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Reforestation is widely proposed for carbon dioxide (CO2) removal but the impact on climate, via atmospheric composition and surface albedo changes, remains relatively unexplored. Using two Earth System models, UKESM1 and CESM2, we compare scenarios where existing forests expand to a near biophysical limit (with croplands fixed at 2015 to preserve food production) with SSP1-2.6 and SSP3-7.0 at 2050 and 2095. In the reforestation scenario, global BVOC emissions are 18% (35%) higher than SSP3-7.0 at 2050 (2095) and 8% (12%) higher than SSP1-2.6. The resulting increases to secondary organic aerosols and aerosol scattering, from BVOC emission changes, drive a negative radiative forcing (RF). However, this is outweighed by the positive RF from increases to methane and ozone and decreases to surface albedo.The net RF is equivalent to CO2 increases of 13 (32) ppm relative to SSP3-7.0 at 2050 (2095) and 3 (8) ppm relative to SSP1-2.6. These indirect factors offset ~25% of the additional CO2 removal arising from reforestation relative to SSP3-7.0 and ~10% relative to SSP1-2.6. This highlights the importance of assessing the full response of the Earth System to reforestation, rather than just the potential CO2 removal.

Aerosol radiative forcing uncertainty affects estimates of climate sensitivity and limits model skill at making climate projections. Efforts to improve the representations of physical processes in climate models, including extensive comparisons with observations, have not significantly constrained the range of possible aerosol forcing values. A far stronger constraint, in particular for the lower (most-negative) bound, can be achieved using global mean energy-balance arguments based on observed changes in historical temperature. Here, we show that structural deficiencies in a climate model, revealed as inconsistencies among observationally constrained cloud properties in the model, limit the effectiveness of observational constraint of the uncertain physical processes. We sample uncertainty in 37 model parameters related to aerosols, clouds and radiation in a perturbed parameter ensemble of the UK Earth System Model and evaluate 1 million model variants (different parameter settings from Gaussian Process emulators) against satellite-derived observations over several cloudy regions. We show that it is possible to reduce the parametric uncertainty in global mean aerosol forcing by more than 50 %, constraining it to a range in close agreement with energy-balance constraints (around −1.3 to −0.1 W m−2). However, our analysis of a very large set of model variants exposes model internal inconsistencies that would not be apparent in a small set of model simulations. Incorporating observations associated with these inconsistencies weakens the forcing constraint because they require a wider range of parameter values to accommodate conflicting information. Our estimated aerosol forcing range is the maximum feasible constraint using our structurally imperfect model and the chosen observations. Structural model developments targeted at the identified inconsistencies would enable a larger set of observations to be used for constraint, which would then narrow the uncertainty further. Such an approach provides a rigorous pathway to improved model realism and reduced uncertainty that has so far not been achieved through the normal model development approach.

Cloud microphysics parametrizations control the transfer of water between phases and hydrometeor species in numerical weather prediction and climate models. As a fundamental component of weather modelling systems cloud microphysics can determine the intensity and timing of precipitation, the extent and longevity of cloud cover and its impact on radiative balance, and directly influence near surface weather metrics such as temperature and wind. In this paper we introduce and demonstrate the performance of a double moment cloud microphysical scheme (CASIM: Cloud AeroSol Interacting Microphysics) in both midlatitude and tropical settings using the same model configuration. Comparisons are made against a control configuration using the current operational single moment cloud microphysics, and CASIM configurations that use fixed in-cloud droplet number or compute cloud droplet number concentration from the aerosol environment. We demonstrate that configuring CASIM as a single moment scheme results in precipitation rate histograms that match the operational single moment microphysics. In the midlatitude setting, results indicate that CASIM performs as well as the single moment microphysics configuration, but improves certain aspects of the surface precipitation field such as greater extent of light ({-1} $$) rain around frontal precipitation features. In the tropical setting, CASIM outperforms the single moment cloud microphysics as evident from improved comparison with radar derived precipitation rates.

The data have been generated by UM-UKCA-CASIM from the nested regional model simulations. It contains data from17 to 18 September, 2014,excluding the spinup day (16 September, 2014), and all the fieldsalso excludethe regional model boundary data. The file names are composed of 'regional_'+variable name+simulation name+'.nc'.Each field is separately saved as netCDF file and isshown on geographic coordinates, which are longitude, latitude, verticalaltitude level. The dimensions in each fields are: lat: latitude lon: longitude lev: altitude hours_since_0UTC_17th: the number of hours in the simulations since 0UTC 17 September 2014. Profiles of gases, rates andaerosol concentrations are plotted with the following files (Fig. 5):regional_nucleation_rate_Bio/BioOx/BioOxEm/Bn/Bnx10.nc,regional_growth_rate_Bio/BioOx/BioOxEm.nc, regional_nucleation_mode_Bio/BioOx/BioOxEm/Bn/Bnx10.nc, regional_aitken_mode_Bio/BioOx/BioOxEm/Bn/Bnx10.nc,regional_accumulation_mode_Bio/BioOx/BioOxEm/Bn/Bnx10.nc,regional_SO2_Bio/BioOx/BioOxEm/Bn/Bnx10.nc,regional_H2SO4_Bio/BioOx/BioOxEm/Bn/Bnx10.nc,regional_monoterpene_Bio/BioOx/BioOxEm/Bn/Bnx10.nc,regional_sec_org_Bio/BioOx/BioOxEm/Bn/Bnx10.nc. The effect of cloud condensation sink on nucleation rate and aerosol concentrationsare evaluated with the following files (Fig. 6 & 7):regional_nucleation_rate_BioOx/BioOxCCS/BioOxEm/BioOxEmCCS.nc,regional_nucleation_mode_BioOx/BioOxCCS/BioOxEm/BioOxEmCCS.nc,regional_aitken_mode_BioOx/BioOxCCS/BioOxEm/BioOxEmCCS.nc. Aerosol concentrations that are caused by new particle formation within the regional domain areplotted with (Fig. 8 & 9):regional_nucleation_mode_BioOxEmCCS/off_regNPF/off_allNPF.nc,regional_aitken_mode_BioOxEmCCS/off_regNPF/off_allNPF.nc,regional_accumulation_mode_BioOxEmCCS/off_regNPF/off_allNPF.nc; regional_nucleation_mode_BioOxCCS.nc, off_regNPF/off_allNPF_BioOxCCS.nc,regional_aitken_mode_BioOxCCS.nc, off_regNPF/off_allNPF_BioOxCCS.nc,regional_accumulation_mode_BioOxCCS.nc,off_regNPF/off_allNPF_BioOxCCS.nc. Vertical transport of aerosol particles is quantifiedwith (Fig.10 & 11):regional_nucleation_mode_off_regNPF/NPF_1-4km/4-7km/7-10km/10-13km/13-16km.nc,regional_aitken_mode_off_regNPF/NPF_1-4km/4-7km/7-10km/10-13km/13-16km.nc,regional_accumulation_mode_off_regNPF/NPF_1-4km/4-7km/7-10km/10-13km/13-16km.nc,regional_W_NPF_13-16km.nc Vertical transport of passive tracers are plotted with (Fig.12 &13):regional_passive_tracer_0/2/4/6/8/10/12/14/16km_NPF_13-16km.nc nuc_NPF_13-16km.mp4, ait_NPF_13-16km.mp4 and acc_NPF_13-16km.mp4 are the videos for nucleation mode, Aitken mode and accumulation mode respectively in NPF_13-16km simulation. They are used to illustrate the vertical motion of aerosol particles.

A Lagrangian framework is developed to show the daily-scale time evolution of low clouds over the eastern subtropical oceans. An identical framework is applied to two general circulation models (GCMs): the CAM5 and UKMET and a set of satellite observations. This approach follows thousands of parcels as they advect downwind in the subtropical trade winds, comparing cloud evolution in time and space. This study tracks cloud cover, in-cloud liquid water path (CLWP), droplet concentration Nd, planetary boundary layer (PBL) depth, and rain rate as clouds transition from regions with predominately stratiform clouds to regions containing mostly trade cumulus. The two models generate fewer clouds with greater Nd relative to observations. Models show stronger Lagrangian cloud cover decline and greater PBL deepening when compared with observations. In comparing frequency distributions of cloud variables over time, it is seen that models generate increasing frequencies of nearly clear conditions at the expense of overcast conditions, whereas observations show transitions from overcast to cloud amounts between 50% and 90%. Lagrangian decorrelation time scales (e-folding time τ) of cloud cover and CLWP are between 11 and 19 h for models and observations, although they are a bit shorter for models. A Lagrangian framework applied here resolves and compares the time evolution of cloud systems as they adjust to environmental perturbations in models and observations. Increasing subsidence in the overlying troposphere leads to declining cloud cover, CLWP, PBL depth, and rain rates in models and observations. Modeled cloud responses to other meteorological variables are less consistent with observations, suggesting a need for continuing mechanical improvements in GCMs.

Representing the number and mass of cloud and aerosol particles independently in a climate, weather prediction or air quality model is important in order to simulate aerosol direct and indirect effects on radiation balance. Here we introduce the first configuration of the UK Met Office Unified Model in which both cloud and aerosol particles have “double-moment” representations with prognostic number and mass. The GLObal Model of Aerosol Processes (GLOMAP) aerosol microphysics scheme, already used in the Hadley Centre Global Environmental Model version 3 (HadGEM3) climate configuration, is coupled to the Cloud AeroSol Interacting Microphysics (CASIM) cloud microphysics scheme. We demonstrate the performance of the new configuration in high-resolution simulations of a case study defined from the CLARIFY aircraft campaign in 2017 near Ascension Island in the tropical southern Atlantic. We improve the physical basis of the activation scheme by representing the effect of existing cloud droplets on the activation of new aerosol, and we also discuss the effect of unresolved vertical velocities. We show that neglect of these two competing effects in previous studies led to compensating errors but realistic droplet concentrations. While these changes lead only to a modest improvement in model performance, they reinforce our confidence in the ability of the model microphysics code to simulate the aerosol–cloud microphysical interactions it was designed to represent. Capturing these interactions accurately is critical to simulating aerosol effects on climate.

The change in planetary albedo due to aerosol-cloud interactions (aci) during the industrial era is the leading source of uncertainty in inferring Earth's climate sensitivity to increased greenhouse gases from the historical record. Examining pristine environments such as the Southern Ocean (SO) helps us to understand the pre-industrial state and constrain the change in cloud brightness over the industrial period associated with aci. This study presents two methods of utilizing observations of pristine environments to examine climate models and our understanding of the pre-industrial state. First, cloud droplet number concentration (Nd) is used as an indicator of aci. Global climate models (GCMs) show that the hemispheric contrast in liquid cloud Nd between the pristine SO and the polluted Northern Hemisphere observed in the present-day can be used as a proxy for the increase in Nd from the pre-industrial. A hemispheric difference constraint developed from MODIS satellite observations indicates that pre-industrial Nd may have been higher than previously thought and provides an estimate of radiative forcing associated with aci between -1.2 and -0.6 Wm-2. Comparisons with MODIS Nd highlight significant GCM discrepancies in pristine, biologically active regions. Second, aerosol and cloud microphysical observations from a recent SO aircraft campaign are used to identify two potentially important mechanisms that are incomplete or missing in GCMs: i) production of new aerosol particles through synoptic uplift, and ii) buffering of Nd against precipitation removal by small, Aitken mode aerosols entrained from the free troposphere. The latter may significantly contribute to the high, summertime SO Nd levels which persist despite precipitation depletion associated with mid-latitude storm systems. Observational comparisons with nudged Community Atmosphere Model version 6 (CAM6) hindcasts show low-biased SO Nd is linked to under-production of free-tropospheric Aitken aerosol which drives low-biases in cloud condensation nuclei number and likely discrepancies in composition. These results have important implications for the ability of current GCMs to capture aci in pristine environments.

Earth system models enable a broad range of climate interactions that physical climate models are unable to simulate. However, the extent to which adding Earth system components changes or improves the simulation of the physical climate is not well understood. Here we present a broad multivariate evaluation of the North Atlantic climate system in historical simulations of the UK Earth System Model (UKESM1) performed for CMIP6. In particular, we focus on the mean state and the decadal time scale evolution of important variables that span the North Atlantic climate system. In general, UKESM1 performs well and realistically simulates many aspects of the North Atlantic climate system. Like the physical version of the model, we find that changes in external forcing, and particularly aerosol forcing, are an important driver of multidecadal change in UKESM1, especially for Atlantic Multidecadal Variability and the Atlantic Meridional Overturning Circulation. However, many of the shortcomings identified are similar to common biases found in physical climate models, including the physical climate model that underpins UKESM1. For example, the summer jet is too weak and too far poleward; decadal variability in the winter jet is underestimated; intraseasonal stratospheric polar vortex variability is poorly represented; and Arctic sea ice is too thick. Forced shortwave changes may be also too strong in UKESM1, which, given the important role of historical aerosol forcing in shaping the evolution of the North Atlantic in UKESM1, motivates further investigation. Therefore, physical model development, alongside Earth system development, remains crucial in order to improve climate simulations.

Significance Enhancement of aerosol that can nucleate cloud droplets increases the droplet number concentration and albedo of clouds. This increases the amount of sunlight reflected to space. Uncertainty in how aerosol−cloud interactions over the industrial period have increased planetary albedo by this mechanism leads to significant uncertainty in climate projections. Our work presents a method for observationally constraining the change in albedo due to anthropogenic aerosol emissions: a hemispheric difference in remotely sensed cloud droplet number between the pristine Southern Ocean (a preindustrial proxy) and the polluted Northern Hemisphere. Application of this constraint to climate models reduces the range of estimated albedo change since industrialization and suggests current models underpredict cloud droplet number concentration in the preindustrial era., The change in planetary albedo due to aerosol−cloud interactions during the industrial era is the leading source of uncertainty in inferring Earth’s climate sensitivity to increased greenhouse gases from the historical record. The variable that controls aerosol−cloud interactions in warm clouds is droplet number concentration. Global climate models demonstrate that the present-day hemispheric contrast in cloud droplet number concentration between the pristine Southern Hemisphere and the polluted Northern Hemisphere oceans can be used as a proxy for anthropogenically driven change in cloud droplet number concentration. Remotely sensed estimates constrain this change in droplet number concentration to be between 8 cm−3 and 24 cm−3. By extension, the radiative forcing since 1850 from aerosol−cloud interactions is constrained to be −1.2 W⋅m−2 to −0.6 W⋅m−2. The robustness of this constraint depends upon the assumption that pristine Southern Ocean droplet number concentration is a suitable proxy for preindustrial concentrations. Droplet number concentrations calculated from satellite data over the Southern Ocean are high in austral summer. Near Antarctica, they reach values typical of Northern Hemisphere polluted outflows. These concentrations are found to agree with several in situ datasets. In contrast, climate models show systematic underpredictions of cloud droplet number concentration across the Southern Ocean. Near Antarctica, where precipitation sinks of aerosol are small, the underestimation by climate models is particularly large. This motivates the need for detailed process studies of aerosol production and aerosol−cloud interactions in pristine environments. The hemispheric difference in satellite estimated cloud droplet number concentration implies preindustrial aerosol concentrations were higher than estimated by most models.

Climate variability in the North Atlantic influences processes such as hurricane activity and droughts. Global model simulations have identified aerosol-cloud interactions (ACIs) as an important driver of sea surface temperature variability via surface aerosol forcing. However, ACIs are a major cause of uncertainty in climate forcing, therefore caution is needed in interpreting the results from coarse resolution, highly parameterized global models. Here we separate and quantify the components of the surface shortwave effective radiative forcing (ERF) due to aerosol in the atmosphere-only version of the UK Earth System Model (UKESM1) and evaluate the cloud properties and their radiative effects against observations. We focus on a northern region of the North Atlantic (NA) where stratocumulus clouds dominate (denoted the northern NA region) and a southern region where trade cumulus and broken stratocumlus dominate (southern NA region). Aerosol forcing was diagnosed using a pair of simulations in which the meteorology is approximately fixed via nudging to analysis; one simulation has pre-industrial (PI) and one has present-day (PD) aerosol emissions. Contributions to the surface ERF from changes in cloud fraction (fc), in-cloud liquid water path (LWPic) and droplet number concentration (Nd) were quantified. Over the northern NA region increases in Nd and LWPic dominate the forcing. This is likely because the high fc there precludes further large increases in fc and allows cloud brightening to act over a larger region. Over the southern NA region increases in fc dominate due to the suppression of rain by the additional aerosols. Aerosol-driven increases in macrophysical cloud properties (LWPic and fc) will rely on the response of the boundary layer parameterization, along with input from the cloud microphysics scheme, which are highly uncertain processes. Model gridboxes with low-altitude clouds present in both the PI and PD dominate the forcing in both regions. In the northern NA the brightening of completely overcast low cloud scenes (100 % cloud cover, likely stratocumlus) contributes the most, whereas in the southern NA the creation of clouds with fc of around 20 % from clear skies in the PI was the largest single contributor, suggesting that trade cumulus clouds are created in response to increases in aerosol. The creation of near-overcast clouds was also important there. The correct spatial pattern, coverage and properties of clouds are important for determining the magnitude of aerosol forcing so we also assess the realism of the modelled PD clouds against satellite observations. We find that the model reproduces the spatial pattern of all the observed cloud variables well, but that there are biases. The shortwave top-of-the-atmosphere (SWTOA) flux is overestimated by 5.8 % in the northern NA region and 1.7 % in the southern NA, which we attribute mainly to positive biases in low-altitude fc. Nd is too low by −20.6 % in the northern NA and too high by by 21.5 % in the southern NA, but does not contribute greatly to the main SWTOA biases. Cloudy-sky liquid water path mainly shows biases north of Scandinavia that reach up to between 50 and 100 % and dominate the SWTOA bias in that region. The large contribution to aerosol forcing in the UKESM1 model from highly uncertain macrophysical adjustments suggests that further targeted observations are needed to assess rain formation processes, how they depend on aerosols and the model response to precipitation in order to reduce uncertainty in climate projections.

Representing the number and mass of cloud and aerosol particles independently in a climate, weather prediction or air quality model is important in order to simulate aerosol direct and indirect effects on radiation balance. Here we introduce the first configuration of the UK Met Office Unified Model in which both cloud and aerosol particles have double-moment representations with prognostic number and mass. The GLOMAP aerosol microphysics scheme, already used in the HadGEM3 climate configuration, is coupled to the CASIM cloud microphysics scheme. We demonstrate the performance of the new configuration in cloud-resolving simulations of a case study defined from the CLARIFY aircraft campaign in 2017 near Ascension Island in the tropical south Atlantic. We improve the physical basis of the activation scheme by representing the effect of existing cloud droplets on the activation of new aerosol, and we also attempt to account for the effect of unresolved vertical velocities. The first of these improvements should be applicable to the representation of aerosol activation in other microphysics schemes. While these changes lead only to a modest improvement in model performance, they reinforce our confidence in the ability of the model to simulate aerosol-cloud microphysical interactions. Capturing these interactions accurately is critical to simulating aerosol effects on climate.

Major changes are occurring across the North Atlantic climate system, including in the atmosphere, ocean and cryosphere, and many observed changes are unprecedented in instrumental records. As the changes in the North Atlantic directly affect the climate and air quality of the surrounding continents, it is important to fully understand how and why the changes are taking place, not least to predict how the region will change in the future. To this end, this article characterizes the recent observed changes in the North Atlantic region, especially in the period 2005–2016, across many different aspects of the system including: atmospheric circulation; atmospheric composition; clouds and aerosols; ocean circulation and properties; and the cryosphere. Recent changes include: an increase in the speed of the North Atlantic jet stream in winter; a southward shift in the North Atlantic jet stream in summer, associated with a weakening summer North Atlantic Oscillation; increases in ozone and methane; increases in net absorbed radiation in the mid‐latitude western Atlantic, linked to an increase in the abundance of high level clouds and a reduction in low level clouds; cooling of sea surface temperatures in the North Atlantic subpolar gyre, concomitant with increases in the western subtropical gyre, and a decline in the Atlantic Ocean's overturning circulation; a decline in Atlantic sector Arctic sea ice and rapid melting of the Greenland Ice Sheet. There are many interactions between these changes, but these interactions are poorly understood. This article concludes by highlighting some of the key outstanding questions.

Representing large-scale co-variability between variables related to aerosols, clouds and radiation is one of many aspects of agreement with observations desirable for a climate model. In this study such relations are investigated in terms of temporal correlations on monthly mean scale, to identify points of agreement and disagreement with observations. Ten regions with different meteorological characteristics and aerosol signatures are studied and correlation matrices for the selected regions offer an overview of model ability to represent present day climate variability. Global climate models with different levels of detail and sophistication in their representation of aerosols and clouds are compared with satellite observations and reanalysis assimilating meteorological fields as well as aerosol optical depth from observations. One example of how the correlation comparison can guide model evaluation and development is the often studied relation between cloud droplet number and water content. Reanalysis, with no parameterized aerosol–cloud coupling, shows weaker correlations than observations, indicating that microphysical couplings between cloud droplet number and water content are not negligible for the co-variations emerging on larger scale. These observed correlations are, however, not in agreement with those expected from dominance of the underlying microphysical aerosol–cloud couplings. For instance, negative correlations in subtropical stratocumulus regions show that suppression of precipitation and subsequent increase in water content due to aerosol is not a dominating process on this scale. Only in one of the studied models are cloud dynamics able to overcome the parameterized dependence of rain formation on droplet number concentration, and negative correlations in the stratocumulus regions are reproduced.

Aerosol–cloud interactions are explored using 1 km simulations of a case study of predominantly closed-cell SE Pacific stratocumulus clouds. The simulations include realistic meteorology along with newly implemented cloud microphysics and sub-grid cloud schemes. The model was critically assessed against observations of liquid water path (LWP), broadband fluxes, cloud fraction (fc), droplet number concentrations (Nd), thermodynamic profiles, and radar reflectivities.Aerosol loading sensitivity tests showed that at low aerosol loadings, changes to aerosol affected shortwave fluxes equally through changes to cloud macrophysical characteristics (LWP, fc) and cloud albedo changes due solely to Nd changes. However, at high aerosol loadings, only the Nd albedo change was important. Evidence was also provided to show that a treatment of sub-grid clouds is as important as order of magnitude changes in aerosol loading for the accurate simulation of stratocumulus at this grid resolution.Overall, the control model demonstrated a credible ability to reproduce observations, suggesting that many of the important physical processes for accurately simulating these clouds are represented within the model and giving some confidence in the predictions of the model concerning stratocumulus and the impact of aerosol. For example, the control run was able to reproduce the shape and magnitude of the observed diurnal cycle of domain mean LWP to within ∼ 10 g m−2 for the nighttime, but with an overestimate for the daytime of up to 30 g m−2. The latter was attributed to the uniform aerosol fields imposed on the model, which meant that the model failed to include the low-Nd mode that was observed further offshore, preventing the LWP removal through precipitation that likely occurred in reality. The boundary layer was too low by around 260 m, which was attributed to the driving global model analysis. The shapes and sizes of the observed bands of clouds and open-cell-like regions of low areal cloud cover were qualitatively captured. The daytime fc frequency distribution was reproduced to within Δfc = 0.04 for fc > ∼ 0.7 as was the domain mean nighttime fc (at a single time) to within Δfc = 0.02. Frequency distributions of shortwave top-of-the-atmosphere (TOA) fluxes from the satellite were well represented by the model, with only a slight underestimate of the mean by 15 %; this was attributed to near–shore aerosol concentrations that were too low for the particular times of the satellite overpasses. TOA long-wave flux distributions were close to those from the satellite with agreement of the mean value to within 0.4 %. From comparisons of Nd distributions to those from the satellite, it was found that the Nd mode from the model agreed with the higher of the two observed modes to within ∼ 15 %.

Aerosol-cloud interactions (ACI) represent a significant source of forcing uncertainty in global climate models (GCMs). Estimates of radiative forcing due to ACI in Fifth Assessment Report range fr ...

Aerosol-cloud interactions represent the leading uncertainty in our ability to infer climate sensitivity from the observational record. The forcing from changes in cloud albedo driven by increases in cloud droplet number (Nd) (the first indirect effect) is confidently negative and has narrowed its probable range in the last decade, but the sign and strength of forcing associated with changes in cloud macrophysics in response to aerosol (aerosol-cloud adjustments) remain uncertain. This uncertainty reflects our inability to accurately quantify variability not associated with a causal link flowing from the cloud microphysical state to cloud macrophysical state. Once variability associated with meteorology has been removed, covariance between the liquid water path averaged across cloudy and clear regions (LWP, here, characterizing the macrophysical state) and Nd (characterizing the microphysical) is the sum of two causal pathways linking Nd to LWP: Nd altering LWP (adjustments) and precipitation scavenging aerosol and thus depleting Nd. Only the former term is relevant to constraining adjustments, but disentangling these terms in observations is challenging. We hypothesize that the diversity of constraints on aerosol-cloud adjustments in the literature may be partly due to not explicitly characterizing covariance flowing from cloud to aerosol, and aerosol to cloud. Here, we restrict our analysis to the regime of extratropical clouds outside of low-pressure centers associated with cyclonic activity. Observations from MAC-LWP, and MODIS are compared to simulations in the MetOffice Unified Model (UM) GA7.1 (the atmosphere model of HadGEM3-GC3.1 and UKESM1). The meteorological predictors of LWP are found to be similar between the model and observations. There is also agreement with previous literature on cloud-controlling factors finding that increasing stability, moisture, and sensible heat flux enhance LWP, while increasing subsidence, and sea surface temperature decrease it. A simulation where cloud microphysics are insensitive to changes in Nd is used to characterize covariance between Nd and LWP that is induced by factors other than aerosol-cloud adjustments. By removing variability associated with meteorology and scavenging we infer the sensitivity of LWP to changes in Nd. Application of this technique to UM GA7.1 simulations reproduces the true model adjustment strength. Observational constraints developed using simulated covariability not induced by adjustments and observed covariability between Nd and LWP predict a 25–30 % overestimate by the UM GA7.1 in LWP change and a 30–35% overestimate in associated radiative forcing.

Aerosol processes and, in particular, aerosol-cloud interactions cut across the traditional physical-Earth system boundary of coupled Earth system models and remain one of the key uncertainties in estimating anthropogenic radiative forcing of climate. Here we calculate the historical aerosol effective radiative forcing (ERF) in the HadGEM3-GA7 climate model in order to assess the suitability of this model for inclusion in the UK Earth system model, UKESM1. The aerosol ERF, calculated for the year 2000 relative to 1850, is large and negative in the standard GA7 model leading to an unrealistic negative total anthropogenic forcing over the twentieth century. We show how underlying assumptions and missing processes in both the physical model and aerosol parameterizations lead to this large aerosol ERF. A number of model improvements are investigated to assess their impact on the aerosol ERF. These include an improved representation of cloud droplet spectral dispersion, updates to the aerosol activation scheme, and black carbon optical properties. One of the largest contributors to the aerosol forcing uncertainty is insufficient knowledge of the preindustrial aerosol climate. We evaluate the contribution of uncertainties in the natural marine emissions of dimethyl sulfide and organic aerosol to the ERF. The combination of model improvements derived from these studies weakens the aerosol ERF by up to 50% of the original value and leads to a total anthropogenic historical forcing more in line with assessed values.

The cloud droplet number concentration (N d) is of central interest to improve the understanding of cloud physics and for quantifying the effective radiative forcing by aerosol‐cloud interactions. Current standard satellite retrievals do not operationally provide N d, but it can be inferred from retrievals of cloud optical depth (τ c) cloud droplet effective radius (r e) and cloud top temperature. This review summarizes issues with this approach and quantifies uncertainties. A total relative uncertainty of 78% is inferred for pixel‐level retrievals for relatively homogeneous, optically thick and unobscured stratiform clouds with favorable viewing geometry. The uncertainty is even greater if these conditions are not met. For averages over 1° ×1° regions the uncertainty is reduced to 54% assuming random errors for instrument uncertainties. In contrast, the few evaluation studies against reference in situ observations suggest much better accuracy with little variability in the bias. More such studies are required for a better error characterization. N d uncertainty is dominated by errors in r e, and therefore, improvements in r e retrievals would greatly improve the quality of the N d retrievals. Recommendations are made for how this might be achieved. Some existing N d data sets are compared and discussed, and best practices for the use of N d data from current passive instruments (e.g., filtering criteria) are recommended. Emerging alternative N d estimates are also considered. First, new ideas to use additional information from existing and upcoming spaceborne instruments are discussed, and second, approaches using high‐quality ground‐based observations are examined., Key Points Satellite cloud droplet concentration uncertainties of 78% for pixel‐level retrievals and 54% for 1 by 1 degree retrievals are estimatedThe effective radius retrieval is the most important aspect for improvement, and more in situ evaluation is neededPotential improvements using passive and active satellite, and ground‐based instruments are discussed

A 1200 km-square area of the tropical south Atlantic Ocean near Ascension Island is studied with the HadGEM climate model at convection-permitting and global resolutions for a ten-day case study period in August 2016. During the simulation period, a plume of biomass burning smoke from Africa moves into the area and mixes into the clouds. We examine the interaction of the smoke with clouds and find it has substantial instantaneous direct, indirect and semi-direct radiative effects, which vary in magnitude and sometimes sign between model configurations. The region of interest is simulated at 4 km resolution, with no parameterised convection scheme. The simulations are driven by, and compared to, the HadGEM global model, running at approximately 65 km resolution. For the first time, the UK Chemistry and Aerosol model UKCA is included in a regional model with prognostic aerosol number concentrations advecting in from the global model at the boundaries of the region. The smoke aerosol is simulated realistically, and is found to affect dynamical, microphysical and radiative properties of the atmosphere across the region. The model captures the large-scale horizontal transport of the aerosol adequately, approximately reproducing a transition from pristine to polluted conditions. However, for some of the simulation, the smoke is around 1 km too low in altitude and therefore mixes into the clouds earlier than observed. Fire emissions increase the total aerosol burden by a factor 3.7 and cloud droplet number concentrations by a factor of 3, which is consistent with MODIS observations. Strong localised perturbations to heating and cooling rates due to the smoke affect the dynamics: in the regional model, the inversion height is reduced by up to 200 m when smoke is included. The smoke also affects precipitation, to an extent which depends on the model microphysics. The microphysical and dynamical changes lead to an increase in liquid water path of 60 g m−2 relative to a simulation without smoke aerosol, when averaged over the polluted period. This increase is mostly due to radiatively driven dynamical changes: the reduced entrainment of dry air from above the cloud layer, rather than precipitation suppression by aerosol. The smoke has substantial direct radiative effects of +11.4 W m−2 in the regional model, when averaged over the polluted five days of our case study. The semi-direct radiative effect of the smoke, −30.5 W m−2, is larger than the indirect radiative effect, −10.1 W m−2. However, the radiative effects are sensitive to the model set-up: the semi-direct effect is smaller in the global model, and also in a simulation with the Kogan (2013) parameterisation of autoconversion and accretion instead of the default, from Khairoutdinov and Kogan (2002). Furthermore, we simulate a liquid water path that is biased high compared to satellite observations by 22 % on average, and this leads to high estimates of the domain-averaged aerosol direct effect and the effect of the aerosol on cloud albedo. With these caveats, we simulate a large net cooling across the region, of −27.6 W m−2.

Significance Simulated clouds over the Southern Ocean reflect too little solar radiation compared with observations, which results in errors in simulated surface temperatures and in many other important features of the climate system. Our results show that the radiative properties of the most biased types of clouds in cyclonic systems are highly sensitive to the concentration of ice-nucleating particles. The uniquely low concentrations of ice-nucleating particles in this remote marine environment strongly inhibit precipitation and allow much brighter clouds to be sustained., Large biases in climate model simulations of cloud radiative properties over the Southern Ocean cause large errors in modeled sea surface temperatures, atmospheric circulation, and climate sensitivity. Here, we combine cloud-resolving model simulations with estimates of the concentration of ice-nucleating particles in this region to show that our simulated Southern Ocean clouds reflect far more radiation than predicted by global models, in agreement with satellite observations. Specifically, we show that the clouds that are most sensitive to the concentration of ice-nucleating particles are low-level mixed-phase clouds in the cold sectors of extratropical cyclones, which have previously been identified as a main contributor to the Southern Ocean radiation bias. The very low ice-nucleating particle concentrations that prevail over the Southern Ocean strongly suppress cloud droplet freezing, reduce precipitation, and enhance cloud reflectivity. The results help explain why a strong radiation bias occurs mainly in this remote region away from major sources of ice-nucleating particles. The results present a substantial challenge to climate models to be able to simulate realistic ice-nucleating particle concentrations and their effects under specific meteorological conditions.

Droplet concentration (N d ) retrievals from passive satellite retrievals of cloud optical depth ( τ ) and effective radius ( r e ) usually assume the model of an idealised cloud in which the liquid water content (LWC) increases linearly between cloud base and cloud top (i.e., at a fixed fraction of the adiabatic LWC) with a constant N d profile. Generally it is assumed that the retrieved r e value is that at the top of the cloud. In reality, barring r e retrieval biases due to cloud heterogeneity, etc., the retrieved r e is representative of that lower down in the cloud due to the vertical penetration of photons at the shortwave infra-red wavelengths used to retrieve r e . This inconsistency will cause an overestimate of N d (referred to here as the penetration depth bias ), which this paper quantifies. Here we estimate penetration depths in terms of optical depth below cloud top ( dτ ) for a range of idealised modelled adiabatic clouds using bispectral retrievals and plane-parallel radiative transfer. We find a tight relationship between dτ and τ and that a 1-D relationship approximates the modelled data well. Using this relationship we find that dτ values and hence N d biases are higher for the 2.1 μm channel r e retrieval ( r e2.1 ) compared to the 3.7 μm one ( r e3.7 ). The theoretical bias in the retrieved N d is likely to be very large for optically thin clouds, nominally approaching infinity for clouds whose τ is close to the penetration depth. The relative N d bias rapidly reduces as cloud thickness increases, although still remains above 20 % for τ 19.8 and τ 7.7 for r e2.1 and r e3.7 , respectively. The magnitude of the N d bias upon climatological N d data sets is estimated globally using one year of daily MODIS (MODerate Imaging Spectroradiometer) data. Screening criteria are applied that are consistent with those required to help ensure accurate N d retrievals. The results show that the SE Atlantic, SE Pacific (where the VOCALS field campaign took place) and Californian stratocumulus regions produce fairly large overestimates due to the penetration depth bias with mean biases of 35–38 % for r e2.1 and 17–20 % for r e3.7 . For the other stratocumulus regions examined the errors are smaller (25–30 % for r e2.1 and 11–14 % for r e3.7 ). Significant time variability in the percentage errors is also found with regional mean standard deviations of 20–40 % of the regional mean percentage error for r e2.1 and 40–60 % for r e3.7 . This shows that it is important to apply a daily correction to N d for the penetration depth error rather than a time-mean correction when examining daily data. We also examine the seasonal variation of the bias and find that the biases in the SE Atlantic, SE Pacific and Californian stratocumulus regions exhibit the most seasonality with the largest errors occurring in the December, January, February (DJF) season. We show that this effect can be corrected for by simply removing dτ from the observed τ and provide a function to allow the calculation of dτ from τ . However, in reality this error will be combined with a number of other errors that affect both the r e and τ , which are potentially larger and may compensate or enhance the bias due to vertical penetration depth.

The sensitivity of the reflection of shortwave radiation over the Southern Ocean to the cloud properties there is estimated using observations from a suite of passive and active satellite instruments in combination with radiative transfer modeling. A composite cloud property observational data description is constructed that consistently incorporates mean cloud liquid water content, ice water content, liquid and ice particle radius information, vertical structure, vertical overlap, and spatial aggregation of cloud water as measured by optical depth versus cloud-top pressure histograms. The observational datasets used are Moderate Resolution Imaging Spectroradiometer (MODIS) effective radius filtered to mitigate solar zenith angle bias, the Multiangle Imaging Spectroradiometer (MISR) cloud-top height–optical depth (CTH–OD) histogram, the liquid water path from the University of Wisconsin dataset, and ice cloud properties from CloudSat. This cloud database is used to compute reflected shortwave radiation as a function of month and location over the ocean from 40° to 60°S, which compares well with observations of reflected shortwave radiation. This calculation is then used to test the sensitivity of the seasonal variation of shortwave reflection to the observed seasonal variation of cloud properties. Effective radius decreases during the summer season, which results in an increase in reflected solar radiation of 4–8 W m−2 during summer compared to what would be reflected if the effective radius remained constant at its annual-mean value. Summertime increases in low cloud fraction similarly increase the summertime reflection of solar radiation by 9–11 W m−2. In-cloud liquid water path is less in summertime, causing the reflected solar radiation to be 1–4 W m−2 less.

Climate models produce an increase in cloud optical depth in midlatitudes associated with climate warming, but the magnitude of this increase and its impact on reflected solar radiation vary from model to model. Transition from ice to liquid in midlatitude clouds is thought to be one mechanism for producing increased cloud optical depth. Here observations of cloud properties are used from a suite of remote sensing instruments to estimate the effect of conversion of ice to liquid associated with warming on reflected solar radiation in the latitude band from 40° to 60°S. The calculated increase in upwelling shortwave radiation (SW↑) is found to be important and of comparable magnitude to the increase in SW↑ associated with warming-induced increases of optical depth in climate models. The region where the authors' estimate increases SW↑ extends farther equatorward than the region where optical depth increases with warming in models. This difference is likely caused by other mechanisms at work in the models but is also sensitive to the amount of ice present in climate models and its susceptibility to warming.

Aerosol-cloud interactions are a major source of uncertainty in predicting 21st century climate change. Using high-resolution, convection-permitting global simulations we predict that increased cloud condensation nuclei (CCN) interacting with midlatitude cyclones will increase their cloud droplet number concentration (CDNC), liquid water (CLWP), and albedo. For the first time this effect is shown with 13 years of satellite observations. Causality between enhanced CCN and enhanced cyclone liquid content is supported by the 2014 eruption of Holuhraun. The change in midlatitude cyclone albedo due to enhanced CCN in a surrogate climate model is around 70 % of the change in a high-resolution convection-permitting model, indicating that climate models may underestimate this indirect effect.

International audience; Aerosols have a potentially large effect on climate, particularly through their interactions with clouds, but the magnitude of this effect is highly uncertain. Large volcanic eruptions produce sulfur dioxide, which in turn produces aerosols; these eruptions thus represent a natural experiment through which to quantify aerosol-cloud interactions. Here we show that the massive 2014-2015 fissure eruption in Holuhraun, Iceland, reduced the size of liquid cloud droplets—consistent with expectations—but had no discernible effect on other cloud properties. The reduction in droplet size led to cloud brightening and global-mean radiative forcing of around -0.2 watts per square metre for September to October 2014. Changes in cloud amount or cloud liquid water path, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes. This result will reduce uncertainties in future climate projections, because we are now able to reject results from climate models with an excessive liquid-water-path response.

In this paper we use a novel observational approach to investigate MODIS satellite retrieval biases of τ and re (using three different MODIS bands: 1.6, 2.1 and 3.7 μm, denoted as re1.6, re2.1 and re3.7, respectively) that occur at high solar zenith angles (θ0) and how they affect retrievals of cloud droplet concentration (Nd). Utilizing the large number of overpasses for polar regions and the diurnal variation of θ0 we estimate biases in the above quantities for an open ocean region that is dominated by low level stratiform clouds. We find that the mean τ is fairly constant between θ0 = 50° and ~65–70°, but then increases rapidly with an increase of over 70 % between the lowest and highest θ0. The re2.1 and re3.7 decrease with θ0, with effects also starting at around θ0 = 65–70°. At low θ0, the re values from the three different MODIS bands agree to within around 0.2 μm, whereas at high θ0 the spread is closer to 1 μm. The percentage changes of re with θ0 are considerably lower than those for τ, being around 5 % and 7% for re2.1 and re3.7. For re1.6 there was very little change with θ0. Evidence is provided that these changes are unlikely to be due to any physical diurnal cycle. The increase in τ and decrease in re both contribute to an overall increase in Nd of 40–70% between low and high θ0. Whilst the overall re changes are quite small, they are not insignificant for the calculation of Nd; we find that the contributions to Nd biases from the τ and re biases were roughly comparable for re3.7, although for the other re bands the τ changes were considerably more important. Also, when considering only the clouds with the more heterogeneous tops, the importance of the re biases was considerably enhanced for both re2.1 and re3.7. When using the variability of 1 km resolution τ data (γτ) as a heterogeneity parameter we obtained the expected result of increasing differences in τ between high and low θ0 as heterogeneity increased, which was not the case when using the variability of 5 km resolution cloud top temperature (σCTT), suggesting that γτ is a better predictor of τ biases at high θ0 than σCTT. For a given θ0, large decreases in re were observed as the cloud top heterogeneity changed from low to high values, although it is possible that physical changes to the clouds associated with cloud heterogeneity variation may account for some of this. However, for a given cloud top heterogeneity we find that the value of θ0 affects the sign and magnitude of the relative differences between re1.6, re2.1 and re3.7, which has implications for attempts to retrieve vertical cloud information using the different MODIS bands. The relatively larger decrease in re3.7 and the lack of change of re1.6 with both θ0 and cloud top heterogeneity suggest that re3.7 is more prone to retrieval biases due to high θ0 than the other bands. We discuss some possible reasons for this. Our results have important implications for individual MODIS swaths at high θ0, which may be used for case studies for example. θ0 values > 65° can occur at latitudes as low as 28° in mid-winter and for higher latitudes the problem will be more acute. Also, Level-3 daily averaged MODIS cloud property data consist of the averages of several overpasses for the high latitudes, which will occur at a range of θ0 values. Thus, some biased data are likely to be included. It is also likely that some of the θ0 effects described here would apply to τ and re retrievals from satellite instruments that use visible light at similar wavelengths along with forward retrieval models that assume plane parallel clouds, such as the GOES imagers, SEVIRI, etc.

Aerosol-cloud interactions are explored using 1 km resolution simulations of SE Pacific stratocumulus clouds that include realistic meteorology along with newly implemented cloud microphysics and sub-grid cloud schemes. The model was critically assessed against observations of Liquid Water Path (LWP), broadband fluxes, cloud fraction (fc), droplet number concentrations (Nd) and radar reflectivities. Aerosol loading sensitivity tests showed that at low aerosol loadings, changes to aerosol affected shortwave fluxes equally through changes to cloud macrophysical charateristics (LWP, fc) and cloud albedo changes due solely to Nd changes. However, at high aerosol loadings, only the Nd albedo change was important. Evidence was also provided to show that a treatment of sub-grid clouds is as important as order of magnitude changes in aerosol loading for the accurate simulation of stratocumulus at this grid resolution. Overall, the control model demonstrated a credible ability to reproduce observations suggesting that many of the important physical processes for accurately simulating these clouds are represented within the model and giving some confidence in the predictions of the model concerning stratocumulus and the impact of aerosol. For example, the control run was able to reproduce the shape and magnitude of the observed diurnal cycle of domain mean LWP to within ~ 10 g m−2 for the nighttime, but with an overestimate for the daytime of up to 30 g m−2. The latter was attributed to the uniform aerosol fields imposed on the model, which meant that the model failed to include the low Nd mode that was observed further offshore, preventing the LWP removal through precipitation that likely occurred in reality. The boundary layer was too low by around 260 m, which was attributed to the driving global model analysis. The shapes and sizes of the observed bands of clouds and open-cell-like regions of low areal cloud cover were qualitatively captured. The daytime fc frequency distribution was reproduced to within fc = 0.04 for fc > ~ 0.7 as was the domain mean nighttime fc (at a single time) to within fc = 0.02. Frequency distributions of shortwave top-of-the-atmosphere (TOA) fluxes from satellite were well represented by the model with only a slight underestimate of the mean by 15 %; this was attributed to near--shore aerosol concentrations that were too low for the particular times of the satellite overpasses. TOA longwave flux distributions were close to those from satellite with agreement of the mean value to within 0.4 %. From comparisons of Nd distributions to those from satellite it was found that the Nd mode from the model agreed with the higher of the two observed modes to within ~ 15 %.

Increasing optical depth poleward of 45° is a robust response to warming in global climate models. Much of this cloud optical depth increase has been hypothesized to be due to transitions from ice-dominated to liquid-dominated mixed-phase cloud. In this study, the importance of liquid-ice partitioning for the optical depth feedback is quantified for 19 Coupled Model Intercomparison Project Phase 5 models. All models show a monotonic partitioning of ice and liquid as a function of temperature, but the temperature at which ice and liquid are equally mixed (the glaciation temperature) varies by as much as 40 K across models. Models that have a higher glaciation temperature are found to have a smaller climatological liquid water path (LWP) and condensed water path and experience a larger increase in LWP as the climate warms. The ice-liquid partitioning curve of each model may be used to calculate the response of LWP to warming. It is found that the repartitioning between ice and liquid in a warming climate contributes at least 20% to 80% of the increase in LWP as the climate warms, depending on model. Intermodel differences in the climatological partitioning between ice and liquid are estimated to contribute at least 20% to the intermodel spread in the high-latitude LWP response in the mixed-phase region poleward of 45°S. It is hypothesized that a more thorough evaluation and constraint of global climate model mixed-phase cloud parameterizations and validation of the total condensate and ice-liquid apportionment against observations will yield a substantial reduction in model uncertainty in the high-latitude cloud response to warming.

Sulfate and organic mass in sea spray explain more than half of the variability in Southern Ocean cloud droplet concentration., Atmospheric aerosols, suspended solid and liquid particles, act as nucleation sites for cloud drop formation, affecting clouds and cloud properties—ultimately influencing the cloud dynamics, lifetime, water path, and areal extent that determine the reflectivity (albedo) of clouds. The concentration Nd of droplets in clouds that influences planetary albedo is sensitive to the availability of aerosol particles on which the droplets form. Natural aerosol concentrations affect not only cloud properties themselves but also modulate the sensitivity of clouds to changes in anthropogenic aerosols. It is shown that modeled natural aerosols, principally marine biogenic primary and secondary aerosol sources, explain more than half of the spatiotemporal variability in satellite-observed Nd. Enhanced Nd is spatially correlated with regions of high chlorophyll a, and the spatiotemporal variability in Nd is found to be driven primarily by high concentrations of sulfate aerosol at lower Southern Ocean latitudes (35o to 45oS) and by organic matter in sea spray aerosol at higher latitudes (45o to 55oS). Biogenic sources are estimated to increase the summertime mean reflected solar radiation in excess of 10 W m–2 over parts of the Southern Ocean, which is comparable to the annual mean increases expected from anthropogenic aerosols over heavily polluted regions of the Northern Hemisphere.

This corrects the article DOI: 10.1038/nature22974.

Mesoscale model simulations are presented of a westerly föhn event over the Antarctic Peninsula mountain ridge and onto the Larsen C ice shelf, just south of the recently collapsed Larsen B ice shelf. Aircraft observations showed the presence of föhn jets descending near the ice shelf surface with maximum wind speeds at 250–350 m in height. Surface flux measurements suggested that melting was occurring. Simulated profiles of wind speed, temperature and wind direction were very similar to the observations. However, the good match only occurred at a model time corresponding to ~9 h before the aircraft observations were made since the model föhn jets died down after this. This was despite the fact that the model was nudged towards analysis for heights greater than ~1.15 km above the surface. Timing issues aside, the otherwise good comparison between the model and observations gave confidence that the model flow structure was similar to that in reality. Details of the model jet structure are explored and discussed and are found to have ramifications for the placement of automatic weather station (AWS) stations on the ice shelf in order to detect föhn flow. Cross sections of the flow are also examined and were found to compare well to the aircraft measurements. Gravity wave breaking above the mountain crest likely created a~situation similar to hydraulic flow and allowed föhn flow and ice shelf surface warming to occur despite strong upwind blocking, which in previous studies of this region has generally not been considered. Our results therefore suggest that reduced upwind blocking, due to wind speed increases or stability decreases, might not result in an increased likelihood of föhn events over the Antarctic Peninsula, as previously suggested. The surface energy budget of the model during the melting periods showed that the net downwelling short-wave surface flux was the largest contributor to the melting energy, indicating that the cloud clearing effect of föhn events is likely to be the most important factor for increased melting relative to non-föhn days. The results also indicate that the warmth of the föhn jets through sensible heat flux ("SH") may not be critical in causing melting beyond boundary layer stabilisation effects (which may help to prevent cloud cover and suppress loss of heat by convection) and are actually cancelled by latent heat flux ("LH") effects (snow ablation). It was found that ground heat flux ("GRD") was likely to be an important factor when considering the changing surface energy budget for the southern regions of the ice shelf as the climate warms.

In this paper we use a novel observational approach to investigate MODIS satellite retrieval biases of τ and re (using three different MODIS bands: 1.6, 2.1 and 3.7 μm, denoted as re1.6, re2.1 and re3.7, respectively) that occur at high solar zenith angles (θ0) and how they affect retrievals of cloud droplet concentration (Nd). Utilizing the large number of overpasses for polar regions and the diurnal variation of θ0 we estimate biases in the above quantities for the open ocean region north of Scandinavia that is dominated by low level stratiform clouds. We find that the mean τ is fairly constant between θ0 = 50° and ~65°, but then increases rapidly with an increase of over 70% between the lowest and highest θ0. re2.1 and re3.7 decrease with θ0, with effects also starting at around θ0 =65°. At low θ0, the re values from the three different MODIS bands agree to within around 0.2 μm, whereas at high θ0 the spread is closer to 1 μm. The percentage changes of re with θ0 are somewhat lower than those for τ being around 5% and 7% for re2.1 and re3.7. For re1.6 there was very little change with θ0. The increase in τ and decrease in re both contribute to an overall increase in Nd of 40–70% between low and high θ0. We argue that such a change is highly unlikely to be due to any physical diurnal cycle, which is supported by the finding that the retrieved Nd is constant at local times at either side of noon for which θ0 < 65°. Whilst the overall re changes are quite small, they are not insignificant for the calculation of Nd; we find that the contributions to Nd biases from the τ and re biases were roughly comparable for re3.7, although for the other re bands the τ changes were considerably more important (roughly twice the contribution for re2.1 and six times for re1.6). However, when considering only the clouds with the more heterogeneous tops, the importance of the re biases was considerably enhanced for both re2.1 and re3.7; τ and re bias contributions were comparable for re2.1 and for re3.7re bias contributions were ~50% greater. For a given θ0, large decreases in re were observed as the cloud top heterogeneity changed from low to high values: decreases of 25–30% for re3.7, ~20% for re2.1 and 10% for re1.6, although, it is possible that physical changes to the clouds associated with cloud heterogeneity variation may account for some of this. However, for a given cloud top heterogeneity we find that the value of θ0 affects the sign and magnitude of the relative differences between re1.6, re2.1 and re3.7, which has implications for attempts to retrieve vertical cloud information using the different MODIS bands. The relatively larger decrease in re3.7 and the lack of change of re1.6 with both θ0 and cloud top heterogeneity suggest that re3.7 is more prone to retrieval biases due to high θ0 than the other bands, which is interesting since re3.7 has generally been shown to be less prone to other retrieval biases (e.g. due to sub-pixel heterogeneity) at low θ0. We discuss some possible reasons for this. Our results have important implications for individual MODIS swaths at high θ0, which may be used for case studies for example. θ0 values >65° can occur at latitudes as low as 28° in mid-winter and for higher latitudes the problem will be more acute. Also, Level 3 daily averaged MODIS cloud property data consists of the averages of several overpasses for the high latitudes, which will occur at a range of θ0 values. Thus, some biased data is likely to be included.

Compared to other regions, little is known about clouds in Antarctica. This arises in part from the challenging deployment of instrumentation in this remote and harsh environment and from the limitations of traditional satellite passive remote sensing over the polar regions. Yet clouds have a critical influence on the ice sheet's radiation budget and its surface mass balance. The extremely low temperatures, absolute humidity levels, and aerosol concentrations found in Antarctica create unique conditions for cloud formation that greatly differ from those encountered in other regions, including the Arctic. During the first decade of the 21st century, new results from field studies, the advent of cloud observations from spaceborne active sensors, and improvements in cloud parameterizations in numerical models have contributed to significant advances in our understanding of Antarctic clouds. This review covers four main topics: (1) observational methods and instruments, (2) the seasonal and interannual variability of cloud amounts, (3) the microphysical properties of clouds and aerosols, and (4) cloud representation in global and regional numerical models. Aside from a synthesis of the existing literature, novel insights are also presented. A new climatology of clouds over Antarctica and the Southern Ocean is derived from combined measurements of the CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellites. This climatology is used to assess the forecast cloud amounts in 20th century global climate model simulations. While cloud monitoring over Antarctica from space has proved essential to the recent advances, the review concludes by emphasizing the need for additional in situ measurements. © Copyright 2012 by the American Geophysical Union. ispartof: Reviews of Geophysics vol:50 issue:1 status: published

In-situ aircraft observations of ice crystal concentrations in Antarctic clouds are presented for the first time. Orographic, layer and wave clouds around the Antarctic Peninsula and Larsen Ice shelf regions were penetrated by the British Antarctic Survey's Twin Otter aircraft, which was equipped with modern cloud physics probes. The clouds studied were mostly in the free troposphere and hence ice crystals blown from the surface are unlikely to have been a major source for the ice phase. The temperature range covered by the experiments was 0 to −21 °C. The clouds were found to contain supercooled liquid water in most regions and at heterogeneous ice formation temperatures ice crystal concentrations (60 s averages) were often less than 0.07 l−1, although values up to 0.22 l−1 were observed. Estimates of observed aerosol concentrations were used as input into the DeMott et al. (2010) ice nuclei (IN) parameterisation. The observed ice crystal number concentrations were generally in broad agreement with the IN predictions, although on the whole the predicted values were higher. Possible reasons for this are discussed and include the lack of IN observations in this region with which to characterise the parameterisation, and/or problems in relating ice concentration measurements to IN concentrations. Other IN parameterisations significantly overestimated the number of ice particles. Generally ice particle concentrations were much lower than found in clouds in middle latitudes for a given temperature. Higher ice crystal concentrations were sometimes observed at temperatures warmer than −9 °C, with values of several per litre reached. These were attributable to secondary ice particle production by the Hallett Mossop process. Even in this temperature range it was observed that there were regions with little or no ice that were dominated by supercooled liquid water. It is likely that in some cases this was due to a lack of seeding ice crystals to act as rimers to initiate secondary ice particle production. This highlights the chaotic and spatially inhomogeneous nature of this process and indicates that the accurate representation of it in global models is likely to represent a challenge. However, the contrast between Hallett Mossop zone ice concentrations and the fairly low concentrations of heterogeneously nucleated ice suggests that the Hallet Mossop process has the potential to be very important in remote, pristine regions such as around the Antarctic coast.

Made available in DSpace on 2013-08-28T14:09:36Z (GMT). No. of bitstreams: 1 WOS000288368900028.pdf: 40348187 bytes, checksum: e0704479142ec2a7b5f82477e124c95d (MD5) Made available in DSpace on 2013-09-30T19:22:39Z (GMT). No. of bitstreams: 2 WOS000288368900028.pdf: 40348187 bytes, checksum: e0704479142ec2a7b5f82477e124c95d (MD5) WOS000288368900028.pdf.txt: 178435 bytes, checksum: 87f5ef11a2b9a968bace460f4d888b33 (MD5) Previous issue date: 2011-01-01 Submitted by Vitor Silverio Rodrigues (vitorsrodrigues@reitoria.unesp.br) on 2014-05-20T15:33:54Z No. of bitstreams: 2 WOS000288368900028.pdf: 40348187 bytes, checksum: e0704479142ec2a7b5f82477e124c95d (MD5) WOS000288368900028.pdf.txt: 178435 bytes, checksum: 87f5ef11a2b9a968bace460f4d888b33 (MD5) Made available in DSpace on 2014-05-20T15:33:54Z (GMT). No. of bitstreams: 2 WOS000288368900028.pdf: 40348187 bytes, checksum: e0704479142ec2a7b5f82477e124c95d (MD5) WOS000288368900028.pdf.txt: 178435 bytes, checksum: 87f5ef11a2b9a968bace460f4d888b33 (MD5) Previous issue date: 2011-01-01 European Commission European Space Agency Programme National de Chimie de l'Atmosphere (PNCA) CNES, France Natural Environment Research Council (NERC), UK The EU HIBISCUS project consisted of a series of field campaigns during the intense convective summers in 2001, 2003 and 2004 in the State of São Paulo in Brazil. Its objective was to investigate the impact of deep convection on the Tropical Tropopause Layer (TTL) and the lower stratosphere by providing a new set of observational data on meteorology, tracers of horizontal and vertical transport, water vapour, clouds, and chemistry in the tropical Upper Troposphere/Lower Stratosphere (UT/LS). This was achieved using short duration research balloons to study local phenomena associated with convection over land, and long-duration balloons circumnavigating the globe to study the contrast between land and oceans.Analyses of observations of short-lived tracers, ozone and ice particles show strong episodic local updraughts of cold air across the lapse rate tropopause up to 18 or 19 km (420-440 K) in the lower stratosphere by overshooting towers. The long duration balloon and satellite measurements reveal a contrast between the composition of the lower stratosphere over land and oceanic areas, suggesting significant global impact of such events. The overshoots are shown to be well captured by non-hydrostatic meso-scale Cloud Resolving Models indicating vertical velocities of 50-60 m s(-1) at the top of the Neutral Buoyancy Level (NBL) at around 14 km, but, in contrast, are poorly represented by global Chemistry-Transport Models (CTM) forced by Numerical Weather Forecast Models (NWP) underestimating the overshooting process. Finally, the data collected by the HIBISCUS balloons have allowed a thorough evaluation of temperature NWP analyses and reanalyses, as well as satellite ozone, nitrogen oxide, water vapour and bromine oxide measurements in the tropics. Univ Versailles, CNRS Serv Aeron SA, Versailles, France UNESP, Inst Pesquisas Meteorol, Bauru, Brazil CNES, Toulouse, France CNRS, LMD, Palaiseau, France Univ Cambridge UCAM, Dept Chem, Cambridge, England Natl Phys Lab, Teddington TW11 0LW, Middx, England Danish Meteorol Inst, Copenhagen, Denmark CNR, ISAC, Rome, Italy CNRS, LPCE, F-45071 Orleans, France Univ Manchester, Manchester, Lancs, England Norwegian Inst Air Res NILU, Kjeller, Norway ENEA, Agenzia Nazl Nuove Tecnol Energia & Sviluppo Econ, Rome, Italy CNRS, LA, Toulouse, France Univ Versailles, Ctr Etud Environm Terr & Planetaires CETP, St Maur, France CPTEC, Cachoeira Paulista, Brazil UNESP, Inst Pesquisas Meteorol, Bauru, Brazil European Commission: EVK2-2001-000111 European Space Agency: 713