Marine Stratocumulus Clouds Research Articles

Total Heating Characteristics of the ISCCP Tropical and Subtropical Cloud Regimes

AbstractComposite profiles of the apparent heat source Q1 and moisture sink Q2 are calculated for the International Satellite Cloud Climatology Project (ISCCP) cloud regimes (or “weather states”) using sounding observations from 10 field campaigns comprising both tropical and subtropical domains. Distinct heating profiles were determined for each ISCCP cloud regime, ranging from strong, upper-tropospheric heating for mesoscale convective systems (WS1) to integrated cooling for populations typically associated with marine stratus and stratocumulus clouds (WS5, WS6, and WS7). Despite being primarily associated with thin cirrus, the corresponding regime (WS4) has heating maxima in the lower and midtroposphere due to the presence of underlying clouds. Regime-averaged Q2 profiles showed similar transitions with strong drying observed for deep convection and low-level moistening for marine boundary layer clouds. The derived profiles were generally similar over land and ocean with the notable exception of the fair-weather cumulus regime (WS8). Additional midlevel moistening was identified for several weather states over land, suggesting enhanced detrainment and more frequent congestus clouds compared to oceanic domains.A control simulation using the Community Atmosphere Model, version 4 (CAM4), was similar to the large-scale patterns of diabatic heating at low levels produced by the ISCCP composites. Differences were more pronounced at middle and upper levels and are largely attributed to the uncertainty in the heating profiles for the cumulus regime (WS8). Low-level heating anomalies were calculated for each phase of the Madden–Julian oscillation (MJO) and they precede upper-tropospheric heating from deep convection by 3–4 phases. Implications for future research using ISCCP heating reconstructions are also discussed.

The Intercomparison of Satellite-Derived and In Situ Profiles of Droplet Effective Radii in Marine Stratocumulus Clouds

It was found that the satellite spectral measurements can be used not only for the determination of cloud top effective radii but also for retrieving the effective radii vertical profile in some specific cases (e.g., for low marine stratocumulus clouds with negligible effects of horizontal variability). The inversion algorithm was applied to Moderate Resolution Imaging Spectroradiometer (MODIS) radiances (at the wavelengths of 0.865, 1.24, 1.6, and 2.1 μm), and the results were validated against in situ airborne measurements of cloud vertical profiles of the effective radius. We found that the results were more reliable for the cases with large vertical gradients of droplet sizes. The new retrieval method, based on the optimal estimation approach, enabled a higher accuracy of the liquid water path estimate than that produced with the standard MODIS vertically homogeneous algorithm.

Diagnosis and testing of low-level cloud parameterizations for the NCEP/GFS model using satellite and ground-based measurements

The objective of this study is to investigate the quality of clouds simulated by the National Centers for Environmental Prediction global forecast system (GFS) model and to examine the causes for some systematic errors seen in the simulations through use of satellite and ground-based measurements. In general, clouds simulated by the GFS model had similar spatial patterns and seasonal trends as those retrieved from passive and active satellite sensors, but large systematic biases exist for certain cloud regimes especially underestimation of low-level marine stratocumulus clouds in the eastern Pacific and Atlantic oceans. This led to the overestimation (underestimation) of outgoing longwave (shortwave) fluxes at the top-of-atmosphere. While temperature profiles from the GFS model were comparable to those obtained from different observational sources, the GFS model overestimated the relative humidity field in the upper and lower troposphere. The cloud condensed water mixing ratio, which is a key input variable in the current GFS cloud scheme, was largely underestimated due presumably to excessive removal of cloud condensate water through strong turbulent diffusion and/or an improper boundary layer scheme. To circumvent the problem associated with modeled cloud mixing ratios, we tested an alternative cloud parameterization scheme that requires inputs of atmospheric dynamic and thermodynamic variables. Much closer agreements were reached in cloud amounts, especially for marine stratocumulus clouds. We also evaluate the impact of cloud overlap on cloud fraction by applying a linear combination of maximum and random overlap assumptions with a de-correlation length determined from satellite products. Significantly better improvements were found for high-level clouds than for low-level clouds, due to differences in the dominant cloud geometry between these two distinct cloud types.

The effects of timing and rate of marine cloud brightening aerosol injection on albedo changes during the diurnal cycle of marine stratocumulus clouds

Abstract. The marine-cloud brightening geoengineering technique has been suggested as a possible means of counteracting the positive radiative forcing associated with anthropogenic atmospheric CO2 increases. The focus of this study is to quantify the albedo response to aerosols injected into marine stratocumulus cloud from a point source at different times of day. We use a cloud-resolving model to investigate both weakly precipitating and non-precipitating regimes. Injection into both regimes induces a first indirect aerosol effect. Additionally, the weakly precipitating regime shows evidence of liquid water path gain associated with a second indirect aerosol effect that contributes to a more negative radiative forcing, and cloud changes indicative of a regime change to more persistent cloud. This results in a cloud albedo increase up to six times larger than in the non-precipitating case. These indirect effects show considerable variation with injection at different times in the diurnal cycle. For the weakly precipitating case, aerosol injection results in domain average increases in cloud albedo of 0.28 and 0.17 in the early and mid morning (03:00:00 local time (LT) and 08:00:00 LT respectively) and 0.01 in the evening (18:00:00 LT). No cloud develops when injecting into the cloud-free early afternoon (13:00:00 LT). However, the all-sky albedo increases (which include both the indirect and direct aerosol effects) are highest for early morning injection (0.11). Mid-morning and daytime injections produce increases of 0.06, with the direct aerosol effect compensating for the lack of cloud albedo perturbation during the cloud-free early afternoon. Evening injection results in an increase of 0.04. For the weakly precipitating case considered, the optimal injection time for planetary albedo response is the early morning. Here, the cloud has more opportunity develop into a more persistent non-precipitating regime prior to the dissipative effects of solar heating. The effectiveness of the sea-spray injection method is highly sensitive to diurnal injection time and the direct aerosol effect of an intense aerosol point source. Studies which ignore these factors could overstate the effectiveness of the marine cloud brightening technique.

Climate‐induced changes in sea salt aerosol number emissions: 1870 to 2100

AbstractGlobal climate model output is combined with an emission parameterization to estimate the change in the global and regional sea salt aerosol number emission from 1870 to 2100. Global average results suggest a general increase in sea salt aerosol number emission due to increasing surface wind speed. However, the emission changes are not uniform over the aerosol size spectrum due to an increase in sea surface temperature. From 1870 to 2100 the emission of coarse mode particles (dry diameter DP>655 nm) increase by approximately 10 % (global average), whereas no significant change in the emission of ultrafine mode aerosols (dry diameter Dp<76 nm) was found over the same period. Significant regional differences in the number emission trends were also found. Based on CAM‐Oslo global climate model output, no straight‐forward relationship was found between the change in the number emissions and changes in the sea salt aerosol burden or optical thickness. This is attributed to a change in the simulated residence time of the sea salt aerosol. For the 21st century, a decrease in the residence time leads to a weaker sea salt aerosol‐climate feedback that what would be inferred based on changes in number emissions alone. Finally, quantifying any potential impact on marine stratocumulus cloud microphysical and radiative properties due to changes in sea salt aerosol number emissions is likely to be complicated by commensurate changes in anthropogenic aerosol emissions and changes in meteorology.

Interannual Tropospheric Aerosol Variability in the Late Twentieth Century and Its Impact on Tropical Atlantic and West African Climate by Direct and Semidirect Effects

Abstract A new high-resolution global tropospheric aerosol dataset with monthly resolution is generated using version 4 of the Community Atmosphere Model (CAM4) coupled to a bulk aerosol model and forced with recent estimates of surface emissions for the period 1961–2000 to identify tropospheric aerosol-induced interannual climate variations. The surface emissions dataset is constructed from phase 5 of the Coupled Model Intercomparison Project (CMIP5) decadal-resolution surface emissions dataset to include reanalysis of tropospheric chemical composition [40-yr Reanalysis of Tropospheric Chemical Composition (RETRO)] wildfire monthly emissions data. A four-member ensemble run is conducted using the spectral configuration of CAM4, forced with the new tropospheric aerosol dataset and prescribed with observed sea surface temperature, sea ice, and greenhouse gases. CAM4 only simulates the direct and semidirect effects of aerosols on the climate. The simulations reveal that variations in tropospheric aerosol levels can induce significant regional climate variability on the interannual time scales. Regression analyses over tropical Atlantic and Africa suggest that increasing dust aerosols can cool the North African landmass and shift convection southward from West Africa into the Gulf of Guinea in the spring season. Further, it is found that carbonaceous aerosols emanating from the southwestern African savannas can significantly cool the region and increase the marine stratocumulus cloud cover over the southeast tropical Atlantic Ocean by aerosol-induced diabatic heating of the free troposphere above the low clouds. Experiments conducted with CAM4 coupled to a slab ocean model suggest that present-day aerosols can cool the tropical North Atlantic and shift the intertropical convergence zone southward and can reduce the ocean mixed layer temperature beneath the increased marine stratocumulus clouds in the southeastern tropical Atlantic.

Marine cloud brightening

The idea behind the marine cloud-brightening (MCB) geoengineering technique is that seeding marine stratocumulus clouds with copious quantities of roughly monodisperse sub-micrometre sea water particles might significantly enhance the cloud droplet number concentration, and thereby the cloud albedo and possibly longevity. This would produce a cooling, which general circulation model (GCM) computations suggest could—subject to satisfactory resolution of technical and scientific problems identified herein—have the capacity to balance global warming up to the carbon dioxide-doubling point. We describe herein an account of our recent research on a number of critical issues associated with MCB. This involves (i) GCM studies, which are our primary tools for evaluating globally the effectiveness of MCB, and assessing its climate impacts on rainfall amounts and distribution, and also polar sea-ice cover and thickness; (ii) high-resolution modelling of the effects of seeding on marine stratocumulus, which are required to understand the complex array of interacting processes involved in cloud brightening; (iii) microphysical modelling sensitivity studies, examining the influence of seeding amount, seed-particle salt-mass, air-mass characteristics, updraught speed and other parameters on cloud–albedo change; (iv) sea water spray-production techniques; (v) computational fluid dynamics studies of possible large-scale periodicities in Flettner rotors; and (vi) the planning of a three-stage limited-area field research experiment, with the primary objectives of technology testing and determining to what extent, if any, cloud albedo might be enhanced by seeding marine stratocumulus clouds on a spatial scale of around 100×100 km. We stress that there would be no justification for deployment of MCB unless it was clearly established that no significant adverse consequences would result. There would also need to be an international agreement firmly in favour of such action.

Occurrence of lower cloud albedo in ship tracks

Abstract. The concept of geoengineering by marine cloud brightening is based on seeding marine stratocumulus clouds with sub-micrometer sea-salt particles to enhance the cloud droplet number concentration and cloud albedo, thereby producing a climate cooling effect. The efficacy of this as a strategy for global cooling rests on the extent to which aerosol-perturbed marine clouds will respond with increased albedo. Ship tracks, quasi-linear cloud features prevalent in oceanic regions impacted by ship exhaust, are a well-known manifestation of the effect of aerosol injection on marine clouds. We present here an analysis of the albedo responses in ship tracks, based on in situ aircraft measurements and three years of satellite observations of 589 individual ship tracks. It is found that the sign (increase or decrease) and magnitude of the albedo response in ship tracks depends on the mesoscale cloud structure, the free tropospheric humidity, and cloud top height. In a closed cell structure (cloud cells ringed by a perimeter of clear air), nearly 30% of ship tracks exhibited a decreased albedo. Detailed cloud responses must be accounted for in global studies of the potential efficacy of sea-spray geoengineering as a means to counteract global warming.

Anthropogenic radiative forcing of marine stratocumulus clouds under different thermodynamic conditions—An LES model study

A LES model with bin microphysics was used to investigate the aerosol indirect effects of marine stratocumulus clouds that develop under different thermodynamic conditions. The diurnal contrasts of cloud development were also examined in detail. Three observed CCN spectra that represent maritime, continental and polluted air masses were used as input CCN spectra. Two observed thermodynamic soundings and two derived ones from the observed soundings to vary the inversion altitude were used as initial thermodynamic conditions. With these initial conditions the model was run for the daytime and nocturnal conditions to make the total number of model runs to be 24. For both daytime and nocturnal conditions, the cloud depth and liquid water path (LWP) varied with the thermodynamic soundings. For a given thermodynamic sounding, LWP tended to be similar or slightly smaller for polluted. However, cloud top radiative cooling is stronger for polluted due to smaller sizes of cloud droplets and therefore turbulent mixing is stronger for polluted. However, there were significant differences in LWP between the daytime and nocturnal clouds. For the daytime condition, the cloud became decoupled from the surface layer and moisture supply was limited for all soundings. In contrast, with the absence of solar radiation, cloud top radiative cooling was much stronger, turbulent mixing was also much stronger and therefore no decoupling occurred and clouds were thicker with greater LWP in the nocturnal runs. To note is the derived thermodynamic sounding (named MH) that produced the thickest clouds: the clouds were too thick to be maintained as a single layer cloud and became multi-layered for the daytime condition while the maritime stratocumulus cloud broke up into a cumuliform cloud due to heavy drizzle for the nocturnal condition. From the daytime run results, the anthropogenic cloud radiative forcing was calculated by subtracting the net cloud radiative forcing for maritime from that for polluted. It amounted to be −21.6Wm−2 for MH but was more than a factor of two larger for all three other soundings, −56.4, −55.4 and −55.7Wm−2. For the MH sounding, the LWP was noticeably smaller for polluted than for maritime, the relative difference of the effective radii between maritime and polluted was small and therefore albedo difference between the two was also small, compared to those for the other three soundings. Notable is the similar magnitude of the anthropogenic cloud radiative forcing for these three soundings despite the significant differences in cloud depths among the clouds produced by these soundings. This may imply that there may be an optimal range of cloud depth that can produce a strong anthropogenic cloud radiative forcing. The cloud depths were smaller than 150m for the thermodynamic sounding that produced the shallowest clouds but it seemed to be within the optimal range.

Microphysical and macrophysical responses of marine stratocumulus polluted by underlying ships: 2. Impacts of haze on precipitating clouds

The 94‐GHz cloud profiling radar on the CloudSat satellite was used to examine the precipitation response of marine stratocumulus clouds to the increased aerosol burden from oceangoing vessels. Aerosol plumes generated by ships sometimes influence cloud microphysics and, to a largely undetermined extent, precipitation. To assess this response, the locations of over one thousand ship tracks coinciding with the radar were meticulously logged by hand from the Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. Although precipitation detectable by the radar was infrequent, drizzle rates were often suppressed (72% of cases) and lighter in the ship tracks compared to the clouds adjacent to them. Ship plumes primarily decreased average rain rates through reducing the spatial coverage of precipitation. However, larger liquid water paths in ship tracks seldom coincided with the reduced cloud water sink from the suppression of precipitation (in less than 20% of cases). The sign and strength of the precipitation response was primarily tied to the mesoscale convective structure of the clouds. When closed cellular clouds were identified, significant decreases in the relative average liquid water path, rain rate (an average relative decrease of 68%), and rain cover fraction were observed in ship tracks compared to the surrounding clouds. The opposite occurred in the open cell regime where it was more common to find enhanced precipitation in the perturbed clouds. Ship plumes ingested into this regime resulted in significantly deeper and brighter clouds with higher liquid water amounts and rain rates (an average relative increase of 89%).

Correction to “On the dependence of albedo on cloud microphysics over marine stratocumulus clouds regimes determined from Clouds and the Earth's Radiant Energy System (CERES) data”

[1] The dependence of the top-of-the-atmosphere (TOA) albedo A on cloud microphysical properties was investigated for the three largest maritime stratocumulus clouds regimes: off California, Southeast Pacific (Chile-Peru), and southwest Africa (Namibia-Angola). Absolute S and relative SR albedo susceptibilities to perturbations in cloud droplet number concentrations Nd, defined as dA/dNd and dA/dln(Nd) respectively, were calculated for the season having maximum cloud cover during the period 2006–2010. Satellite-based susceptibilities were computed by combining an adiabatically based Nd estimate and liquid water path (LWP) derived from Terra Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals matched with TOA A from the Clouds and the Earth's Radiant Energy System. Empirical susceptibility maps were calculated for three constant LWP intervals at 25, 50, and 90 g−2. It was found that S increases with LWP, with small and spatially homogeneous values for low LWP, and a contrasting increase far offshore for larger LWP values. An overall increase of SR with LWP was also observed, with larger values near the coast for LWP = 25 and 50 g−2. A relatively homogeneous spatial pattern of maximum SR values covered most of each regime's domain for a LWP of 90 g−2. These results highlight the importance of LWP in modulating the albedo susceptibility. The dependencies of S and SR on LWP are mostly explained by variations in the mean Nd and cloud optical thickness (τ), with an increase of S with LWP linked to a decrease in Nd, whereas SR increased with τ and A, until reaching a maximum for A and τ near 0.36–0.4 and 12–14 respectively, and decreasing thereafter, consistent with expectations based on two-stream estimates. Larger SR values in the Southeast Pacific are thought to be the consequence of a drier and more pristine atmosphere. Radiative transfer simulations with realistic values of above-cloud water vapor path and aerosol optical thickness showed that differing atmospheric compositions could explain why the Chile-Peru regime was the marine stratocumulus cloud deck most susceptible to change its TOA albedo due to fractional changes in Nd.

On the dependence of albedo on cloud microphysics over marine stratocumulus clouds regimes determined from Clouds and the Earth's Radiant Energy System (CERES) data

The dependence of the top‐of‐the‐atmosphere (TOA) albedo A on cloud microphysical properties was investigated for the three largest maritime stratocumulus clouds regimes: off California, Southeast Pacific (Chile‐Peru), and southwest Africa (Namibia‐Angola). Absolute S and relative SR albedo susceptibilities to perturbations in cloud droplet number concentrations Nd, defined as dA/dNd and dA/dln(Nd) respectively, were calculated for the season having maximum cloud cover during the period 2006–2010. Satellite‐based susceptibilities were computed by combining an adiabatically based Nd estimate and liquid water path (LWP) derived from Terra Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals matched with TOA A from the Clouds and the Earth's Radiant Energy System. Empirical susceptibility maps were calculated for three constant LWP intervals at 25, 50, and 90 g−2. It was found that S increases with LWP, with small and spatially homogeneous values for low LWP, and a contrasting increase far offshore for larger LWP values. An overall increase of SR with LWP was also observed, with larger values near the coast for LWP = 25 and 50 g−2. A relatively homogeneous spatial pattern of maximum SR values covered most of each regime's domain for a LWP of 90 g−2. These results highlight the importance of LWP in modulating the albedo susceptibility. The dependencies of S and SR on LWP are mostly explained by variations in the mean Nd and cloud optical thickness (τ), with an increase of S with LWP linked to a decrease in Nd, whereas SR increased with τ and A, until reaching a maximum for A and τ near 0.36–0.4 and 12–14 respectively, and decreasing thereafter, consistent with expectations based on two‐stream estimates. Larger SR values in the Southeast Pacific are thought to be the consequence of a drier and more pristine atmosphere. Radiative transfer simulations with realistic values of above‐cloud water vapor path and aerosol optical thickness showed that differing atmospheric compositions could explain why the Chile‐Peru regime was the marine stratocumulus cloud deck most susceptible to change its TOA albedo due to fractional changes in Nd.

Direct and semi-direct radiative forcing of smoke aerosols over clouds

Abstract. Observations from Earth observing satellites indicate that dark carbonaceous aerosols that absorb solar radiation are widespread in the tropics and subtropics. When these aerosols mix with clouds, there is generally a reduction of cloudiness owing to absorption of solar energy in the aerosol layer. Over the subtropical South Atlantic Ocean, where smoke from savannah burning in southern Africa resides above a persistent deck of marine stratocumulus clouds, radiative heating of the smoke layer leads to a thickening of the cloud layer. Here, satellite observations of the albedo of overcast scenes of 25 km2 size or larger are combined with additional satellite observations of clouds and aerosols to estimate the top-of-atmosphere direct radiative forcing attributable to presence of dark aerosol above bright cloud, and the negative semi-direct forcing attributable to the thickening of the cloud layer. The average positive direct radiative forcing by smoke over an overcast scene is 9.2±6.6 W m−2 for cases with an unambiguous signal of absorbing aerosol over cloud in passive ultraviolet remote sensing observations. However, cloud liquid water path is enhanced by 16.3±7.7 g m−2 across the range of values for sea surface temperature for cases of smoke over cloud. The negative radiative forcing associated with this semi-direct effect of smoke over clouds is estimated to be −5.9±3.5 W m−2. Therefore, the cooling associated with the semi-direct cloud thickening effect compensates for greater than 60 % of the direct radiative effect. Accounting for the frequency of occurrence of significant absorbing aerosol above overcast scenes leads to an estimate of the average direct forcing of 1.0±0.7 W m−2 contributed by these scenes averaged over the subtropical southeast Atlantic Ocean during austral winter. The regional average of the negative semi-direct forcing is −0.7±0.4 W m−2. Therefore, smoke aerosols overlaying the decks of overcast marine stratocumulus clouds considered here yield a small net positive radiative forcing, which results from the difference of two larger effects.

Tropical Atlantic Biases in CCSM4

This paper focuses on diagnosing biases in the seasonal climate of the tropical Atlantic in the twentieth-century simulation of the Community Climate System Model, version 4 (CCSM4). The biases appear in both atmospheric and oceanic components. Mean sea level pressure is erroneously high by a few millibars in the subtropical highs and erroneously low in the polar lows (similar to CCSM3). As a result, surface winds in the tropics are ~1 m s−1 too strong. Excess winds cause excess cooling and depressed SSTs north of the equator. However, south of the equator SST is erroneously high due to the presence of additional warming effects. The region of highest SST bias is close to southern Africa near the mean latitude of the Angola–Benguela Front (ABF). Comparison of CCSM4 to ocean simulations of various resolutions suggests that insufficient horizontal resolution leads to the insufficient northward transport of cool water along this coast and an erroneous southward stretching of the ABF. A similar problem arises in the coupled model if the atmospheric component produces alongshore winds that are too weak. Erroneously warm coastal SSTs spread westward through a combination of advection and positive air–sea feedback involving marine stratocumulus clouds. This study thus highlights three aspects to improve to reduce bias in coupled simulations of the tropical Atlantic: 1) large-scale atmospheric pressure fields; 2) the parameterization of stratocumulus clouds; and 3) the processes, including winds and ocean model resolution, that lead to errors in seasonal SST along southwestern Africa. Improvements of the latter require horizontal resolution much finer than the 1° currently used in many climate models.

Observation of a Self-Limiting, Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus

High-resolution measurements of thermodynamic, microphysical, and turbulence properties inside a turbulent inversion layer above a marine stratocumulus cloud layer are presented. The measurements are performed with the helicopter-towed measurement payload Airborne Cloud Turbulence Observation System (ACTOS), which allows for sampling with low true air speeds and steep profiles through cloud top. Vertical profiles show that the turbulent inversion layer consists of clear air above the cloud top, with nearly linear profiles of potential temperature, horizontal wind speed, absolute humidity, and concentration of interstitial aerosol. The layer is turbulent, with an energy dissipation rate nearly the same as that in the lower cloud, suggesting that the two are actively coupled, but with significant anisotropic turbulence at the large scales within the turbulent inversion layer. The turbulent inversion layer is traversed six times and the layer thickness is observed to vary between 37 and 85 m, whereas the potential temperature and horizontal wind speed differences at the top and bottom of the layer remain essentially constant. The Richardson number therefore increases with increasing layer thickness, from approximately 0.2 to 0.7, suggesting that the layer develops to the point where shear production of turbulence is sufficiently weak to be balanced by buoyancy suppression. This picture is consistent with prior numerical simulations of the evolution of turbulence in localized stratified shear layers. It is observed that the large eddy scale is suppressed by buoyancy and is on the order of the Ozmidov scale, much less than the thickness of the turbulent inversion layer, such that direct mixing between the cloud top and the free troposphere is inhibited, and the entrainment velocity tends to decrease with increasing turbulent inversion-layer thickness. Qualitatively, the turbulent inversion layer likely grows through nibbling rather than engulfment.

Variations in Cloud Cover and Cloud Types over the Ocean from Surface Observations, 1954–2008

Abstract Synoptic weather observations from ships throughout the World Ocean have been analyzed to produce a climatology of total cloud cover and the amounts of nine cloud types. About 54 million observations contributed to the climatology, which now covers 55 years from 1954 to 2008. In this work, interannual variations of seasonal cloud amounts are analyzed in 10° grid boxes. Long-term variations O(5–10 yr), coherent across multiple latitude bands, remain present in the updated cloud data. A comparison to coincident data on islands indicates that the coherent variations are probably spurious. An exact cause for this behavior remains elusive. The globally coherent variations are removed from the gridbox time series using a Butterworth filter before further analysis. Before removing the spurious variation, the global average time series of total cloud cover over the ocean shows low-amplitude, long-term variations O(2%) over the 55-yr span. High-frequency, year-to-year variation is seen O(1%–2%). Among the cloud types, the most widespread and consistent relationship is found for the extensive marine stratus and stratocumulus clouds (MSC) over the eastern parts of the subtropical oceans. Substantiating and expanding upon previous work, strong negative correlation is found between MSC and sea surface temperature (SST) in the eastern North Pacific, eastern South Pacific, eastern South Atlantic, eastern North Atlantic, and the Indian Ocean west of Australia. By contrast, a positive correlation between cloud cover and SST is seen in the central Pacific. High clouds show a consistent low-magnitude positive correlation with SST over the equatorial ocean. In regions of persistent MSC, time series show decreasing MSC amount. This decrease could be due to further spurious variation within the data. However, the decrease combined with observed increases in SST and the negative correlation between marine stratus and sea surface temperature suggests a positive cloud feedback to the warming sea surface. The observed decrease of MSC has been partly but not completely offset by increasing cumuliform clouds in these regions; a similar decrease in stratiform and increase in cumuliform clouds had previously been seen over land. Interannual variations of cloud cover in the tropics show strong correlation with an ENSO index.

Effect of coarse marine aerosols on stratocumulus clouds

[1] In contrast to fine anthropogenic aerosols (radii ∼<0.5 μm), large aerosol particles are thought to enhance cloud droplet growth, promote precipitation formation and reduce cloud albedo. While shown in cloud simulation models, the impact of coarse aerosols on marine stratocumulus clouds lacks observational evidence. Here, by combining data from AMSR-E and MODIS, both aboard NASA's satellite Aqua, we link the amount of coarse marine aerosols emitted to the atmosphere through wind-driven processes with the size of cloud droplets, at the world's largest deck of marine stratocumulus clouds over the southeastern Pacific. For constrained meteorological conditions, approximately 1/2 of the change in droplet effective radius (reff) is attributed to increase in coarse marine aerosol optical depth (τcm), as surface winds intensify. Accordingly, a twofold increase in τcm is associated with a 1.4 μm +/−0.11 increase in reff. Our results suggest that climatic changes in surface winds may play an important role not only over land for wind-power estimation but also over the oceans by changing clouds reflectance and lifetime.

A comprehensive numerical study of aerosol-cloud-precipitation interactions in marine stratocumulus

Abstract. Three-dimensional large-eddy simulations (LES) with detailed bin-resolved microphysics are performed to explore the diurnal variation of marine stratocumulus (MSc) clouds under clean and polluted conditions. The sensitivity of the aerosol-cloud-precipitation interactions to variation of sea surface temperature, free tropospheric humidity, large-scale divergence rate, and wind speed is assessed. The comprehensive set of simulations corroborates previous studies that (1) with moderate/heavy drizzle, an increase in aerosol leads to an increase in cloud thickness; and (2) with non/light drizzle, an increase in aerosol results in a thinner cloud, due to the pronounced effect on entrainment. It is shown that for higher SST, stronger large-scale divergence, drier free troposphere, or lower wind speed, the cloud thins and precipitation decreases. The sign and magnitude of the Twomey effect, droplet dispersion effect, cloud thickness effect, and cloud optical depth susceptibility to aerosol perturbations (i.e., change in cloud optical depth to change in aerosol number concentration) are evaluated by LES experiments and compared with analytical formulations. The Twomey effect emerges as dominant in total cloud optical depth susceptibility to aerosol perturbations. The dispersion effect, that of aerosol perturbations on the cloud droplet size spectrum, is positive (i.e., increase in aerosol leads to spectral narrowing) and accounts for 3% to 10% of the total cloud optical depth susceptibility at nighttime, with greater influence in heavier drizzling clouds. The cloud thickness effect is negative (i.e., increase in aerosol leads to thinner cloud) for non/light drizzling cloud and positive for a moderate/heavy drizzling clouds; the cloud thickness effect contributes 5% to 22% of the nighttime total cloud susceptibility. Overall, the total cloud optical depth susceptibility ranges from ~0.28 to 0.53 at night; an increase in aerosol concentration enhances cloud optical depth, especially with heavier precipitation and in a more pristine environment. During the daytime, the range of magnitude for each effect is more variable owing to cloud thinning and decoupling. The good agreement between LES experiments and analytical formulations suggests that the latter may be useful in evaluations of the total cloud susceptibility. The ratio of the magnitude of the cloud thickness effect to that of the Twomey effect depends on cloud base height and cloud thickness in unperturbed (clean) clouds.

Simultaneous retrieval of aerosol and cloud properties during the MILAGRO field campaign

Abstract. Estimation of Direct Climate Forcing (DCF) due to aerosols in cloudy areas has historically been a difficult task, mainly because of a lack of appropriate measurements. Recently, passive remote sensing instruments have been developed that have the potential to retrieve both cloud and aerosol properties using polarimetric, multiple view angle, and multi spectral observations, and therefore determine DCF from aerosols above clouds. One such instrument is the Research Scanning Polarimeter (RSP), an airborne prototype of a sensor on the NASA Glory satellite, which unfortunately failed to reach orbit during its launch in March of 2011. In the spring of 2006, the RSP was deployed on an aircraft based in Veracruz, Mexico, as part of the Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign. On 13 March, the RSP over flew an aerosol layer lofted above a low altitude marine stratocumulus cloud close to shore in the Gulf of Mexico. We investigate the feasibility of retrieving aerosol properties over clouds using these data. Our approach is to first determine cloud droplet size distribution using the angular location of the cloud bow and other features in the polarized reflectance. The selected cloud was then used in a multiple scattering radiative transfer model optimization to determine the aerosol optical properties and fine tune the cloud size distribution. In this scene, we were able to retrieve aerosol optical depth, the fine mode aerosol size distribution parameters and the cloud droplet size distribution parameters to a degree of accuracy required for climate modeling. This required assumptions about the aerosol vertical distribution and the optical properties of the coarse aerosol size mode. A sensitivity study was also performed to place this study in the context of future systematic scanning polarimeter observations, which found that the aerosol complex refractive index can also be observed accurately if the aerosol optical depth is larger than roughly 0.8 at a wavelength of (0.555 μm).

The aerosol–Bénard cell effect on marine stratocumulus clouds and its contribution to glacial-interglacial cycles

[1] Aerosol-cloud interactions, such as aerosol loading in convective clouds resulting in either precipitation suppression or cloud invigoration, in higher cloud tops, and in longer-lived clouds, are well known. Here we investigate a new aerosol-cloud interaction, the effect of aerosol loading on Bénard cells, on the stratocumulus cloud fraction, and ultimately on the climate over glacial-interglacial cycles, using a two-dimensional model running a million year continuous simulation. This radiative effect is observed only in marine boundary layer stratocumulus clouds that have a convective cellular structure. Recent research suggests that aerosols can switch the direction of convection in Bénard cells (from open cells to closed cells) by suppressing precipitation and therefore dramatically change the cloud fraction. The effect investigated in this work differs from previously known aerosol effects on convective clouds by its intensity and magnitude and has never been taken into account in past climate simulations. The results show that accounting for the aerosol–Bénard cell effect alone contributes a negative radiative forcing, affecting both the Northern Hemisphere mean annual surface temperature and ice volume. Adding the aerosol–Bénard cell effect to the direct radiative effect of dust and to the effect of dust on snow and ice albedo shows that the aerosol–Bénard cell effect plays a significant role in glacial-interglacial climate change, strengthening the earlier glacial cycles and creating a larger glacial-interglacial surface temperature amplitude while preserving the continental ice volume amplitude. Because of the model limitations, there are a number of uncertainties involved. However, the results serve to give a preliminary evaluation of the aerosol–Bénard cell effect at least qualitatively if not quantitatively.