Cloud droplet number concentration (Nd) is a key microphysical property that is largely controlled by the balance between sources and sinks of aerosols that serve as cloud condensation nuclei (CCN). ...Despite being a key sink of CCN, the impact of coalescence scavenging on Southern Ocean (SO) cloud is poorly known. We apply a simple source‐and‐sink budget model based on parameterizations to austral summer aircraft observations to test model behavior and examine the relative influence of processes that determine Nd in SO stratocumulus clouds. The model predicts Nd with little bias and a correlation coefficient of ∼0.7 compared with observations. Coalescence scavenging is found to be an important sink of CCN in both liquid and mixed‐phase precipitating stratocumulus and reduces the predicted Nd by as much as 90% depending on the precipitation rate. The free tropospheric aerosol source controls Nd more strongly than the surface aerosol source during austral summer.
Plain Language Summary
Low altitude stratiform clouds are ubiquitous over the Southern Ocean (SO) and have a profound climate impact through reflecting sunlight back to space, cooling the earth. The number of water droplets in a given volume (the concentration) is a key variable that determines the reflective ability of the cloud. The cloud droplet number concentration (Nd) is largely set by the number of aerosols (tiny airborne particles) on which water can condense and is controlled by the balance between sources that generate aerosols (e.g., ocean biology, sea spray) and sinks which remove aerosols from the atmosphere. The formation of liquid precipitation in clouds (drizzle) is a strong sink of aerosols because each precipitation droplet is created through the merger of many cloud droplets (i.e., coalescence) where each cloud droplet contains at least one aerosol particle. Here, we test a simple source‐and‐sink model to predict SO Nd using aircraft measurements from a recent aircraft campaign in the SO during austral summer. We find this model can predict cloud droplet number well and that liquid precipitation processes are important in controlling droplet number concentration (and thus cloud reflectance) over the SO.
Key Points
A simple source‐and‐sink budget model accurately predicts in situ observations of Southern Ocean (SO) cloud droplet number concentration (Nd)
Coalescence scavenging by liquid droplets is a dominant sink of SO cloud condensation nuclei
Free tropospheric aerosol controls Nd more strongly than surface aerosol during austral summer
The cloud-base temperature (CBT) is one of the parameters that dominates the cloudy sky surface downward longwave radiation (SDLR). However, CBT is rarely available at regional and global scales, and ...its application in estimating cloud sky SDLR is limited. In this study, a framework to globally estimate cloud sky SDLR during both daytime and nighttime is proposed. This framework is composed of three parts. First, a global cloudy property database was constructed by combing the extracted cloud vertical structure (CVS) parameters from the active CloudSat data and cloud properties from passive MODIS data. Second, the empirical methods for estimating cloud thickness (CT) under ISCCP cloud classification system and MODIS cloud classification system were developed. Additionally, the coefficients of CERES CT estimate models were refitted using the constructed cloud property database. With the estimated CT and reanalysis data, calculating the CBT is straightforward. The accuracy of the estimated CT for ISCCP cloud type is compared with the existing studies that were conducted at local scales. Our CT estimate accuracy is comparable to that of the existing studies. According to the validation results at ARM NSA and SGP stations, the CT estimated by the developed CT model for MODIS cloud type is better than that estimated by the original CERES CT model. Finally, the cloudy sky SDLR values were derived by feeding the estimated CBT and other parameters to the single-layer cloud model (SLCM). When validated by the ground measured SDLR collected from the SURFRAD network, the bias and RMSE are 5.42 W∙m−2 and 30.3 W∙m−2, respectively. This accuracy is comparable to the evaluation results of the mainstream SDLR products (Gui et al. 2010), the new evaluation results of SLCMs (Yu et al. 2018), and the accuracy of a new cloudy sky SDLR estimate method (Wang et al. 2018). All the derived CBTs improve the SDLR estimate accuracy more than the SLCM that directly uses cloud-top temperature (CTT). We will collect more ground measurements and continue to validate the developed framework in the future.
•We propose a framework to globally estimate cloud sky SDLR during both daytime and nighttime.•The CT estimation models are developed for ISCCP cloud type and MODIS cloud type.•The cloudy sky SDLR is retrieved by the SLCM using the CBT derived from CT.•The bias and RMSE of the derived cloudy sky SDLR over SURFRAD are 5.42 W∙m−2 and 30.3 W∙m−2.
Abstract
Flight data from the Cloud System Evolution over the Trades (CSET) campaign over the Pacific stratocumulus-to-cumulus transition are organized into 18 Lagrangian cases suitable for study and ...future modeling, made possible by the use of a track-and-resample flight strategy. Analysis of these cases shows that 2-day Lagrangian coherence of long-lived species (CO and O3) is high (r = 0.93 and 0.73, respectively), but that of subcloud aerosol, MBL depth, and cloud properties is limited. Although they span a wide range in meteorological conditions, most sampled air masses show a clear transition when considering 2-day changes in cloudiness (−31% averaged over all cases), MBL depth (+560 m), estimated inversion strength (EIS; −2.2 K), and decoupling, agreeing with previous satellite studies and theory. Changes in precipitation and droplet number were less consistent. The aircraft-based analysis is augmented by geostationary satellite retrievals and reanalysis data along Lagrangian trajectories between aircraft sampling times, documenting the evolution of cloud fraction, cloud droplet number concentration, EIS, and MBL depth. An expanded trajectory set spanning the summer of 2015 is used to show that the CSET-sampled air masses were representative of the season, with respect to EIS and cloud fraction. Two Lagrangian case studies attractive for future modeling are presented with aircraft and satellite data. The first features a clear Sc–Cu transition involving MBL deepening and decoupling with decreasing cloud fraction, and the second undergoes a much slower cloud evolution despite a greater initial depth and decoupling state. Potential causes for the differences in evolution are explored, including free-tropospheric humidity, subsidence, surface fluxes, and microphysics.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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.
Deep convective clouds and associated anvils exert opposite radiative effects. The impact of different aerosol types on these two categories of clouds remains a major challenge in understanding ...aerosol‐cloud interactions. Using 11‐year satellite retrievals, we find that cloud top height (CTH) and ice cloud fraction of deep convective clouds and anvil cirrus identified by Cloud‐Aerosol Lidar with Orthogonal Polarization increase with small aerosol loadings and level off or even decrease with further aerosol increase. Compared with continental aerosols, CTH affected by marine aerosols starts to decrease at smaller aerosol loadings. Moreover, cloud optical depth (COD) of deep convective clouds decreases with aerosol loadings. COD of anvil cirrus increases with increased loadings of most aerosol types but decreases with smoke aerosol. These relationships are mainly attributed to the aerosol effect rather than the meteorological effects. Our findings contribute to the development of models and better assessment of aerosol‐cloud radiative forcing.
Plain Language Summary
By acting as the seeds of clouds, aerosols affect the formation and development of clouds, thereby affecting climate. Deep convective clouds and associated anvil cirrus are often accompanied with severe weather events. These two types of clouds have opposite climate effects: the former generally cools the Earth system while the latter warms the Earth system. Using 11 years of satellite data, we find that with the increase of aerosol loadings, the cloud top height (CTH) and cloud fraction of deep convective clouds and anvil cirrus identified by Cloud‐Aerosol Lidar with Orthogonal Polarization first increase and then remain virtually unchanged or even decrease. We also analyze the effects of different types of aerosols on deep convective clouds and anvil cirrus. We find that, compared with continental aerosols, CTH affected by marine aerosols starts to decrease at smaller aerosol loadings. As the aerosol loadings increase, the cloud optical depth of deep convective clouds decreases while the optical thickness of the anvil cirrus increases. Therefore, these two categories of clouds as well as the effects from various aerosol types should be carefully considered when quantifying the aerosol effects on deep convective cloud systems.
Key Points
The height and amount of Cloud‐Aerosol Lidar with Orthogonal Polarization‐identified deep convective clouds and anvil cirrus first increase and then level off with aerosol increase
Marine aerosols start to decrease cloud top height when aerosol optical depth is relatively smaller than continental aerosols
With increased aerosol loadings, deep convective clouds have a decreased cloud optical depth (COD) while anvils have an increased COD. One exception is for smoke
Abstract
The suggested impact of pollution on deep convection dynamics, referred to as the convective invigoration, is investigated in simulations applying microphysical piggybacking and a ...comprehensive double-moment bulk microphysics scheme. The setup follows the case of daytime convective development over land based on observations during the Large-Scale Biosphere–Atmosphere (LBA) experiment in Amazonia. In contrast to previous simulations with single-moment microphysics schemes and in agreement with results from bin microphysics simulations by others, the impact of pollution simulated by the double-moment scheme is large for the upper-tropospheric convective anvils that feature higher cloud fractions in polluted conditions. The increase comes from purely microphysical considerations: namely, the increased cloud droplet concentrations in polluted conditions leading to the increased ice crystal concentrations and, consequently, smaller fall velocities and longer residence times. There is no impact on convective dynamics above the freezing level and thus no convective invigoration. Polluted deep convective clouds precipitate about 10% more than their pristine counterparts. The small enhancement comes from smaller supersaturations below the freezing level and higher buoyancies inside polluted convective updrafts with velocities between 5 and 10 m s−1. The simulated supersaturations are large, up to several percent in both pristine and polluted conditions, and they call into question results from deep convection simulations applying microphysical schemes with saturation adjustment. Sensitivity simulations show that the maximum supersaturations and the upper-tropospheric anvil cloud fractions strongly depend on the details of small cloud condensation nuclei (CCN) that can be activated in strong updrafts above the cloud base.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Stratocumulus (Sc) clouds and stratocumulus‐to‐cumulus transitions (SCTs) are challenging to represent in global models and they contribute to a large spread in modeled subtropical cloud feedbacks. ...We evaluate the impact of increasing the horizontal model resolution (∼135, 60 and 25 km, respectively) and increasing the complexity of the aerosol–cloud interaction parameterization (interactive versus non‐interactive at medium resolution) on springtime subtropical marine Sc properties and SCTs in the atmosphere‐only version of HadGEM3‐GC3.1. No significant impact on the spatial location of the SCT could be found between the different model versions. Increasing horizontal resolution led to small but significant increases in liquid water content and a stronger (more negative) shortwave (SW) cloud radiative effect (CRE), in particular over the southern‐hemisphere Sc regions. However, for two out of the four studied regions, the stronger SW CRE also brought the model outside the range of satellite‐derived values of the SW CRE. Applying non‐interactive aerosols instead of interactive aerosols also led to significantly higher liquid water content and a stronger SW CRE over the southern‐hemisphere Sc regions, while over the northern‐hemisphere Sc regions, a competition between a substantial increase in the cloud droplet number concentration and small changes in the liquid water content resulted in a weaker SW CRE or non‐significant changes. In general, using interactive instead of non‐interactive aerosol–cloud interactions brought the model closer to satellite‐retrieved mean values of the SW CRE. Our results suggest that increasing the horizontal resolution or the complexity of the aerosol–cloud parameterization has a small but statistically significant effect on the SW CRE of marine Sc, in particular over regions with high liquid water content. For these regions, the effect of introducing non‐interactive versus interactive aerosol–cloud interactions is about as large as increasing the horizontal resolution from medium to high.
Stratocumulus clouds are common over cold subtropical oceans. They tend to break up into shallow cumulus as they are transported by the tradewinds toward the equator, a process called stratocumulus‐to‐cumulus transition (SCT). We investigate if an increased spatial resolution or an explicit representation of aerosol–cloud interactions improves the representation of stratocumulus and SCTs in a climate model. We find that both model upgrades result in a better representation of the cloud radiative effects, but that the improvement is small.
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